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Replace 'disto' with 'distro' since they are all functions about distro booting. Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
675 lines
27 KiB
ReStructuredText
675 lines
27 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+:
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U-Boot Standard Boot
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====================
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Introduction
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------------
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Standard boot provides a built-in way for U-Boot to automatically boot
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an Operating System without custom scripting and other customisation. It
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introduces the following concepts:
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- bootdev - a device which can hold or access a distro (e.g. MMC, Ethernet)
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- bootmeth - a method to scan a bootdev to find bootflows (e.g. distro boot)
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- bootflow - a description of how to boot (provided by the distro)
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For Linux, the distro (Linux distribution, e.g. Debian, Fedora) is responsible
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for creating a bootflow for each kernel combination that it wants to offer.
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These bootflows are stored on media so they can be discovered by U-Boot. This
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feature is typically called `distro boot` (see :doc:`distro`) because it is
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a way for distributions to boot on any hardware.
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Traditionally U-Boot has relied on scripts to implement this feature. See
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distro_bootcmd_ for details. This is done because U-Boot has no native support
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for scanning devices. While the scripts work remarkably well, they can be hard
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to understand and extend, and the feature does not include tests. They are also
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making it difficult to move away from ad-hoc CONFIGs, since they are implemented
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using the environment and a lot of #defines.
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Standard boot is a generalisation of distro boot. It provides a more built-in
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way to boot with U-Boot. The feature is extensible to different Operating
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Systems (such as Chromium OS) and devices (beyond just block and network
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devices). It supports EFI boot and EFI bootmgr too.
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Finally, standard boot supports the operation of :doc:`vbe`.
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Bootflow
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--------
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A bootflow is a file that describes how to boot a distro. Conceptually there can
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be different formats for that file but at present U-Boot only supports the
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BootLoaderSpec_ format. which looks something like this::
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menu autoboot Welcome to Fedora-Workstation-armhfp-31-1.9. Automatic boot in # second{,s}. Press a key for options.
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menu title Fedora-Workstation-armhfp-31-1.9 Boot Options.
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menu hidden
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label Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
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kernel /vmlinuz-5.3.7-301.fc31.armv7hl
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append ro root=UUID=9732b35b-4cd5-458b-9b91-80f7047e0b8a rhgb quiet LANG=en_US.UTF-8 cma=192MB cma=256MB
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fdtdir /dtb-5.3.7-301.fc31.armv7hl/
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initrd /initramfs-5.3.7-301.fc31.armv7hl.img
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As you can see it specifies a kernel, a ramdisk (initrd) and a directory from
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which to load devicetree files. The details are described in distro_bootcmd_.
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The bootflow is provided by the distro. It is not part of U-Boot. U-Boot's job
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is simply to interpret the file and carry out the instructions. This allows
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distros to boot on essentially any device supported by U-Boot.
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Typically the first available bootflow is selected and booted. If that fails,
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then the next one is tried.
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Bootdev
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-------
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Where does U-Boot find the media that holds the operating systems? That is the
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job of bootdev. A bootdev is simply a layer on top of a media device (such as
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MMC, NVMe). The bootdev accesses the device, including partitions and
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filesystems that might contain things related to an operating system.
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For example, an MMC bootdev provides access to the individual partitions on the
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MMC device. It scans through these to find filesystems, then provides a list of
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these for consideration.
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Bootmeth
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--------
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Once the list of filesystems is provided, how does U-Boot find the bootflow
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files in these filesystems. That is the job of bootmeth. Each boot method has
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its own way of doing this.
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For example, the distro bootmeth simply looks through the provided filesystem
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for a file called `extlinux/extlinux.conf`. This files constitutes a bootflow.
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If the distro bootmeth is used on multiple partitions it may produce multiple
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bootflows.
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Note: it is possible to have a bootmeth that uses a partition or a whole device
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directly, but it is more common to use a filesystem.
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Note that some bootmeths are 'global', meaning that they select the bootdev
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themselves. Examples include VBE and EFI boot manager. In this case, they
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provide a `read_bootflow()` method which checks whatever bootdevs it likes, then
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returns the bootflow, if found. Some of these bootmeths may be very slow, if
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they scan a lot of devices.
