mirror of
https://github.com/AsahiLinux/u-boot
synced 2024-12-01 17:10:11 +00:00
c2ee5ee7b3
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+:
|
|
|
|
U-Boot Standard Boot
|
|
====================
|
|
|
|
Introduction
|
|
------------
|
|
|
|
Standard boot provides a built-in way for U-Boot to automatically boot
|
|
an Operating System without custom scripting and other customisation. It
|
|
introduces the following concepts:
|
|
|
|
- bootdev - a device which can hold or access a distro (e.g. MMC, Ethernet)
|
|
- bootmeth - a method to scan a bootdev to find bootflows (e.g. distro boot)
|
|
- bootflow - a description of how to boot (provided by the distro)
|
|
|
|
For Linux, the distro (Linux distribution, e.g. Debian, Fedora) is responsible
|
|
for creating a bootflow for each kernel combination that it wants to offer.
|
|
These bootflows are stored on media so they can be discovered by U-Boot. This
|
|
feature is typically called `distro boot` (see :doc:`distro`) because it is
|
|
a way for distributions to boot on any hardware.
|
|
|
|
Traditionally U-Boot has relied on scripts to implement this feature. See
|
|
distro_bootcmd_ for details. This is done because U-Boot has no native support
|
|
for scanning devices. While the scripts work remarkably well, they can be hard
|
|
to understand and extend, and the feature does not include tests. They are also
|
|
making it difficult to move away from ad-hoc CONFIGs, since they are implemented
|
|
using the environment and a lot of #defines.
|
|
|
|
Standard boot is a generalisation of distro boot. It provides a more built-in
|
|
way to boot with U-Boot. The feature is extensible to different Operating
|
|
Systems (such as Chromium OS) and devices (beyond just block and network
|
|
devices). It supports EFI boot and EFI bootmgr too.
|
|
|
|
Finally, standard boot supports the operation of :doc:`vbe`.
|
|
|
|
Bootflow
|
|
--------
|
|
|
|
A bootflow is a file that describes how to boot a distro. Conceptually there can
|
|
be different formats for that file but at present U-Boot only supports the
|
|
BootLoaderSpec_ format. which looks something like this::
|
|
|
|
menu autoboot Welcome to Fedora-Workstation-armhfp-31-1.9. Automatic boot in # second{,s}. Press a key for options.
|
|
menu title Fedora-Workstation-armhfp-31-1.9 Boot Options.
|
|
menu hidden
|
|
|
|
label Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
|
|
kernel /vmlinuz-5.3.7-301.fc31.armv7hl
|
|
append ro root=UUID=9732b35b-4cd5-458b-9b91-80f7047e0b8a rhgb quiet LANG=en_US.UTF-8 cma=192MB cma=256MB
|
|
fdtdir /dtb-5.3.7-301.fc31.armv7hl/
|
|
initrd /initramfs-5.3.7-301.fc31.armv7hl.img
|
|
|
|
As you can see it specifies a kernel, a ramdisk (initrd) and a directory from
|
|
which to load devicetree files. The details are described in distro_bootcmd_.
|
|
|
|
The bootflow is provided by the distro. It is not part of U-Boot. U-Boot's job
|
|
is simply to interpret the file and carry out the instructions. This allows
|
|
distros to boot on essentially any device supported by U-Boot.
|
|
|
|
Typically the first available bootflow is selected and booted. If that fails,
|
|
then the next one is tried.
|
|
|
|
|
|
Bootdev
|
|
-------
|
|
|
|
Where does U-Boot find the media that holds the operating systems? That is the
|
|
job of bootdev. A bootdev is simply a layer on top of a media device (such as
|
|
MMC, NVMe). The bootdev accesses the device, including partitions and
|
|
filesystems that might contain things related to an operating system.
|
|
|
|
For example, an MMC bootdev provides access to the individual partitions on the
|
|
MMC device. It scans through these to find filesystems, then provides a list of
|
|
these for consideration.
