Re: [RFC][PATCH] Enable async suspend/resume of i2c devices

[Sonny Rao <sonnyrao-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>]

On Thu, Apr 7, 2011 at 12:55 AM, Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org> wrote:
> Hi Mark,
>
> On Thu, 7 Apr 2011 07:31:24 +0900, Mark Brown wrote:
>> On Wed, Apr 06, 2011 at 10:49:17AM -0400, Alan Stern wrote:
>>
>> > Neither is the case.  For these subsystems, the PM dependencies _are_
>> > known.
>>
>> ...
>>
>> > Now, I have no idea what the situation is with regard to I2C...
>>
>> You definitely don't know *anything* about the relationships for I2C,
>> especially in embedded systems.
>
> Can you please elaborate? The i2c subsystem uses a standard
> parent-children relationship. It seems fairly similar to USB for
> example. The only special case I can think of is with bus multiplexing,
> but it would be easy enough to switch off async suspend/resume in this
> case.
>
> Sonny, I would expect you to obtain the time saving by only switching
> the i2c client device to async suspend/resume. wasn't it the case? i2c
> bus device suspend/resume shouldn't matter, as the operation should be
> handled by the hardware (e.g. PCI) layer.

I tried that first, and it looked like the client resume wouldn't start on the
async thread until the kernel resumed the i2c master which was
happening very late for some reason.  Maybe this was just a fluke of
the device ordering, but enabling it for both master and client allowed
the device resume to consistently happen much earlier.

Sonny