On Tue, 2020-07-07 at 16:14 +0200, Ulf Hansson wrote: > On Thu, 11 Jun 2020 at 13:20, Matthias Schiffer > <matthias.schiffer@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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