|
|
|
|
|
|
Bootmeth
|
|
--------
|
|
|
|
Once the list of filesystems is provided, how does U-Boot find the bootflow
|
|
files in these filesystems. That is the job of bootmeth. Each boot method has
|
|
its own way of doing this.
|
|
|
|
For example, the distro bootmeth simply looks through the provided filesystem
|
|
for a file called `extlinux/extlinux.conf`. This files constitutes a bootflow.
|
|
If the distro bootmeth is used on multiple partitions it may produce multiple
|
|
bootflows.
|
|
|
|
Note: it is possible to have a bootmeth that uses a partition or a whole device
|
|
directly, but it is more common to use a filesystem.
|
|
|
|
Note that some bootmeths are 'global', meaning that they select the bootdev
|
|
themselves. Examples include VBE and EFI boot manager. In this case, they
|
|
provide a `read_bootflow()` method which checks whatever bootdevs it likes, then
|
|
returns the bootflow, if found. Some of these bootmeths may be very slow, if
|
|
they scan a lot of devices.
|
|
|
|
|
|
Boot process
|
|
------------
|
|
|
|
U-Boot tries to use the 'lazy init' approach whereever possible and distro boot
|
|
is no exception. The algorithm is::
|
|
|
|
while (get next bootdev)
|
|
while (get next bootmeth)
|
|
while (get next bootflow)
|
|
try to boot it
|
|
|
|
So U-Boot works its way through the bootdevs, trying each bootmeth in turn to
|
|
obtain bootflows, until it either boots or exhausts the available options.
|
|
|
|
Instead of 500 lines of #defines and a 4KB boot script, all that is needed is
|
|
the following command::
|
|
|
|
bootflow scan -lb
|
|
|
|
which scans for available bootflows, optionally listing each find it finds (-l)
|
|
and trying to boot it (-b).
|
|
|
|
When global bootmeths are available, these are typically checked before the
|
|
above bootdev scanning.
|
|
|
|
|
|
Controlling ordering
|
|
--------------------
|
|
|
|
Several options are available to control the ordering of boot scanning:
|
|
|
|
|
|
boot_targets
|
|
~~~~~~~~~~~~
|
|
|
|
This environment variable can be used to control the list of bootdevs searched
|
|
and their ordering, for example::
|
|
|
|
setenv boot_targets "mmc0 mmc1 usb pxe"
|
|
|
|
Entries may be removed or re-ordered in this list to affect the boot order. If
|
|
the variable is empty, the default ordering is used, based on the priority of
|
|
bootdevs and their sequence numbers.
|
|
|
|
|
|
bootmeths
|
|
~~~~~~~~~
|
|
|
|
This environment variable can be used to control the list of bootmeths used and
|
|
their ordering for example::
|
|
|
|
setenv bootmeths "syslinux efi"
|
|
|
|
Entries may be removed or re-ordered in this list to affect the order the
|
|
bootmeths are tried on each bootdev. If the variable is empty, the default
|
|
ordering is used, based on the bootmeth sequence numbers, which can be
|
|
controlled by aliases.
|
|
|
|
The :ref:`usage/cmd/bootmeth:bootmeth command` (`bootmeth order`) operates in
|
|
the same way as setting this variable.
|
|
|
|
|
|
Bootdev uclass
|
|
--------------
|
|
|
|
The bootdev uclass provides an simple API call to obtain a bootflows from a
|
|
device::
|
|
|
|
int bootdev_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
|
|
struct bootflow *bflow);
|
|
|
|
This takes a iterator which indicates the bootdev, partition and bootmeth to
|
|
use. It returns a bootflow. This is the core of the bootdev implementation. The
|
|
bootdev drivers that implement this differ depending on the media they are
|
|
reading from, but each is responsible for returning a valid bootflow if
|
|
available.
