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

[Marek Vasut <marex@denx.de>]

On 3/17/19 4:43 PM, Russell King - ARM Linux admin wrote:
> On Sun, Mar 17, 2019 at 04:05:14PM +0100, Stefan Agner wrote:
>> On 16.03.2019 16:39, Russell King - ARM Linux admin wrote:
>>> On Sat, Mar 16, 2019 at 01:33:58PM +0100, Marek Vasut wrote:
>>>> If you have a FS or partition table there, it does.
>>>> If you don't, I agree ... that's a problem.
>>>
>>> eMMC boot partitions are called mmcblkXbootY, and unless you have more
>>> than one eMMC device on the system, they can be found either by looking
>>> for /dev/mmcblk*boot* or by querying udev.  The advantage of using udev
>>> is you can discover the physical device behind it by looking at DEVPATH,
>>> ID_PATH, etc, but you may not have that installed on an embedded device.
>>>
>>> However, as I say, just looking for /dev/mmcblk*boot* is sufficient to
>>> find the eMMC boot partitions where there is just one eMMC device
>>> present (which seems to be the standard setup.)
>>>
>>>>> I don't care the slightest what the numbering is, as long as it is
>>>>> stable.  On some hardware, with an unpatched kernel, the mmc device
>>>>> numbering changes depending on whether or not an SD card is inserted on
>>>>> boot.  Getting rid of that behaviour is really all I want.
>>>>
>>>> Agreed, that would be an improvement.
>>>
>>> The mmc device numbering was tied to the mmc host numbering a while back
>>> and the order that the hosts are probed should be completely independent
>>> of whether a card is inserted or not:
>>>
>>>         snprintf(md->disk->disk_name, sizeof(md->disk->disk_name),
>>>                  "mmcblk%u%s", card->host->index, subname ? subname : "");
>>>
>>>         snprintf(rpmb_name, sizeof(rpmb_name),
>>>                  "mmcblk%u%s", card->host->index, subname ? subname : "");
>>>
>>> I suspect that Mans is quoting something from the dim and distant past
>>> to confuse the issue - as shown above, it is now dependent on the host
>>> numbering order not the order in which cards are inserted.
>>
>> Commit 9aaf3437aa72 ("mmc: block: Use the mmc host device index as the
>> mmcblk device index") which came in with v4.6 enables constant mmc block
>> device numbering. I can confirm that it works nicely, and it improved
>> the situation a lot.
>>
>> That being said, we still use a patch downstream which allows
>> renumbering using an alias. We deal with a bunch of different boards
>> with different SoC's. I have a couple of SD cards with various rootfs
>> and use internal eMMC boot quite often as well. Remembering which board
>> uses which numbering is a pain. Maintaining a patch is just easier...
>> Furthermore, U-Boot allows reordering and all boards I deal with use mmc
>> 0 for the internal eMMC. The aliases allow consistency.
> 
> Maybe eMMC should've been given a different block device name?

I presume that's because they have hardware and software partitioning
and that's why you want to discern those two.

-- 
Best regards,
Marek Vasut