Re: adding aliases to mmc ... again

[Stephen Warren <swarren@wwwdotorg.org>]

On 05/22/2014 09:30 AM, Sascha Hauer wrote:
> Hi all,
> 
> The wish to have persistent MMC block device names for passing a suitable
> root=/dev/mmcblkX option came up several times already and has been discussed
> at least in these threads:
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2012-July/109984.html
> https://www.mail-archive.com/linux-mmc@vger.kernel.org/msg22104.html
> http://comments.gmane.org/gmane.linux.kernel.mmc/21519
> 
> Several patches have been proposed to nail the slot index to a known number.
> Arguments against these patches were:
> 
> - Use an initrd and locate the correct root device there
>   even Thomas who suggested this admitted this would be painful to do
> - use root=UUID= or root=PARTUUID=
>   This generally works but has an important downside. With the UUID
>   approach devices which should boot from the internal eMMC may start
>   booting from an external SD slot when somebody deliberately inserts
>   a SD card with the same UUID.
> 
> The following patches should have the technical issues fixed. It works
> by counting the mmc aliases in the devicetree during initialization of
> the mmc framework. Those slot numbers will never be assigned to other
> hosts.

Does it solve the following, which AFAIK has always been the primary
argument against aligning block device IDs with controller IDs:

- User inserts SD card into MMC controller ID (or alias) 1.
- /dev/mmcblk1 now exists
- Mount /dev/mmcblk1 on /mnt/tmp
- User removes that SD card
- /dev/mmcblk1 still exists, since it's mounted so can't be deleted
after the card removal.
- User inserts SD card into MMC controller ID (or alias) 1.
- /dev/mmcblkN (N is something other than 1) now exists

Now, the block device ID must be != the original ID, since two block
devices exist.