|
|
|
|
A helper called `bootdev_find_in_blk()` makes it fairly easy to implement this
|
|
function for each media device uclass, in a few lines of code.
|
|
|
|
|
|
Bootdev drivers
|
|
---------------
|
|
|
|
A bootdev driver is typically fairly simple. Here is one for mmc::
|
|
|
|
static int mmc_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
|
|
struct bootflow *bflow)
|
|
{
|
|
struct udevice *mmc_dev = dev_get_parent(dev);
|
|
struct udevice *blk;
|
|
int ret;
|
|
|
|
ret = mmc_get_blk(mmc_dev, &blk);
|
|
/*
|
|
* If there is no media, indicate that no more partitions should be
|
|
* checked
|
|
*/
|
|
if (ret == -EOPNOTSUPP)
|
|
ret = -ESHUTDOWN;
|
|
if (ret)
|
|
return log_msg_ret("blk", ret);
|
|
assert(blk);
|
|
ret = bootdev_find_in_blk(dev, blk, iter, bflow);
|
|
if (ret)
|
|
return log_msg_ret("find", ret);
|
|
|
|
return 0;
|
|
}
|
|
|
|
static int mmc_bootdev_bind(struct udevice *dev)
|
|
{
|
|
struct bootdev_uc_plat *ucp = dev_get_uclass_plat(dev);
|
|
|
|
ucp->prio = BOOTDEVP_0_INTERNAL_FAST;
|
|
|
|
return 0;
|
|
}
|
|
|
|
struct bootdev_ops mmc_bootdev_ops = {
|
|
.get_bootflow = mmc_get_bootflow,
|
|
};
|
|
|
|
static const struct udevice_id mmc_bootdev_ids[] = {
|
|
{ .compatible = "u-boot,bootdev-mmc" },
|
|
{ }
|
|
};
|
|
|
|
U_BOOT_DRIVER(mmc_bootdev) = {
|
|
.name = "mmc_bootdev",
|
|
.id = UCLASS_BOOTDEV,
|
|
.ops = &mmc_bootdev_ops,
|
|
.bind = mmc_bootdev_bind,
|
|
.of_match = mmc_bootdev_ids,
|
|
};
|
|
|
|
The implementation of the `get_bootflow()` method is simply to obtain the
|
|
block device and call a bootdev helper function to do the rest. The
|
|
implementation of `bootdev_find_in_blk()` checks the partition table, and
|
|
attempts to read a file from a filesystem on the partition number given by the
|
|
`@iter->part` parameter.
|
|
|
|
Each bootdev has a priority, which indicates the order in which it is used.
|
|
Faster bootdevs are used first, since they are more likely to be able to boot
|
|
the device quickly.
|
|
|
|
|
|
Device hierarchy
|
|
----------------
|
|
|
|
A bootdev device is a child of the media device. In this example, you can see
|
|
that the bootdev is a sibling of the block device and both are children of
|
|
media device::
|
|
|
|
mmc 0 [ + ] bcm2835-sdhost | |-- mmc@7e202000
|
|
blk 0 [ + ] mmc_blk | | |-- mmc@7e202000.blk
|
|
bootdev 0 [ ] mmc_bootdev | | `-- mmc@7e202000.bootdev
|
|
mmc 1 [ + ] sdhci-bcm2835 | |-- sdhci@7e300000
|
|
blk 1 [ ] mmc_blk | | |-- sdhci@7e300000.blk
|
|
bootdev 1 [ ] mmc_bootdev | | `-- sdhci@7e300000.bootdev
|
|
|
|
The bootdev device is typically created automatically in the media uclass'
|
|
`post_bind()` method by calling `bootdev_setup_for_dev()`. The code typically
|
|
something like this::
|
|
|
|
ret = bootdev_setup_for_dev(dev, "eth_bootdev");
|
|
if (ret)
|
|
return log_msg_ret("bootdev", ret);
|
|
|
|
Here, `eth_bootdev` is the name of the Ethernet bootdev driver and `dev`
|
|
is the ethernet device. This function is safe to call even if standard boot is
|
|
not enabled, since it does nothing in that case. It can be added to all uclasses
|
|
which implement suitable media.
