linux-kernel - Re: Consistent block device references for root= cmdline

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Message-ID: <CAPDyKFq+RiwbDj+58+W5GTcT7=ZOpZFmc02+FxjRGYwbBgA8oQ@mail.gmail.com>
Date:   Wed, 10 Jun 2020 16:52:05 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Consistent block device references for root= cmdline

On Wed, 10 Jun 2020 at 15:15, Matthias Schiffer
<matthias.schiffer@...tq-group.com> wrote:
>
> Hello all,
>
> there have been numerous attempts to make the numbering of mmcblk
> devices consistent, mostly by using aliases from the DTS ([1], [2],
> [3]), but all have been (rightfully) rejected. Unless I have overlooked
> a more recent development, no attempts for a different solution were
> made.

According to aliases attempts, I think those have failed, mainly
because of two reasons.

1. Arguments stating that LABELs/UUIDs are variable alternatives. This
isn't the case, which I think was also concluded from the several
earlier discussions.
2. Patches that tried adding support for mmc aliases, were not
correctly coded. More precisely, what needs to be addressed is that
the mmc core also preserves the same ids to be set for the host class
as the block device, mmc[n] must correspond to mmcblk[n].

>
> As far as I can tell, the core of the issue seems to be the following:
>
> The existing solutions like LABELs and UUIDs are viable alternatives in
> many cases, but in particular on embedded systems, this is not quite
> sufficient: In addition to the problem that more knowledge about the
> system to boot is required in the bootloader, this approach fails
> completely when the same firmware image exists on multiple devices, for
> example on an eMMC and an SD card - not an entirely uncommon situation
> during the development of embedded systems.
>
> With udev, I can refer to a specific partition using a path like
> /dev/disk/by-path/platform-2194000.usdhc-part2. In [4] it was proposed
> to add a way to refer to a device path/phandle from the kernel command
> line. Has there been any progress on this proposal?

Lots of time during the years I have been approached, both publicly
and offlist, about whether it would be possible to add support for
"consistent" mmcblk devices. To me, I am fine with the aliases
approach, as long as it gets implemented correctly.

>
> Kind regards,
> Matthias
>
>
> [1] https://patchwork.kernel.org/patch/8685711/
> [2] https://lore.kernel.org/patchwork/cover/674381/
> [3] https://www.spinics.net/lists/linux-mmc/msg26586.html
> [4] https://www.spinics.net/lists/linux-mmc/msg26708.html
>

Kind regards
Uffe