[PATCH v2 0/4] Patches to allow consistent mmc / mmcblk numbering w/ device tree

Russell King - ARM Linux linux at arm.linux.org.uk
Fri Apr 29 12:57:41 PDT 2016

On Fri, Apr 29, 2016 at 12:43:39PM -0700, Doug Anderson wrote:
> Russell,
> On Fri, Apr 29, 2016 at 11:12 AM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Fri, Apr 29, 2016 at 10:32:15AM -0700, Douglas Anderson wrote:
> >> This series picks patches from various different places to produce what
> >> I consider the best solution to getting consistent mmc and mmcblk
> >> ordering.
> >>
> >> Why consistent ordering and why not just use UUIDs?  IMHO consistent
> >> ordering solves a few different problems:
> >
> > NAK.  Really.  Use UUIDs, that's the proper solution here.
> Un-NAK.  UUIDs don't solve point #1.

Re-NAK.  I don't think your point #1 is valid.  See my other reply.

> * Presumably on a PC you've got an extra bit in the middle (like grub
> or something like that) that can help you resolve your UUIDs even if
> you get your kernel from somewhere else.

You are over-estimating what grub does.  Grub doesn't resolve UUIDs at
all.  Grub just passes the kernel arguments in its configuration file
for the entry it is booting to the kernel.  It's a static configuration
found in /boot/grub/grub.conf.

It doesn't probe devices for UUIDs.

> * Presumably in the non-embedded world kernel hackers have a different
> workflow.  They probably don't swap between different devices with
> different configurations on an hourly basis.  They're not in the habit
> of totally reimaging their system periodically.  Etc.  Trying to force
> the workflow of a PC kernel hacker and an embedded kernel hacker to be
> the same doesn't seem like a worthwhile goal.

In _my_ world with the "embedded" devices I have, I mount by UUID on
platforms which have multiple MMC devices to avoid exactly the problem
you're having.  This works fine.

If I were to switch the SD card, and I wanted to avoid changing the
boot loader configuration, I'd use label instead, and I'd label all
the SD card rootfs using the same label so I could just swap the cards.

> * Presumably an embedded kernel hacker running with ATA / SCSI could
> _usually_ assume that "sda" is his/her root filesystem.  It's unlikely
> an embedded system would have more than one "sda" disk builtin and
> it's nearly guaranteed (I think) that a builtin ATA / SCSI controller
> would probe before any USB based devices.

You've got a funny view again.  N2100 has two hard disks.  The clearfog
board from SolidRun has two mini-PCIe slots, each of which can have two
SATA interfaces... If you want to use it as a server-type platform with
lots of disks...

> Sure, if your root
> filesystem is USB based (really?) and you've got additional USB
> storage devices then you're SOL.  Sorry.

One of my Versatile Express platforms boots from USB, and has a MMC
slot...  So this argument does not stack up.


RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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