|
|
|
|
|
|
The bootstd device
|
|
------------------
|
|
|
|
Standard boot requires a single instance of the bootstd device to make things
|
|
work. This includes global information about the state of standard boot. See
|
|
`struct bootstd_priv` for this structure, accessed with `bootstd_get_priv()`.
|
|
|
|
Within the devicetree, if you add bootmeth devices, they should be children of
|
|
the bootstd device. See `arch/sandbox/dts/test.dts` for an example of this.
|
|
|
|
|
|
.. _`Automatic Devices`:
|
|
|
|
Automatic devices
|
|
-----------------
|
|
|
|
It is possible to define all the required devices in the devicetree manually,
|
|
but it is not necessary. The bootstd uclass includes a `dm_scan_other()`
|
|
function which creates the bootstd device if not found. If no bootmeth devices
|
|
are found at all, it creates one for each available bootmeth driver.
|
|
|
|
If your devicetree has any bootmeth device it must have all of them that you
|
|
want to use, since no bootmeth devices will be created automatically in that
|
|
case.
|
|
|
|
|
|
Using devicetree
|
|
----------------
|
|
|
|
If a bootdev is complicated or needs configuration information, it can be
|
|
added to the devicetree as a child of the media device. For example, imagine a
|
|
bootdev which reads a bootflow from SPI flash. The devicetree fragment might
|
|
look like this::
|
|
|
|
spi@0 {
|
|
flash@0 {
|
|
reg = <0>;
|
|
compatible = "spansion,m25p16", "jedec,spi-nor";
|
|
spi-max-frequency = <40000000>;
|
|
|
|
bootdev {
|
|
compatible = "u-boot,sf-bootdev";
|
|
offset = <0x2000>;
|
|
size = <0x1000>;
|
|
};
|
|
};
|
|
};
|
|
|
|
The `sf-bootdev` driver can implement a way to read from the SPI flash, using
|
|
the offset and size provided, and return that bootflow file back to the caller.
|
|
When distro boot wants to read the kernel it calls distro_getfile() which must
|
|
provide a way to read from the SPI flash. See `distro_boot()` at distro_boot_
|
|
for more details.
|
|
|
|
Of course this is all internal to U-Boot. All the distro sees is another way
|
|
to boot.
|
|
|
|
|
|
Configuration
|
|
-------------
|
|
|
|
Standard boot is enabled with `CONFIG_BOOTSTD`. Each bootmeth has its own CONFIG
|
|
option also. For example, `CONFIG_BOOTMETH_DISTRO` enables support for distro
|
|
boot from a disk.
|
|
|
|
|
|
Available bootmeth drivers
|
|
--------------------------
|
|
|
|
Bootmeth drivers are provided for:
|
|
|
|
- distro boot from a disk (syslinux)
|
|
- distro boot from a network (PXE)
|
|
- EFI boot using bootefi
|
|
- VBE
|
|
- EFI boot using boot manager
|
|
|
|
|
|
Command interface
|
|
-----------------
|
|
|
|
Three commands are available:
|
|
|
|
`bootdev`
|
|
Allows listing of available bootdevs, selecting a particular one and
|
|
getting information about it. See :doc:`../usage/cmd/bootdev`
|
|
|
|
`bootflow`
|
|
Allows scanning one or more bootdevs for bootflows, listing available
|
|
bootflows, selecting one, obtaining information about it and booting it.
