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From:   Ulf Hansson <ulf.hansson@linaro.org>
Date:   Wed, 5 Aug 2020 09:50:06 +0200
Message-ID: <CAPDyKFrA+P=G+LQLSJXZhPyAn_jUzv7tSfaphiKVMuenmKEs+w@mail.gmail.com>
Subject: Re: (EXT) Re: (EXT) Re: Consistent block device references for root= cmdline
To:     Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, Jens Axboe <axboe@kernel.dk>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        "linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 29 Jul 2020 at 10:43, Matthias Schiffer
<matthias.schiffer@ew.tq-group.com> wrote:
>
> On Tue, 2020-07-07 at 16:14 +0200, Ulf Hansson wrote:
> > On Thu, 11 Jun 2020 at 13:20, Matthias Schiffer
> > <matthias.schiffer@ew.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@ew.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?

Yes, something along those lines.

Kind regards
Uffe