Re: [PATCH v4 3/4] mmc: dw_mmc: Always setup the bus after suspend/resume

From: Doug Anderson
Date: Fri Aug 09 2013 - 11:43:20 EST


On Fri, Aug 9, 2013 at 6:35 AM, Seungwon Jeon <tgih.jun@xxxxxxxxxxx> wrote:
> On Wed, August 07, 2013, Doug Anderson wrote:
>> After suspend/resume all of the dw_mmc registers are reset to
>> defaults. We restore most of them, but specifically don't setup the
>> clock registers after resume unless we've got a powered card. Things
>> still work because the core will eventually call set_ios() and we'll
>> set things up.
> Hmm, I didn't get the need of this call during resume.
> I think set_ios is only valid where core layer calls.
> Besides, important things is ios's parameters.
> If suspend has finished successfully, last call of set_ios() is from mmc_power_off().
> On seeing fields of 'mmc->ios' stored last, these values aren't proper in resume phase.
> Please check mmc_power_off() function.
> In case MMC_PM_KEEP_POWER it could be kept.

Most of my reasoning has to do with the fact that the state of the
system after suspend/resume should not be significantly different than
the state of the system before suspend/resume. If the state of the
system is different in the two cases it points out potential problems
or inefficiencies.

To make this more concrete:

1. Boot up a system with no card in the SD Card slot.
2. Note down the value of registers like CLKDIV, CLKENA, etc.
3. Suspend / resume (S2R)
4. Check the values of CLKDIV, CLKENA, etc.

You will notice that they are different. This is a bad sign and can
be a source of bugs (though I don't know of any). ...or it could mean
that power draw is different (could be better, could be worse) after a
suspend/resume cycle.

Said another way, if the value of CLKDIV, CLKENA, etc is not important
when a card is not inserted, why do they get initialized at boot time?

In general, I think that the mmc core code makes the assumption that
it's up to the driver to make sure that its state is preserved across
S2R. For dw_mmc the driver doesn't do the "brute force" that some
drivers do of just saving and restoring all registers using a copy
loop. Instead, the dw_mmc driver runs code that tries to set the
state back to something reasonable. Without my patch the dw_mmc
driver doesn't run any code that restores these registers.
dw_mci_set_ios() will do so.

Another option would be to forcibly save/restore registers in suspend/resume.

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