Re: [PATCH 0/3] Patches to allow consistent mmc / mmcblk numbering

Ulf Hansson
2016-05-10T11:09:14Z

On 29 April 2016 at 19:39, Arnd Bergmann <arnd@arndb.de> wrote:
> On Thursday 28 April 2016 16:06:42 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:
>>
>> 1. For poor, feeble-minded humans like me, have sane numbering for
>> devices helps a lot. When grepping through dmesg it's terribly handy
>> if a given SDMMC device has a consistent number. I know that I can
>> do "dmesg | grep mmc0" or "dmesg | grep mmcblk0" to find info about
>> the eMMC. I know that I can do "dmesg | grep mmc1" to find info
>> about the SD card slot. I don't want it to matter which one probed
>> first, I don't want it to matter if I'm working on a variant of the
>> hardware that has the SD card slot disabled, and I don't want to care
>> what my boot device was. Worrying about what device number I got
>> increases my cognitive load.
>>
>> 2. There are cases where it's not trivially easy during development to
>> use the UUID. Specifically I work a lot with coreboot / depthcharge
>> as a BIOS. When configured properly, that BIOS has a nice feature to
>> allow you to fetch the kernel and kernel command line from TFTP by
>> pressing Ctrl-N. In this particular case the BIOS doesn't actually
>> know which disk I'd like for my root filesystem, so it's not so easy
>> for it to put the right UUID into the command line. For this
>> purpose, knowing that "mmcblk0" will always refer to eMMC is handy.
>>
>>
>> Jaehoon Chung (1):
>> Documentation: mmc: Document mmc aliases
>>
>> Stefan Agner (2):
>> mmc: read mmc alias from device tree
>> mmc: use SD/MMC host ID for block device name ID
>>
>> Documentation/devicetree/bindings/mmc/mmc.txt | 11 +++++++++++
>> drivers/mmc/card/block.c | 3 ++-
>> drivers/mmc/core/host.c | 25 ++++++++++++++++++++-----
>> 3 files changed, 33 insertions(+), 6 deletions(-)
>
>
> Does this mean we can revert 9aaf343 ("mmc: block: Use the mmc host
> device index as the mmcblk device index") for now and wait until this
> is in as well?

No, as that one fixes an issue for a widely deployed product (family).

>
> The commit I mention here breaks a significant number of boots
> on Olof's test build setup, and it would be nice to avoid breaking
> them again when we get yet another device numbering system.

They don't have to break *again*. He just have to convert *once* to
use UUID/PARTID which is really what most should be doing.

Kind regards
Uffe