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

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Date:	Fri, 9 Aug 2013 08:43:13 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Seungwon Jeon <tgih.jun@...sung.com>
Cc:	Chris Ball <cjb@...top.org>, Olof Johansson <olof@...om.net>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	James Hogan <james.hogan@...tec.com>,
	Grant Grundler <grundler@...omium.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Abhilash Kesavan <a.kesavan@...sung.com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/4] mmc: dw_mmc: Always setup the bus after suspend/resume

Seungwon,

On Fri, Aug 9, 2013 at 6:35 AM, Seungwon Jeon <tgih.jun@...sung.com> 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.

-Doug
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