|
|
See :doc:`../usage/cmd/bootflow`
|
|
|
|
`bootmeth`
|
|
Allow listing of available bootmethds and setting the order in which they
|
|
are tried. See :doc:`../usage/cmd/bootmeth`
|
|
|
|
.. _BootflowStates:
|
|
|
|
Bootflow states
|
|
---------------
|
|
|
|
Here is a list of states that a bootflow can be in:
|
|
|
|
======= =======================================================================
|
|
State Meaning
|
|
======= =======================================================================
|
|
base Starting-out state, indicates that no media/partition was found. For an
|
|
SD card socket it may indicate that the card is not inserted.
|
|
media Media was found (e.g. SD card is inserted) but no partition information
|
|
was found. It might lack a partition table or have a read error.
|
|
part Partition was found but a filesystem could not be read. This could be
|
|
because the partition does not hold a filesystem or the filesystem is
|
|
very corrupted.
|
|
fs Filesystem was found but the file could not be read. It could be
|
|
missing or in the wrong subdirectory.
|
|
file File was found and its size detected, but it could not be read. This
|
|
could indicate filesystem corruption.
|
|
ready File was loaded and is ready for use. In this state the bootflow is
|
|
ready to be booted.
|
|
======= =======================================================================
|
|
|
|
|
|
Theory of operation
|
|
-------------------
|
|
|
|
This describes how standard boot progresses through to booting an operating
|
|
system.
|
|
|
|
To start. all the necessary devices must be bound, including bootstd, which
|
|
provides the top-level `struct bootstd_priv` containing optional configuration
|
|
information. The bootstd device is also holds the various lists used while
|
|
scanning. This step is normally handled automatically by driver model, as
|
|
described in `Automatic Devices`_.
|
|
|
|
Bootdevs are also required, to provide access to the media to use. These are not
|
|
useful by themselves: bootmeths are needed to provide the means of scanning
|
|
those bootdevs. So, all up, we need a single bootstd device, one or more bootdev
|
|
devices and one or more bootmeth devices.
|
|
|
|
Once these are ready, typically a `bootflow scan` command is issued. This kicks
|
|
of the iteration process, which involves looking through the bootdevs and their
|
|
partitions one by one to find bootflows.
|
|
|
|
Iteration is kicked off using `bootflow_scan_first()`, which calls
|
|
`bootflow_scan_bootdev()`.
|
|
|
|
The iterator is set up with `bootflow_iter_init()`. This simply creates an
|
|
empty one with the given flags. Flags are used to control whether each
|
|
iteration is displayed, whether to return iterations even if they did not result
|
|
in a valid bootflow, whether to iterate through just a single bootdev, etc.
|
|
|
|
Then the ordering of bootdevs is determined, by `bootdev_setup_iter_order()`. By
|
|
default, the bootdevs are used in the order specified by the `boot_targets`
|
|
environment variable (e.g. "mmc2 mmc0 usb"). If that is missing then their
|
|
sequence order is used, as determined by the `/aliases` node, or failing that
|
|
their order in the devicetree. For BOOTSTD_FULL, if there is a `bootdev-order`
|
|
property in the bootstd node, then this is used as a final fallback. In any
|
|
case, the iterator ends up with a `dev_order` array containing the bootdevs that
|
|
are going to be used, with `num_devs` set to the number of bootdevs and
|
|
`cur_dev` starting at 0.
|
|
|
|
Next, the ordering of bootmeths is determined, by `bootmeth_setup_iter_order()`.
|
|
By default the ordering is again by sequence number, i.e. the `/aliases` node,
|
|
or failing that the order in the devicetree. But the `bootmeth order` command
|
|
or `bootmeths` environment variable can be used to set up an ordering. If that
|
|
has been done, the ordering is in `struct bootstd_priv`, so that ordering is
|
|
simply copied into the iterator. Either way, the `method_order` array it set up,
|
|
along with `num_methods`.
|
|
|
|
Note that global bootmeths are always put at the end of the ordering. If any are
|
|
present, `cur_method` is set to the first one, so that global bootmeths are done
|
|
first. Once all have been used, these bootmeths are dropped from the iteration.