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Boot process
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------------
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U-Boot tries to use the 'lazy init' approach whereever possible and distro boot
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is no exception. The algorithm is::
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while (get next bootdev)
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while (get next bootmeth)
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while (get next bootflow)
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try to boot it
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So U-Boot works its way through the bootdevs, trying each bootmeth in turn to
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obtain bootflows, until it either boots or exhausts the available options.
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Instead of 500 lines of #defines and a 4KB boot script, all that is needed is
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the following command::
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bootflow scan -lb
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which scans for available bootflows, optionally listing each find it finds (-l)
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and trying to boot it (-b).
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When global bootmeths are available, these are typically checked before the
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above bootdev scanning.
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Controlling ordering
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--------------------
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Several options are available to control the ordering of boot scanning:
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boot_targets
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~~~~~~~~~~~~
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This environment variable can be used to control the list of bootdevs searched
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and their ordering, for example::
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setenv boot_targets "mmc0 mmc1 usb pxe"
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Entries may be removed or re-ordered in this list to affect the boot order. If
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the variable is empty, the default ordering is used, based on the priority of
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bootdevs and their sequence numbers.
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bootmeths
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~~~~~~~~~
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This environment variable can be used to control the list of bootmeths used and
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their ordering for example::
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setenv bootmeths "syslinux efi"
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Entries may be removed or re-ordered in this list to affect the order the
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bootmeths are tried on each bootdev. If the variable is empty, the default
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ordering is used, based on the bootmeth sequence numbers, which can be
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controlled by aliases.
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The :ref:`usage/cmd/bootmeth:bootmeth command` (`bootmeth order`) operates in
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the same way as setting this variable.
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Bootdev uclass
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--------------
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The bootdev uclass provides an simple API call to obtain a bootflows from a
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device::
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int bootdev_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
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struct bootflow *bflow);
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This takes a iterator which indicates the bootdev, partition and bootmeth to
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use. It returns a bootflow. This is the core of the bootdev implementation. The
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bootdev drivers that implement this differ depending on the media they are
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reading from, but each is responsible for returning a valid bootflow if
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available.
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A helper called `bootdev_find_in_blk()` makes it fairly easy to implement this
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function for each media device uclass, in a few lines of code.
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Bootdev drivers
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---------------
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A bootdev driver is typically fairly simple. Here is one for mmc::
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static int mmc_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
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struct bootflow *bflow)
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{
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struct udevice *mmc_dev = dev_get_parent(dev);
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struct udevice *blk;
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int ret;
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ret = mmc_get_blk(mmc_dev, &blk);
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/*
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* If there is no media, indicate that no more partitions should be
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* checked
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*/
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if (ret == -EOPNOTSUPP)
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ret = -ESHUTDOWN;
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if (ret)
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return log_msg_ret("blk", ret);
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assert(blk);
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ret = bootdev_find_in_blk(dev, blk, iter, bflow);
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if (ret)
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return log_msg_ret("find", ret);
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return 0;
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}
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static int mmc_bootdev_bind(struct udevice *dev)
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{
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struct bootdev_uc_plat *ucp = dev_get_uclass_plat(dev);
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ucp->prio = BOOTDEVP_0_INTERNAL_FAST;
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return 0;
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}
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struct bootdev_ops mmc_bootdev_ops = {
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.get_bootflow = mmc_get_bootflow,
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};
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static const struct udevice_id mmc_bootdev_ids[] = {
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{ .compatible = "u-boot,bootdev-mmc" },
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{ }
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};
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U_BOOT_DRIVER(mmc_bootdev) = {
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.name = "mmc_bootdev",
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.id = UCLASS_BOOTDEV,
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.ops = &mmc_bootdev_ops,
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.bind = mmc_bootdev_bind,
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.of_match = mmc_bootdev_ids,
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};
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The implementation of the `get_bootflow()` method is simply to obtain the
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block device and call a bootdev helper function to do the rest. The
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implementation of `bootdev_find_in_blk()` checks the partition table, and
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attempts to read a file from a filesystem on the partition number given by the
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`@iter->part` parameter.
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Each bootdev has a priority, which indicates the order in which it is used.
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Faster bootdevs are used first, since they are more likely to be able to boot
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the device quickly.
