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

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Message-ID: <4ecc41303f9232319cedf414fc466a8c4c0c6dc6.camel@ew.tq-group.com>
Date:   Wed, 29 Jul 2020 10:43:01 +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: (EXT) Re: Consistent block device references for
 root= cmdline

On Tue, 2020-07-07 at 16:14 +0200, Ulf Hansson wrote:
> On Thu, 11 Jun 2020 at 13:20, Matthias Schiffer
> <matthias.schiffer@...tq-group.com> wrote:
> > 
> > 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?
> 
> Yes. The mmc_bus_type holds all cards - all discovered 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.
> 
> Well, if we would pre-parse the DTB to look for all "mmc block
> aliases" and keep a mark of those ids as being reserved, then we
> should be able to cope with both 1) and 2).

Hello Ulf,

it seems to me like Sascha's patches from 2014 do precisely that:

https://www.spinics.net/lists/linux-mmc/msg26587.html
https://www.spinics.net/lists/linux-mmc/msg26588.html

I haven't looked into porting this to a modern kernel yet, but do you
think that the approach is sound?



> 
> > 
> > > 
> > > > 
> > > > 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