|
|
When there are no global bootmeths, `cur_method` is set to 0.
|
|
|
|
At this point the iterator is ready to use, with the first bootdev and bootmeth
|
|
selected. Most of the other fields are 0. This means that the current partition
|
|
is 0, which is taken to mean the whole device, since partition numbers start at
|
|
1. It also means that `max_part` is 0, i.e. the maximum partition number we know
|
|
about is 0, meaning that, as far as we know, there is no partition table on this
|
|
bootdev.
|
|
|
|
With the iterator ready, `bootflow_scan_bootdev()` checks whether the current
|
|
settings produce a valid bootflow. This is handled by `bootflow_check()`, which
|
|
either returns 0 (if it got something) or an error if not (more on that later).
|
|
If the `BOOTFLOWF_ALL` iterator flag is set, even errors are returned as
|
|
incomplete bootflows, but normally an error results in moving onto the next
|
|
iteration.
|
|
|
|
Note that `bootflow_check()` handles global bootmeths explicitly, but calling
|
|
`bootmeth_get_bootflow()` on each one. The `doing_global` flag indicates when
|
|
the iterator is in that state.
|
|
|
|
The `bootflow_scan_next()` function handles moving onto the next iteration and
|
|
checking it. In fact it sits in a loop doing that repeatedly until it finds
|
|
something it wants to return.
|
|
|
|
The actual 'moving on' part is implemented in `iter_incr()`. This is a very
|
|
simple function. It increments the first counter. If that hits its maximum, it
|
|
sets it to zero and increments the second counter. You can think of all the
|
|
counters together as a number with three digits which increment in order, with
|
|
the least-sigificant digit on the right, counting like this:
|
|
|
|
======== ======= =======
|
|
bootdev part method
|
|
======== ======= =======
|
|
0 0 0
|
|
0 0 1
|
|
0 0 2
|
|
0 1 0
|
|
0 1 1
|
|
0 1 2
|
|
1 0 0
|
|
1 0 1
|
|
...
|
|
======== ======= =======
|
|
|
|
The maximum value for `method` is `num_methods - 1` so when it exceeds that, it
|
|
goes back to 0 and the next `part` is considered. The maximum value for that is
|
|
`max_part`, which is initially zero for all bootdevs. If we find a partition
|
|
table on that bootdev, `max_part` can be updated during the iteration to a
|
|
higher value - see `bootdev_find_in_blk()` for that, described later. If that
|
|
exceeds its maximum, then the next bootdev is used. In this way, iter_incr()
|
|
works its way through all possibilities, moving forward one each time it is
|
|
called.
|
|
|
|
Note that global bootmeths introduce a subtlety into the above description.
|
|
When `doing_global` is true, the iteration takes place only among the bootmeths,
|
|
i.e. the last column above. The global bootmeths are at the end of the list.
|
|
Assuming that they are entries 3 and 4 in the list, the iteration then looks
|
|
like this:
|
|
|
|
======== ======= ======= =======================================
|
|
bootdev part method notes
|
|
======== ======= ======= =======================================
|
|
. . 3 doing_global = true, method_count = 5
|
|
. . 4
|
|
0 0 0 doing_global = false, method_count = 3
|
|
0 0 1
|
|
0 0 2
|
|
0 1 0
|
|
0 1 1
|
|
0 1 2
|
|
1 0 0
|
|
1 0 1
|
|
...
|
|
======== ======= ======= =======================================
|
|
|
|
The changeover of the value of `doing_global` from true to false is handled in
|
|
`iter_incr()` as well.
|
|
|
|
There is no expectation that iteration will actually finish. Quite often a
|
|
valid bootflow is found early on. With `bootflow scan -b`, that causes the
|
|
bootflow to be immediately booted. Assuming it is successful, the iteration never
|
|
completes.
|
|
|
|
Also note that the iterator hold the **current** combination being considered.