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Device hierarchy
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----------------
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A bootdev device is a child of the media device. In this example, you can see
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that the bootdev is a sibling of the block device and both are children of
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media device::
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mmc 0 [ + ] bcm2835-sdhost | |-- mmc@7e202000
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blk 0 [ + ] mmc_blk | | |-- mmc@7e202000.blk
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bootdev 0 [ ] mmc_bootdev | | `-- mmc@7e202000.bootdev
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mmc 1 [ + ] sdhci-bcm2835 | |-- sdhci@7e300000
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blk 1 [ ] mmc_blk | | |-- sdhci@7e300000.blk
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bootdev 1 [ ] mmc_bootdev | | `-- sdhci@7e300000.bootdev
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The bootdev device is typically created automatically in the media uclass'
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`post_bind()` method by calling `bootdev_setup_for_dev()`. The code typically
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something like this::
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ret = bootdev_setup_for_dev(dev, "eth_bootdev");
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if (ret)
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return log_msg_ret("bootdev", ret);
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Here, `eth_bootdev` is the name of the Ethernet bootdev driver and `dev`
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is the ethernet device. This function is safe to call even if standard boot is
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not enabled, since it does nothing in that case. It can be added to all uclasses
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which implement suitable media.
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The bootstd device
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------------------
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Standard boot requires a single instance of the bootstd device to make things
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work. This includes global information about the state of standard boot. See
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`struct bootstd_priv` for this structure, accessed with `bootstd_get_priv()`.
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Within the devicetree, if you add bootmeth devices, they should be children of
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the bootstd device. See `arch/sandbox/dts/test.dts` for an example of this.
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.. _`Automatic Devices`:
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Automatic devices
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-----------------
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It is possible to define all the required devices in the devicetree manually,
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but it is not necessary. The bootstd uclass includes a `dm_scan_other()`
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function which creates the bootstd device if not found. If no bootmeth devices
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are found at all, it creates one for each available bootmeth driver.
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If your devicetree has any bootmeth device it must have all of them that you
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want to use, since no bootmeth devices will be created automatically in that
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case.
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Using devicetree
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----------------
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If a bootdev is complicated or needs configuration information, it can be
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added to the devicetree as a child of the media device. For example, imagine a
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bootdev which reads a bootflow from SPI flash. The devicetree fragment might
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look like this::
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spi@0 {
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flash@0 {
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reg = <0>;
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compatible = "spansion,m25p16", "jedec,spi-nor";
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spi-max-frequency = <40000000>;
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bootdev {
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compatible = "u-boot,sf-bootdev";
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offset = <0x2000>;
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size = <0x1000>;
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};
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};
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};
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The `sf-bootdev` driver can implement a way to read from the SPI flash, using
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the offset and size provided, and return that bootflow file back to the caller.
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When distro boot wants to read the kernel it calls distro_getfile() which must
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provide a way to read from the SPI flash. See `distro_boot()` at distro_boot_
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for more details.
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Of course this is all internal to U-Boot. All the distro sees is another way
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to boot.
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Configuration
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-------------
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Standard boot is enabled with `CONFIG_BOOTSTD`. Each bootmeth has its own CONFIG
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option also. For example, `CONFIG_BOOTMETH_DISTRO` enables support for distro
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boot from a disk.
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Available bootmeth drivers
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--------------------------
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Bootmeth drivers are provided for:
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- distro boot from a disk (syslinux)
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- distro boot from a network (PXE)
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- EFI boot using bootefi
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- VBE
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- EFI boot using boot manager
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Command interface
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-----------------
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Three commands are available:
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`bootdev`
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Allows listing of available bootdevs, selecting a particular one and
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getting information about it. See :doc:`../usage/cmd/bootdev`
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`bootflow`
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Allows scanning one or more bootdevs for bootflows, listing available
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bootflows, selecting one, obtaining information about it and booting it.
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See :doc:`../usage/cmd/bootflow`
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`bootmeth`
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Allow listing of available bootmethds and setting the order in which they
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are tried. See :doc:`../usage/cmd/bootmeth`
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.. _BootflowStates:
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Bootflow states
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---------------
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Here is a list of states that a bootflow can be in:
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======= =======================================================================
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State Meaning
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======= =======================================================================
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base Starting-out state, indicates that no media/partition was found. For an
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SD card socket it may indicate that the card is not inserted.
