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

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Date:   Thu, 11 Jun 2020 13:23:13 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Roger Heflin <rogerheflin@...il.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>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: (EXT) Re: Consistent block device references for root= cmdline

On Wed, 2020-06-10 at 12:33 -0500, Roger Heflin wrote:
> No idea if this would still work, but back before label/uuid and lvm
> in initird I had a staticly linked "C" program that ran inside
> initrd,
> it searched for likely places a boot device could be (mounted them
> and
> looked for a file to confirm it was the right device, then unmounted
> it), and when it found the right one, it then echo's is major/minor
> numbers into /proc/sys/kernel/real-root-dev and that is used for
> root=
> without it being on the command line.  Assuming you could get
> something similar started by sytemd and/or udev inside the initrd it
> might still work.

Using an initramfs is obviously an option, but it complicates both the
build setup and boot process, so we would like to avoid making this a
hard requirement if possible.


> 
> On Wed, Jun 10, 2020 at 11:51 AM Ulf Hansson <ulf.hansson@...aro.org>
> 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.
> > 
> > > 
> > > 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