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

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Message-ID: <cfcec3df57e6dd5ef353ef3a5b4b9793c28eb401.camel@ew.tq-group.com>
Date:   Thu, 11 Jun 2020 13:20:30 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
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: (EXT) Re: Consistent block device references for root= cmdline

On Wed, 2020-06-10 at 16:52 +0200, Ulf Hansson wrote:
> 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.


It seems the principal technical problem is the one described here:

https://www.spinics.net/lists/linux-mmc/msg26602.html

I don't see any way to solve this completely, as there seem to be two
fundamentally conflicting requirements:

1) If a mounted SD card is replaced, it must be assigned a new
/dev/mmcblkN
2) /dev/mmcblkN should always match the configured alias IDs

What is the reason we need 1) - is it possible to have multiple eMMCs
or SD cards on a single bus, with detection at runtime? Otherwise I'd
expect this to be handled like other drives with removable media (CD,
floppy), with static device assignment.

If we can't give up on 1) for some reason, we'll have to accept that we
can't guarantee 2) unconditionally. As far as I can tell, the patches
provided by Sascha and others did that in a reasonable way: The aliases
would work in most cases - in particular for the first assignment on
boot, which is required to find the correct rootfs.

> 
> > 
> > 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