Re: [RFC PATCH 0/3] fastboot: sunxi: Determine MMC device at runtime

[Sean Anderson <sean.anderson@seco.com>]

On 5/24/21 11:15 AM, Andre Przywara wrote:
 > On Mon, 24 May 2021 10:37:31 -0400
 > Sean Anderson <sean.anderson@seco.com> wrote:
 >
 >> On 5/23/21 8:36 PM, Andre Przywara wrote:
 >>   > At the moment the fastboot code relies on the Kconfig variable
 >>   > CONFIG_FASTBOOT_FLASH_MMC_DEV to point to the MMC device to use for the
 >>   > flash command. This value needs to be the *U-Boot device number*, which
 >>   > is picked by the U-Boot device model at runtime. This makes it quite
 >>   > tricky and fragile to fix this variable at compile time, as other DT
 >>   > nodes and aliases influence the enumeration process.
 >>   >
 >>   > To make this process more robust, allow the device number to be picked at
 >>   > runtime, which sounds like a better fit to find this device number. Patch
 >>   > 1/3 introduces a weak function for that purpose.
 >>   > Patch 2/3 then implements this function for the Allwinner platform. The
 >>   > code follows the idea behind the existing Kconfig defaults: Use the eMMC
 >>   > device, if that exists, or the SD card otherwise. This patch is actually
 >>   > not sunxi specific, so might be adopted by other platforms as well.
 >>   > Patch 3/3 then drops the existing Kconfig defaults for sunxi, to clean
 >>   > this up and remove the implicit assumption that the eMMC device is always
 >>   > device 1 (as least for the fastboot code).
 >>   >
 >>   > I would be curious if others think this is the right way forward.
 >>   > The fact that the U-Boot device numbers are determined at runtime
 >>   > seems to conflict with the idea of a Kconfig variable in the first place,
 >>   > hence this series. This brings us one step closer to the end goal of
 >>   > removing the "eMMC is device 1" assumption.
 >>
 >> I would actually favor removing CONFIG_FASTBOOT_FLASH_MMC_DEV
 >> altogether, and just specifying the device explicitly in fastboot
 >> commands. If you need to dynamically change the device, you can create
 >> some aliases. E.g. you could have something like
 >>
 >> "fastboot_aliases=setenv fastboot_partition_alias_user ${mmcdev}.0:0"
 >>
 >> and then run this variable just before calling `fastboot 0` (or whatever
 >> your usb device is).
 >
 > Fine with me. I was actually wondering about this already, but didn't
 > want to disrupt every user, especially since I can't really test this
 > very well.

The ideal way would be to define aliases dynamically mapping old
partition names to new syntax. So if the U-Boot shell were a bit more
expressive, one could do

	for partition in $(mmc part list $mmcdev); do
		env set fastboot_partition_alias_${partition} ${mmcdev}\#${partition}
	done
	fastboot usb 0

which would map old partition names to new-style syntax. Unfortunately,
we don't quite have that capability yet. I think defining a bunch of
aliases when you run fastboot could be... suprising. I'm not sure if
there is a good solution here.

 > So can you use this explicit device naming from the host side already
 > with the current code?  Can you give an example how this would look
 > like?

Flash the partition named "rootfs" on MMC 0 hardware partition 0

$ fastboot flash 0#rootfs root.simg

Flash the entirety of MMC 1 hardware partition 2

$ fastboot flash 1.2:0 boot2.img

For a more thorough treatment of this syntax, see [1]

 > The documentation I could find only speaks of the Android
 > partition names (like "system"), which requires environment variables
 > to work, IIUC?

They require that you use a GPT-formatted disk with partition labels.
Alternatively, you can use aliases. There is one sentence on the
new-style syntax here [2], but I really should expand on it...

--Sean

[1] https://u-boot.readthedocs.io/en/latest/usage/partitions.html#partitions
[2] https://u-boot.readthedocs.io/en/latest/android/fastboot.html#partition-names

 >
 > Thanks,
 > Andre
 >
 >>   >
 >>   > I am looking forward to any comments on this series!
 >>   >
 >>   > Cheers,
 >>   > Andre
 >>   >
 >>   > Andre Przywara (3):
 >>   >    fastboot: Allow runtime determination of MMC device
 >>   >    sunxi: Implement fastboot_get_mmc_device()
 >>   >    sunxi: Drop sunxi FASTBOOT_FLASH_MMC_DEV defaults
 >>   >
 >>   >   board/sunxi/board.c           | 37 +++++++++++++++++++++++++++++++++++
 >>   >   drivers/fastboot/Kconfig      |  4 +---
 >>   >   drivers/fastboot/fb_command.c |  6 +++---
 >>   >   drivers/fastboot/fb_common.c  |  3 ++-
 >>   >   drivers/fastboot/fb_mmc.c     | 12 ++++++++----
 >>   >   include/fastboot.h            |  7 +++++++
 >>   >   6 files changed, 58 insertions(+), 11 deletions(-)
 >>   >
 >