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

[Kevin Hilman <khilman@ti.com>]

"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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