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media Media was found (e.g. SD card is inserted) but no partition information
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was found. It might lack a partition table or have a read error.
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part Partition was found but a filesystem could not be read. This could be
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because the partition does not hold a filesystem or the filesystem is
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very corrupted.
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fs Filesystem was found but the file could not be read. It could be
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missing or in the wrong subdirectory.
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file File was found and its size detected, but it could not be read. This
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could indicate filesystem corruption.
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ready File was loaded and is ready for use. In this state the bootflow is
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ready to be booted.
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======= =======================================================================
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Theory of operation
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-------------------
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This describes how standard boot progresses through to booting an operating
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system.
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To start. all the necessary devices must be bound, including bootstd, which
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provides the top-level `struct bootstd_priv` containing optional configuration
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information. The bootstd device is also holds the various lists used while
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scanning. This step is normally handled automatically by driver model, as
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described in `Automatic Devices`_.
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Bootdevs are also required, to provide access to the media to use. These are not
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useful by themselves: bootmeths are needed to provide the means of scanning
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those bootdevs. So, all up, we need a single bootstd device, one or more bootdev
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devices and one or more bootmeth devices.
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Once these are ready, typically a `bootflow scan` command is issued. This kicks
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of the iteration process, which involves looking through the bootdevs and their
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partitions one by one to find bootflows.
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Iteration is kicked off using `bootflow_scan_first()`, which calls
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`bootflow_scan_bootdev()`.
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The iterator is set up with `bootflow_iter_init()`. This simply creates an
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empty one with the given flags. Flags are used to control whether each
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iteration is displayed, whether to return iterations even if they did not result
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in a valid bootflow, whether to iterate through just a single bootdev, etc.
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Then the ordering of bootdevs is determined, by `bootdev_setup_iter_order()`. By
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default, the bootdevs are used in the order specified by the `boot_targets`
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environment variable (e.g. "mmc2 mmc0 usb"). If that is missing then their
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sequence order is used, as determined by the `/aliases` node, or failing that
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their order in the devicetree. For BOOTSTD_FULL, if there is a `bootdev-order`
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property in the bootstd node, then this is used as a final fallback. In any
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case, the iterator ends up with a `dev_order` array containing the bootdevs that
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are going to be used, with `num_devs` set to the number of bootdevs and
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`cur_dev` starting at 0.
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Next, the ordering of bootmeths is determined, by `bootmeth_setup_iter_order()`.
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By default the ordering is again by sequence number, i.e. the `/aliases` node,
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or failing that the order in the devicetree. But the `bootmeth order` command
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or `bootmeths` environment variable can be used to set up an ordering. If that
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has been done, the ordering is in `struct bootstd_priv`, so that ordering is
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simply copied into the iterator. Either way, the `method_order` array it set up,
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along with `num_methods`.
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Note that global bootmeths are always put at the end of the ordering. If any are
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present, `cur_method` is set to the first one, so that global bootmeths are done
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first. Once all have been used, these bootmeths are dropped from the iteration.
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When there are no global bootmeths, `cur_method` is set to 0.
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At this point the iterator is ready to use, with the first bootdev and bootmeth
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selected. Most of the other fields are 0. This means that the current partition
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is 0, which is taken to mean the whole device, since partition numbers start at
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1. It also means that `max_part` is 0, i.e. the maximum partition number we know
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about is 0, meaning that, as far as we know, there is no partition table on this
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bootdev.
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With the iterator ready, `bootflow_scan_bootdev()` checks whether the current
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settings produce a valid bootflow. This is handled by `bootflow_check()`, which
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either returns 0 (if it got something) or an error if not (more on that later).
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If the `BOOTFLOWF_ALL` iterator flag is set, even errors are returned as
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incomplete bootflows, but normally an error results in moving onto the next
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iteration.
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Note that `bootflow_check()` handles global bootmeths explicitly, but calling
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`bootmeth_get_bootflow()` on each one. The `doing_global` flag indicates when
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the iterator is in that state.
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The `bootflow_scan_next()` function handles moving onto the next iteration and
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checking it. In fact it sits in a loop doing that repeatedly until it finds
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something it wants to return.
