Re: [PATCHv4 2/3] MMC: OMAP: HSMMC: add runtime pm support

From: Kevin Hilman <khilman@ti.com>
To: "S, Venkatraman" <svenkatr@ti.com>
Cc: "Dong, Chuanxiao" <chuanxiao.dong@intel.com>,
	Balaji T K <balajitk@ti.com>,
	"linux-omap@vger.kernel.org" <linux-omap@vger.kernel.org>,
	"linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
	"cjb@laptop.org" <cjb@laptop.org>,
	"tony@atomide.com" <tony@atomide.com>,
	"madhu.cr@ti.com" <madhu.cr@ti.com>,
	"b-cousson@ti.com" <b-cousson@ti.com>,
	"paul@pwsan.com" <paul@pwsan.com>,
	"kishore.kadiyala@ti.com" <kishore.kadiyala@ti.com>
Subject: Re: [PATCHv4 2/3] MMC: OMAP: HSMMC: add runtime pm support
Date: Wed, 13 Jul 2011 08:56:26 -0700	[thread overview]
Message-ID: <871uxuqimt.fsf@ti.com> (raw)
In-Reply-To: <CANfBPZ-M+yrZv2r3sGYFRC4vRmD5AZ1b4FfyFYENmqH+FDgCcg@mail.gmail.com> (Venkatraman S.'s message of "Wed, 13 Jul 2011 21:04:14 +0530")

"S, Venkatraman" <svenkatr@ti.com> writes:

> On Wed, Jul 13, 2011 at 8:29 PM, Kevin Hilman <khilman@ti.com> wrote:

[...]

>>
>> My basic question is this: why does this device need to be runtime
>> resumed during system suspend?  Why can't it just stay runtime
>> suspended?
>>
>
> From my understanding, the runtime suspend is usually implemented to not
> lose the card 'context', i.e. transactions can continue after a
> runtime suspend /
> resume cycle.
>
> For system suspend, the MMC core sends a sleep command (which, in
> itself, is a transaction) to the card to go to sleep state, and for
> all practical purposes, the card is treated as 'removed'. When the
> system resumes, the card is rescanned and re-initialized.
>
> Hence, for system suspend, the MMC controller needs to be enabled to
> actually send the command which puts the card to sleep (and hence the
> resume).

Great, this is the detail I was looking for since my MMC knowledge is
quite limited.  

So, in theory, this same sleep command could be sent during runtime
suspend as well, so that a system suspend would not have to runtime
resume, correct?  But I suppose it would result in a much higher latency
runtime resumes.  It might be worth experimenting with doing this,
possibly in combination with longer auto-suspend delay times.

Thanks for the explanation,

Kevin
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