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doc: Document virtio support
Add REAME.virtio to describe the information about U-Boot support for VirtIO devices, including supported boards, build instructions, driver details etc. Signed-off-by: Bin Meng <bmeng.cn@gmail.com> Reviewed-by: Simon Glass <sjg@chromium.org>
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Simon Glass
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doc/README.virtio
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doc/README.virtio
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# SPDX-License-Identifier: GPL-2.0+
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#
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# Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
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VirtIO Support
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==============
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This document describes the information about U-Boot support for VirtIO [1]
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devices, including supported boards, build instructions, driver details etc.
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What's VirtIO?
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--------------
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VirtIO is a virtualization standard for network and disk device drivers where
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just the guest's device driver "knows" it is running in a virtual environment,
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and cooperates with the hypervisor. This enables guests to get high performance
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network and disk operations, and gives most of the performance benefits of
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paravirtualization. In the U-Boot case, the guest is U-Boot itself, while the
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virtual environment are normally QEMU [2] targets like ARM, RISC-V and x86.
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Status
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------
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VirtIO can use various different buses, aka transports as described in the
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spec. While VirtIO devices are commonly implemented as PCI devices on x86,
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embedded devices models like ARM/RISC-V, which does not normally come with
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PCI support might use simple memory mapped device (MMIO) instead of the PCI
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device. The memory mapped virtio device behaviour is based on the PCI device
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specification. Therefore most operations including device initialization,
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queues configuration and buffer transfers are nearly identical. Both MMIO
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and PCI transport options are supported in U-Boot.
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The VirtIO spec defines a lots of VirtIO device types, however at present only
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network and block device, the most two commonly used devices, are supported.
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The following QEMU targets are supported.
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- qemu_arm_defconfig
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- qemu_arm64_defconfig
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- qemu-riscv32_defconfig
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- qemu-riscv64_defconfig
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- qemu-x86_defconfig
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- qemu-x86_64_defconfig
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Note ARM and RISC-V targets are configured with VirtIO MMIO transport driver,
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and on x86 it's the PCI transport driver.
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Build Instructions
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------------------
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Building U-Boot for pre-configured QEMU targets is no different from others.
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For example, we can do the following with the CROSS_COMPILE environment
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variable being properly set to a working toolchain for ARM:
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$ make qemu_arm_defconfig
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$ make
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You can even create a QEMU ARM target with VirtIO devices showing up on both
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MMIO and PCI buses. In this case, you can enable the PCI transport driver
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from 'make menuconfig':
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Device Drivers --->
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...
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VirtIO Drivers --->
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...
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[*] PCI driver for virtio devices
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Other drivers are at the same location and can be tuned to suit the needs.
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Requirements
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------------
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It is required that QEMU v2.5.0+ should be used to test U-Boot VirtIO support
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on QEMU ARM and x86, and v2.12.0+ on QEMU RISC-V.
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Testing
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-------
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The following QEMU command line is used to get U-Boot up and running with
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VirtIO net and block devices on ARM.
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$ qemu-system-arm -nographic -machine virt -bios u-boot.bin \
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-netdev tap,ifname=tap0,id=net0 \
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-device virtio-net-device,netdev=net0 \
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-drive if=none,file=test.img,format=raw,id=hd0 \
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-device virtio-blk-device,drive=hd0
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On x86, command is slightly different to create PCI VirtIO devices.
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$ qemu-system-i386 -nographic -bios u-boot.rom \
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-netdev tap,ifname=tap0,id=net0 \
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-device virtio-net-pci,netdev=net0 \
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-drive if=none,file=test.img,format=raw,id=hd0 \
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-device virtio-blk-pci,drive=hd0
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Additional net and block devices can be created by appending more '-device'
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parameters. It is also possible to specify both MMIO and PCI VirtIO devices.
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For example, the following commnad creates 3 VirtIO devices, with 1 on MMIO
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and 2 on PCI bus.
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$ qemu-system-arm -nographic -machine virt -bios u-boot.bin \
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-netdev tap,ifname=tap0,id=net0 \
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-device virtio-net-pci,netdev=net0 \
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-drive if=none,file=test0.img,format=raw,id=hd0 \
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-device virtio-blk-device,drive=hd0 \
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-drive if=none,file=test1.img,format=raw,id=hd1 \
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-device virtio-blk-pci,drive=hd1
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By default QEMU creates VirtIO legacy devices by default. To create non-legacy
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(aka modern) devices, pass additional device property/value pairs like below:
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$ qemu-system-i386 -nographic -bios u-boot.rom \
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-netdev tap,ifname=tap0,id=net0 \
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-device virtio-net-pci,netdev=net0,disable-legacy=true,disable-modern=false \
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-drive if=none,file=test.img,format=raw,id=hd0 \
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-device virtio-blk-pci,drive=hd0,disable-legacy=true,disable-modern=false
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A 'virtio' command is provided in U-Boot shell.