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The actual 'moving on' part is implemented in `iter_incr()`. This is a very
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simple function. It increments the first counter. If that hits its maximum, it
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sets it to zero and increments the second counter. You can think of all the
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counters together as a number with three digits which increment in order, with
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the least-sigificant digit on the right, counting like this:
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======== ======= =======
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bootdev part method
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======== ======= =======
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0 0 0
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0 0 1
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0 0 2
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0 1 0
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0 1 1
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0 1 2
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1 0 0
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1 0 1
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...
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======== ======= =======
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The maximum value for `method` is `num_methods - 1` so when it exceeds that, it
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goes back to 0 and the next `part` is considered. The maximum value for that is
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`max_part`, which is initially zero for all bootdevs. If we find a partition
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table on that bootdev, `max_part` can be updated during the iteration to a
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higher value - see `bootdev_find_in_blk()` for that, described later. If that
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exceeds its maximum, then the next bootdev is used. In this way, iter_incr()
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works its way through all possibilities, moving forward one each time it is
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called.
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Note that global bootmeths introduce a subtlety into the above description.
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When `doing_global` is true, the iteration takes place only among the bootmeths,
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i.e. the last column above. The global bootmeths are at the end of the list.
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Assuming that they are entries 3 and 4 in the list, the iteration then looks
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like this:
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======== ======= ======= =======================================
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bootdev part method notes
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======== ======= ======= =======================================
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. . 3 doing_global = true, method_count = 5
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. . 4
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0 0 0 doing_global = false, method_count = 3
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0 0 1
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0 0 2
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0 1 0
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0 1 1
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0 1 2
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1 0 0
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1 0 1
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...
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======== ======= ======= =======================================
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The changeover of the value of `doing_global` from true to false is handled in
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`iter_incr()` as well.
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There is no expectation that iteration will actually finish. Quite often a
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valid bootflow is found early on. With `bootflow scan -b`, that causes the
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bootflow to be immediately booted. Assuming it is successful, the iteration never
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completes.
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Also note that the iterator hold the **current** combination being considered.
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So when `iter_incr()` is called, it increments to the next one and returns it,
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the new **current** combination.
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Note also the `err` field in `struct bootflow_iter`. This is normally 0 and has
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thus has no effect on `iter_inc()`. But if it is non-zero, signalling an error,
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it indicates to the iterator what it should do when called. It can force moving
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to the next partition, or bootdev, for example. The special values
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`BF_NO_MORE_PARTS` and `BF_NO_MORE_DEVICES` handle this. When `iter_incr` sees
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`BF_NO_MORE_PARTS` it knows that it should immediately move to the next bootdev.
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When it sees `BF_NO_MORE_DEVICES` it knows that there is nothing more it can do
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so it should immediately return. The caller of `iter_incr()` is responsible for
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updating the `err` field, based on the return value it sees.
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The above describes the iteration process at a high level. It is basically a
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very simple increment function with a checker called `bootflow_check()` that
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checks the result of each iteration generated, to determine whether it can
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produce a bootflow.
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So what happens inside of `bootflow_check()`? It simply calls the uclass
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method `bootdev_get_bootflow()` to ask the bootdev to return a bootflow. It
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passes the iterator to the bootdev method, so that function knows what we are
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talking about. At first, the bootflow is set up in the state `BOOTFLOWST_BASE`,
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with just the `method` and `dev` intiialised. But the bootdev may fill in more,
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e.g. updating the state, depending on what it finds. For global bootmeths the
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`bootmeth_get_bootflow()` function is called instead of
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`bootdev_get_bootflow()`.
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Based on what the bootdev or bootmeth responds with, `bootflow_check()` either
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returns a valid bootflow, or a partial one with an error. A partial bootflow
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is one that has some fields set up, but did not reach the `BOOTFLOWST_READY`
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state. As noted before, if the `BOOTFLOWF_ALL` iterator flag is set, then all
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bootflows are returned, even partial ones. This can help with debugging.
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So at this point you can see that total control over whether a bootflow can
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be generated from a particular iteration, or not, rests with the bootdev (or
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global bootmeth). Each one can adopt its own approach.
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Going down a level, what does the bootdev do in its `get_bootflow()` method?
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Let us consider the MMC bootdev. In that case the call to
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`bootdev_get_bootflow()` ends up in `mmc_get_bootflow()`. It locates the parent
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device of the bootdev, i.e. the `UCLASS_MMC` device itself, then finds the block
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device associated with it. It then calls the helper function
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`bootdev_find_in_blk()` to do all the work. This is common with just about any
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bootdev that is based on a media device.