|
|
So when `iter_incr()` is called, it increments to the next one and returns it,
|
|
the new **current** combination.
|
|
|
|
Note also the `err` field in `struct bootflow_iter`. This is normally 0 and has
|
|
thus has no effect on `iter_inc()`. But if it is non-zero, signalling an error,
|
|
it indicates to the iterator what it should do when called. It can force moving
|
|
to the next partition, or bootdev, for example. The special values
|
|
`BF_NO_MORE_PARTS` and `BF_NO_MORE_DEVICES` handle this. When `iter_incr` sees
|
|
`BF_NO_MORE_PARTS` it knows that it should immediately move to the next bootdev.
|
|
When it sees `BF_NO_MORE_DEVICES` it knows that there is nothing more it can do
|
|
so it should immediately return. The caller of `iter_incr()` is responsible for
|
|
updating the `err` field, based on the return value it sees.
|
|
|
|
The above describes the iteration process at a high level. It is basically a
|
|
very simple increment function with a checker called `bootflow_check()` that
|
|
checks the result of each iteration generated, to determine whether it can
|
|
produce a bootflow.
|
|
|
|
So what happens inside of `bootflow_check()`? It simply calls the uclass
|
|
method `bootdev_get_bootflow()` to ask the bootdev to return a bootflow. It
|
|
passes the iterator to the bootdev method, so that function knows what we are
|
|
talking about. At first, the bootflow is set up in the state `BOOTFLOWST_BASE`,
|
|
with just the `method` and `dev` intiialised. But the bootdev may fill in more,
|
|
e.g. updating the state, depending on what it finds. For global bootmeths the
|
|
`bootmeth_get_bootflow()` function is called instead of
|
|
`bootdev_get_bootflow()`.
|
|
|
|
Based on what the bootdev or bootmeth responds with, `bootflow_check()` either
|
|
returns a valid bootflow, or a partial one with an error. A partial bootflow
|
|
is one that has some fields set up, but did not reach the `BOOTFLOWST_READY`
|
|
state. As noted before, if the `BOOTFLOWF_ALL` iterator flag is set, then all
|
|
bootflows are returned, even partial ones. This can help with debugging.
|
|
|
|
So at this point you can see that total control over whether a bootflow can
|
|
be generated from a particular iteration, or not, rests with the bootdev (or
|
|
global bootmeth). Each one can adopt its own approach.
|
|
|
|
Going down a level, what does the bootdev do in its `get_bootflow()` method?
|
|
Let us consider the MMC bootdev. In that case the call to
|
|
`bootdev_get_bootflow()` ends up in `mmc_get_bootflow()`. It locates the parent
|
|
device of the bootdev, i.e. the `UCLASS_MMC` device itself, then finds the block
|
|
device associated with it. It then calls the helper function
|
|
`bootdev_find_in_blk()` to do all the work. This is common with just about any
|
|
bootdev that is based on a media device.
|
|
|
|
The `bootdev_find_in_blk()` helper is implemented in the bootdev uclass. It
|
|
names the bootflow and copies the partition number in from the iterator. Then it
|
|
calls the bootmeth device to check if it can support this device. This is
|
|
important since some bootmeths only work with network devices, for example. If
|
|
that check fails, it stops.
|
|
|
|
Assuming the bootmeth is happy, or at least indicates that it is willing to try
|
|
(by returning 0 from its `check()` method), the next step is to try the
|
|
partition. If that works it tries to detect a file system. If that works then it
|
|
calls the bootmeth device once more, this time to read the bootflow.
|
|
|
|
Note: At present a filesystem is needed for the bootmeth to be called on block
|
|
devices, simply because we don't have any examples where this is not the case.
|
|
This feature can be added as needed.
|
|
|
|
If we take the example of the `bootmeth_distro` driver, this call ends up at
|
|
`distro_read_bootflow()`. It has the filesystem ready, so tries various
|
|
filenames to try to find the `extlinux.conf` file, reading it if possible. If
|
|
all goes well the bootflow ends up in the `BOOTFLOWST_READY` state.