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=> virtio
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virtio - virtio block devices sub-system
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Usage:
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virtio scan - initialize virtio bus
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virtio info - show all available virtio block devices
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virtio device [dev] - show or set current virtio block device
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virtio part [dev] - print partition table of one or all virtio block devices
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virtio read addr blk# cnt - read `cnt' blocks starting at block
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`blk#' to memory address `addr'
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virtio write addr blk# cnt - write `cnt' blocks starting at block
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`blk#' from memory address `addr'
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To probe all the VirtIO devices, type:
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=> virtio scan
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Then we can show the connected block device details by:
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=> virtio info
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Device 0: QEMU VirtIO Block Device
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Type: Hard Disk
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Capacity: 4096.0 MB = 4.0 GB (8388608 x 512)
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And list the directories and files on the disk by:
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=> ls virtio 0 /
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<DIR> 4096 .
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<DIR> 4096 ..
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<DIR> 16384 lost+found
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<DIR> 4096 dev
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<DIR> 4096 proc
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<DIR> 4096 sys
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<DIR> 4096 var
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<DIR> 4096 etc
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<DIR> 4096 usr
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<SYM> 7 bin
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<SYM> 8 sbin
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<SYM> 7 lib
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<SYM> 9 lib64
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<DIR> 4096 run
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<DIR> 4096 boot
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<DIR> 4096 home
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<DIR> 4096 media
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<DIR> 4096 mnt
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<DIR> 4096 opt
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<DIR> 4096 root
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<DIR> 4096 srv
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<DIR> 4096 tmp
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0 .autorelabel
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Driver Internals
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----------------
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There are 3 level of drivers in the VirtIO driver family.
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+---------------------------------------+
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| virtio device drivers |
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| +-------------+ +------------+ |
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| | virtio-net | | virtio-blk | |
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| +-------------+ +------------+ |
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+---------------------------------------+
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+---------------------------------------+
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| virtio transport drivers |
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| +-------------+ +------------+ |
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| | virtio-mmio | | virtio-pci | |
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| +-------------+ +------------+ |
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+---------------------------------------+
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+----------------------+
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| virtio uclass driver |
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+----------------------+
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The root one is the virtio uclass driver (virtio-uclass.c), which does lots of
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common stuff for the transport drivers (virtio_mmio.c, virtio_pci.c). The real
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virtio device is discovered in the transport driver's probe() method, and its
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device ID is saved in the virtio uclass's private data of the transport device.
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Then in the virtio uclass's post_probe() method, the real virtio device driver
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(virtio_net.c, virtio_blk.c) is bound if there is a match on the device ID.
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The child_post_bind(), child_pre_probe() and child_post_probe() methods of the
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virtio uclass driver help bring the virtio device driver online. They do things
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like acknowledging device, feature negotiation, etc, which are really common
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for all virtio devices.
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The transport drivers provide a set of ops (struct dm_virtio_ops) for the real
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virtio device driver to call. These ops APIs's parameter is designed to remind
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the caller to pass the correct 'struct udevice' id of the virtio device, eg:
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int virtio_get_status(struct udevice *vdev, u8 *status)
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So the parameter 'vdev' indicates the device should be the real virtio device.
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But we also have an API like:
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struct virtqueue *vring_create_virtqueue(unsigned int index, unsigned int num,
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unsigned int vring_align,
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struct udevice *udev)
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Here the parameter 'udev' indicates the device should be the transport device.
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Similar naming is applied in other functions that are even not APIs, eg:
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static int virtio_uclass_post_probe(struct udevice *udev)
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static int virtio_uclass_child_pre_probe(struct udevice *vdev)
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So it's easy to tell which device these functions are operating on.
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Development Flow
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----------------
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At present only VirtIO network card (device ID 1) and block device (device
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ID 2) are supported. If you want to develop new driver for new devices,
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please follow the guideline below.
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1. add new device ID in virtio.h
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#define VIRTIO_ID_XXX X
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2. update VIRTIO_ID_MAX_NUM to be the largest device ID plus 1
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3. add new driver name string in virtio.h
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#define VIRTIO_XXX_DRV_NAME "virtio-xxx"
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4. create a new driver with name set to the name string above
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U_BOOT_DRIVER(virtio_xxx) = {
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.name = VIRTIO_XXX_DRV_NAME,
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...
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.remove = virtio_reset,
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.flags = DM_FLAG_ACTIVE_DMA,
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}
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Note the driver needs to provide the remove method and normally this can be
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hooked to virtio_reset(). The driver flags should contain DM_FLAG_ACTIVE_DMA
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for the remove method to be called before jumping to OS.
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5. provide bind() method in the driver, where virtio_driver_features_init()
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should be called for driver to negotiate feature support with the device.
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6. do funny stuff with the driver
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References
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----------
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[1] http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.pdf
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[2] https://www.qemu.org
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