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The `bootdev_find_in_blk()` helper is implemented in the bootdev uclass. It
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names the bootflow and copies the partition number in from the iterator. Then it
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calls the bootmeth device to check if it can support this device. This is
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important since some bootmeths only work with network devices, for example. If
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that check fails, it stops.
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Assuming the bootmeth is happy, or at least indicates that it is willing to try
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(by returning 0 from its `check()` method), the next step is to try the
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partition. If that works it tries to detect a file system. If that works then it
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calls the bootmeth device once more, this time to read the bootflow.
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Note: At present a filesystem is needed for the bootmeth to be called on block
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devices, simply because we don't have any examples where this is not the case.
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This feature can be added as needed.
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If we take the example of the `bootmeth_distro` driver, this call ends up at
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`distro_read_bootflow()`. It has the filesystem ready, so tries various
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filenames to try to find the `extlinux.conf` file, reading it if possible. If
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all goes well the bootflow ends up in the `BOOTFLOWST_READY` state.
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At this point, we fall back from the bootmeth driver, to
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`bootdev_find_in_blk()`, then back to `mmc_get_bootflow()`, then to
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`bootdev_get_bootflow()`, then to `bootflow_check()` and finally to its caller,
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either `bootflow_scan_bootdev()` or `bootflow_scan_next()`. In either case,
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the bootflow is returned as the result of this iteration, assuming it made it to
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the `BOOTFLOWST_READY` state.
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That is the basic operation of scanning for bootflows. The process of booting a
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bootflow is handled by the bootmeth driver for that bootflow. In the case of
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distro boot, this parses and processes the `extlinux.conf` file that was read.
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See `distro_boot()` for how that works. The processing may involve reading
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additional files, which is handled by the `read_file()` method, which is
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`distro_read_file()` in this case. All bootmethds should support reading files,
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since the bootflow is typically only the basic instructions and does not include
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the operating system itself, ramdisk, device tree, etc.
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The vast majority of the bootstd code is concerned with iterating through
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partitions on bootdevs and using bootmethds to find bootflows.
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How about bootdevs which are not block devices? They are handled by the same
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methods as above, but with a different implementation. For example, the bootmeth
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for PXE boot (over a network) uses `tftp` to read files rather than `fs_read()`.
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But other than that it is very similar.
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Tests
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-----
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Tests are located in `test/boot` and cover the core functionality as well as
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the commands. All tests use sandbox so can be run on a standard Linux computer
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and in U-Boot's CI.
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For testing, a DOS-formatted disk image is used with a single FAT partition on
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it. This is created in `setup_bootflow_image()`, with a canned one from the
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source tree used if it cannot be created (e.g. in CI).
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Bootflow internals
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------------------
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The bootstd device holds a linked list of scanned bootflows as well as the
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currently selected bootdev and bootflow (for use by commands). This is in
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`struct bootstd_priv`.
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Each bootdev device has its own `struct bootdev_uc_plat` which holds a
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list of scanned bootflows just for that device.
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The bootflow itself is documented in bootflow_h_. It includes various bits of
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information about the bootflow and a buffer to hold the file.
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Future
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------
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Apart from the to-do items below, different types of bootflow files may be
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implemented in future, e.g. Chromium OS support which is currently only
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available as a script in chromebook_coral.
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To do
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-----
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Some things that need to be done to completely replace the distro-boot scripts:
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- add bootdev drivers for dhcp, sata, scsi, ide, virtio
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- PXE boot for EFI
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- support for loading U-Boot scripts
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Other ideas:
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- `bootflow prep` to load everything preparing for boot, so that `bootflow boot`
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can just do the boot.
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- automatically load kernel, FDT, etc. to suitable addresses so the board does
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not need to specify things like `pxefile_addr_r`
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.. _distro_bootcmd: https://github.com/u-boot/u-boot/blob/master/include/config_distro_bootcmd.h
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.. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
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.. _distro_boot: https://github.com/u-boot/u-boot/blob/master/boot/distro.c
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.. _bootflow_h: https://github.com/u-boot/u-boot/blob/master/include/bootflow.h
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