Re: [bisect] Merge tag 'mmc-v4.6' of git://git.linaro.org/people/ulf.hansson/mmc (was [GIT PULL] MMC for v.4.6)

[Ulf Hansson]

On 3 April 2016 at 13:54, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Apr 2, 2016 at 9:56 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>
>> Note how mmc1 => mmcblk0 and mmc0 => mmcblk1.
>>
>> This produces a failure to boot as the wrong partition is mounted as
>> root (/dev/mmcblk0p2 is now on the wrong mmc).
>
> It *looks* very much like somebody is doing asynchronous probing of
> the bus, meaning that the devices get probed in random order.

Correct.

>
> And that "random order" is admittedly probably usually fairly static
> on any particular hardware platform, but then something happens to
> change timing, and...
>
> This is why you should never probe the actual *bus* asynchronously,
> just do the end-point setup async. For example, you'd enumerate ports
> (and assign devices to the ports) synchronously, but then after device
> assignment the actual device probing can be async.

So to do this, we need to tie the mmc/sd/sdio controller to a
dedicated mmcblk id.

There have been some ideas to fix this by using "aliases" in a DT
based configuration.

>
>> The bisect tried all the mmc tree patches which were all good.
>> I double-checked by cloning the mmc tree and building both mmc-v4.6
>> and v4.5-rc6, and both tested good.
>>
>> I interpret that to mean some change in mmc + some new behavior elsewhere
>> for v4.6 is causing this. Any ideas?
>
> Hmm. If it really is just timing, it could have been around forever,
> and just hidden by the fact that normally mmc0 gets probed before
> mmc1, but then some other probing thing slowed down or the exact
> details of the async workqueue  scheduling changed, and now mmc1 just
> *happens* to get probed first..
>
> The thing that changed scheduling order could easily have come from
> some non-mmc change.
>
> NOTE! I have nothing to back this up except that (a) we've had
> problems like this before and (b) it does look from your dmesg that
> mmcX is simply probed in the "wrong" order. I didn't look at exactly
> what mmc does or who does the probing.
>
> Maybe Ulf can explain what it is that is _supposed_ to keep the mmc
> probe order stable. Ulf?
>
>              Linus

The commit that's likely to cause the regression is:
520bd7a8b415 ("mmc: core: Optimize boot time by detecting cards
simultaneously").

This commit further enables asynchronous detection of (e)MMC/SD/SDIO
cards, by converting from an *ordered* work-queue to a *non-ordered*
work-queue for card detection.

Although, one should know that there have *never* been any guarantees
to get a fixed mmcblk id for a card. I expect that's what has been
assumed here.

Let me elaborate a bit on the card detection procedure. When the mmc
controller has been successfully probed, its driver schedules a work
to start enumeration of cards. Only cards that gets detected
successfully becomes registered and those gets an mmcblk id assigned
to it. The picked id, is the first available starting from zero. Now,
as cards can be removable and because drivers for mmc controllers may
sometimes returns -EPROBE_DEFER (for whatever reason), there's never
been support for fixed mmcblk ids.

To deal with this, one should use the so called UUID/PARTUUID. Is
there any reasons to why that can't be done in this case?

Kind regards
Uffe