specifying order of /dev/mmcblk devices via device-tree?

[stefan@agner.ch (Stefan Agner)]

On 2016-11-16 06:45, Ulf Hansson wrote:
> On 28 October 2016 at 17:37, Mark Rutland <mark.rutland@arm.com> wrote:
>> On Fri, Oct 28, 2016 at 08:23:04AM -0700, Tim Harvey wrote:
>>> Greetings,
>>>
>>> I have an IMX6 board that has the following:
>>> sdhc1: mmc0: sdio radio
>>> sdhc2: mmc1: /dev/mmcblk1: microSD connector
>>> sdhc3: mmc2: /dev/mmcblk2: on-board eMMC
>>>
>>> I would like to have sdhc3 registered as /dev/mmcblk0 and sdhc2
>>> registered as /dev/mmcblk1 so that permanent storage is the first
>>> mmcblk device as I think this is more intuitive however currently
>>> these get instanced in the order they appear in the imx6qdl.dtsi
>>> device-tree configuration and are not able to be mapped the way I want
>>> them in my dts file.
>>>
>>> Is there a way, or if not is there a desire for a way, to specify the
>>> order of /dev/mmcblk devices via device-tree?
>>
>> As with many other devices, there is no standard way of controlling the
>> Linux enumeration (and given the ID space is shared with other dynamic
>> devices it's not something that could generally work).
>>
>> These should be refererd to by UUID if possible.
>>
>> If not, we could cosider adding a by-dt-path or something like that.
> 
> So does that mean you think using "DT aliases" would be okay? As
> Javier pointed out, there have been some attempts [1] for that, but
> they didn't make it.
> Perhaps we need to re-consider, and if so please re-review the DT
> bindings patch from that series.

I really would like to see some sort of stable MMC block device
ordering.

by-dt-path solutions requires initramfs, which adds complexity and boot
time. Both not very welcome in simple embedded devices.

PARTUUID is relying on a particular instance of a partition, which is
not the same as to say boot from the second partition of device X.

The main problem I see in my patchset is that it works around the fact
MMC is a bus. Adding the mmc controller number to the block device name
would avoid that issue (mmc0blk0...), not sure if a such a drastic
change would be acceptable.

--
Stefan