Re: [PATCH 1/2] mmc: block: mmcblkN: use slot index instead of dynamic name index

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On 6 August 2012 21:01, Chris Ball <cjb@xxxxxxxxxx> wrote:
> Hi,
>
> On Mon, Aug 06 2012, Dirk Behme wrote:
>> On embedded devices, often there is a combination of removable mmc
>> devices (e.g. MMC/SD cards) and hard wired ones (e.g. eMMC).
>> Depending on the hardware configuration, the 'mmcblkN' node might
>> change if the removable device is available or not at boot time.
>>
>> E.g. if the removable device is attached at boot time, it might
>> become mmxblk0. And the hard wired one mmcblk1. But if the removable
>> device isn't there at boot time, the hard wired one will become
>> mmcblk0. This makes it somehow difficult to hard code the root device
>> to the non-removable device and boot fast.
>>
>> This change does simply associate 'N' of 'mmcblkN' with the slot index
>> instead of the dynamic name index. The slot index is always the same,
>> ensuring that the non-removable mmc device is associated always
>> with the same mmcblkN. Independent of the availability of the removable
>> one.
>
> I like this change in principle, but doesn't it break boot for everyone
> currently using e.g. root=/dev/mmcblk0p1 on slot index 2?  That doesn't
> sound like an acceptable regression.
>
It shouldn't break boot for any user already doing the right thing.
If the user's platform probes only slot-2, it will still be 'mmcblk0'
If the platform probes slot-1 & 2, it is fundamentally unsafe to count
upon the card in slot-2 to be enumerated before the one in slot-1.
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