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

Maxime Ripard maxime at cerno.tech
Mon Jun 7 17:14:07 CEST 2021

On Mon, May 24, 2021 at 11:43:43AM -0400, Sean Anderson wrote:
> On 5/24/21 11:28 AM, Maxime Ripard wrote:
> > On Mon, May 24, 2021 at 10:37:31AM -0400, Sean Anderson 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).
> >
> > If we're going that way, maybe we can just pass the interface on the
> > command line like dfu does?
> That could work, but I don't think it's necessary. At the moment there
> are many different ways to specify partitions (KConfig, Aliases, "U-boot
> syntax", GPT partition labels). I would rather pare things down to the
> minimum necessary ways than add yet another bit of state to specify
> partitions.
> > That way the new requirement would be very obvious instead of
> > introducing a new environment variable no one really expects?
> I'm not sure what you mean here. This alias system has been in place for
> a while, and it's very convenient for mapping a stable name to some
> arbitrary device and partition.

I don't deny the fact that it can be convenient. What I was pointing out
is that it's been optional the whole time, that I've been using fastboot
for like 5 years and didn't realize that this feature was there, and
it's only implemented for mmc.

To make it required, without any warning, would be fairly confusing.

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