|
|
|
|
At this point, we fall back from the bootmeth driver, to
|
|
`bootdev_find_in_blk()`, then back to `mmc_get_bootflow()`, then to
|
|
`bootdev_get_bootflow()`, then to `bootflow_check()` and finally to its caller,
|
|
either `bootflow_scan_bootdev()` or `bootflow_scan_next()`. In either case,
|
|
the bootflow is returned as the result of this iteration, assuming it made it to
|
|
the `BOOTFLOWST_READY` state.
|
|
|
|
That is the basic operation of scanning for bootflows. The process of booting a
|
|
bootflow is handled by the bootmeth driver for that bootflow. In the case of
|
|
distro boot, this parses and processes the `extlinux.conf` file that was read.
|
|
See `distro_boot()` for how that works. The processing may involve reading
|
|
additional files, which is handled by the `read_file()` method, which is
|
|
`distro_read_file()` in this case. All bootmethds should support reading files,
|
|
since the bootflow is typically only the basic instructions and does not include
|
|
the operating system itself, ramdisk, device tree, etc.
|
|
|
|
The vast majority of the bootstd code is concerned with iterating through
|
|
partitions on bootdevs and using bootmethds to find bootflows.
|
|
|
|
How about bootdevs which are not block devices? They are handled by the same
|
|
methods as above, but with a different implementation. For example, the bootmeth
|
|
for PXE boot (over a network) uses `tftp` to read files rather than `fs_read()`.
|
|
But other than that it is very similar.
|
|
|
|
|
|
Tests
|
|
-----
|
|
|
|
Tests are located in `test/boot` and cover the core functionality as well as
|
|
the commands. All tests use sandbox so can be run on a standard Linux computer
|
|
and in U-Boot's CI.
|
|
|
|
For testing, a DOS-formatted disk image is used with a single FAT partition on
|
|
it. This is created in `setup_bootflow_image()`, with a canned one from the
|
|
source tree used if it cannot be created (e.g. in CI).
|
|
|
|
|
|
Bootflow internals
|
|
------------------
|
|
|
|
The bootstd device holds a linked list of scanned bootflows as well as the
|
|
currently selected bootdev and bootflow (for use by commands). This is in
|
|
`struct bootstd_priv`.
|
|
|
|
Each bootdev device has its own `struct bootdev_uc_plat` which holds a
|
|
list of scanned bootflows just for that device.
|
|
|
|
The bootflow itself is documented in bootflow_h_. It includes various bits of
|
|
information about the bootflow and a buffer to hold the file.
|
|
|
|
|
|
Future
|
|
------
|
|
|
|
Apart from the to-do items below, different types of bootflow files may be
|
|
implemented in future, e.g. Chromium OS support which is currently only
|
|
available as a script in chromebook_coral.
|
|
|
|
|
|
To do
|
|
-----
|
|
|
|
Some things that need to be done to completely replace the distro-boot scripts:
|
|
|
|
- add bootdev drivers for dhcp, sata, scsi, ide, virtio
|
|
- PXE boot for EFI
|
|
- support for loading U-Boot scripts
|
|
|
|
Other ideas:
|
|
|
|
- `bootflow prep` to load everything preparing for boot, so that `bootflow boot`
|
|
can just do the boot.
|
|
- automatically load kernel, FDT, etc. to suitable addresses so the board does
|
|
not need to specify things like `pxefile_addr_r`
|
|
|
|
|
|
.. _distro_bootcmd: https://github.com/u-boot/u-boot/blob/master/include/config_distro_bootcmd.h
|
|
.. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
|
|
.. _distro_boot: https://github.com/u-boot/u-boot/blob/master/boot/distro.c
|
|
.. _bootflow_h: https://github.com/u-boot/u-boot/blob/master/include/bootflow.h
|