Re: [DistroKit] [PATCH v3 3/6] v7a: barebox: pass root=mmcblkXpN instead of root=PARTUUID=… to kernel

[Roland Hieber <rhi@pengutronix.de>]

On Wed, Nov 22, 2023 at 02:27:00PM +0100, Ahmad Fatoum wrote:
> Hello Roland,
> 
> On 20.10.23 11:05, Roland Hieber wrote:
> > Kernels prior to v5.10-rc1 could not ensure deterministic ordering for
> > MMC devices. For this reason, the default 'append-root' behaviour of
> > barebox for MMC block devices is to append a 'root=PARTUUID=XYZ' option
> > to the kernel command line. The PARTUUID is derived from the partition
> > table and is expected to be unique system-wide. This is however
> > susceptible to breakage if the exact same image is written to different
> > boot media in the system, e.g. SD and eMMC, so the PARTUUID will no
> > longer uniquely refer to a single partition.
> > 
> > Newer kernel versions honor the numbering used for 'mmc' aliases,
> > which makes 'root=mmcblkXpN' deterministic as long as barebox and
> > kernel aliases are identical. Since we're using kernel 6.4, enable this
> > option in barebox.
> 
> As you mention this feature requires that barebox and kernel aliases
> are identical. At least for the STM32MP157C-EV1, this is not the case
> as the kernel device tree has no aliases at all.

Oh. How come? Can we patch the kernel DT to make them the same?

> Can you send a revert?

I can send a revert, but I don't like the implications. Incidentally,
this topic was discussed in length last week on a systemd pull request
of mine [1], with the TL;DR that filesystem and partition UUIDs should
not generally be considered unique. While that might be the case in the
DistroKit test runs, we also have some boards with eMMCs that can hold
the same image as the SD cards that we boot from. I suspect this might
come back to bite us some time in the future.

[1]: https://github.com/systemd/systemd/pull/29219#issuecomment-1812226959

 - Roland

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