linux-kernel - Re: [RFC PATCH] mmc: Enable wakeup

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Date:	Fri, 14 Jun 2013 11:42:33 -0700
From:	Zoran Markovic <zoran.markovic@...aro.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>, linux-mmc@...r.kernel.org,
	San Mehat <san@...gle.com>, Colin Cross <ccross@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Chris Ball <cjb@...top.org>,
	Johan Rudholm <johan.rudholm@...ricsson.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Konstantin Dorfman <kdorfman@...eaurora.org>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] mmc: Enable wakeup_sources for mmc core

> I am not sure I understand why this patch is needed. When a new card
> is inserted/removed and the upper levels gets notification about the
> new card, triggering the mounting/un-mounting of the file system, why
> should it be the lowest layer (mmc) that prevents the platform from
> enter suspend/sleep? Why do we need to prevent it at all?
>
> Note that notifier handling in mmc_pm_notify, was if I remember
> correctly, not completely developed when the original version of this
> patch was being discussed. mmc_pm_notify prevents cards from being
> inserted/removed in the middle of suspend->resume sequence, is that
> not enough?

I will try to speak on behalf of the original implementers in a hope
they would provide the original motivation for the patch.

My understanding is that any preemption in the procedure could be an
opportunity to suspend, as there may be a suspend request racing with
this code. This is why the calls to __pm_stay_awake() and
queue_delayed_work() are so tightly coupled. It would be up to the
delayed work procedure (mmc_rescan()) to decide whether or not it is
safe to suspend. If there are no changes in the MMC state or all
changes can be handled by mmc_rescan(), it is safe to call
__pm_relax(). Otherwise, userland may take over processing of this
event, and this is why the awake state needs to be extended by 1/2
second.

Regards, Zoran
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