Re: [PATCH][RFC] mfd: intel-lpss: Avoid resuming runtime-suspended lpss unnecessarily

[Chen Yu]

Hi,
On Mon, Sep 12, 2016 at 04:17:04PM +0100, Lee Jones wrote:
> On Sun, 04 Sep 2016, Chen Yu wrote:
> 
> > We have report that the intel_lpss_prepare() takes too much time during
> > suspend, and this is because we first resume the devices from runtime
> > suspend by resume_lpss_device(), to make sure they are in proper state
> > before system suspend, which takes 100ms for each LPSS devices(PCI power
> > state from D3_cold to D0). And since resume_lpss_device() resumes the
> > devices synchronously, we might get huge latency if we have many
> > LPSS devices.
> > 
> > So first try is to use pm_request_resume() instead, to make the runtime
> > resume process asynchronously. Unfortunately the asynchronous runtime
> > resume relies on pm_wq, which is freezed at early stage. So we choose
> > another method, that is to avoid resuming runtime-suspended devices,
> > if they are already runtime suspended. This is safe because for LPSS
> > driver, the runtime suspend and system suspend are of the same
> > hook - i.e., intel_lpss_suspend(). And moreover, this device is
> > neither runtime wakeup source nor system wakeup source.
> > 
> > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > Cc: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > ---
> >  drivers/mfd/intel-lpss.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/mfd/intel-lpss.c b/drivers/mfd/intel-lpss.c
> > index 41b1138..6dcc9a0 100644
> > --- a/drivers/mfd/intel-lpss.c
> > +++ b/drivers/mfd/intel-lpss.c
> > @@ -485,6 +485,15 @@ static int resume_lpss_device(struct device *dev, void *data)
> >  int intel_lpss_prepare(struct device *dev)
> >  {
> >  	/*
> > +	 * This is safe because:
> > +	 * 1. The runtime suspend and system suspend
> > +	 * are of the same hook.
> > +	 * 2. This device is neither runtime wakeup source
> > +	 * nor system wakeup source.
> > +	 */
> > +	if (pm_runtime_status_suspended(dev))
> > +		return 1;
> 
> What's '1'?
> 
According to the comment in device_prepare():

A positive return value from ->prepare() means "this device appears
to be runtime-suspended and its state is fine, so if it really is
runtime-suspended, you can leave it in that state provided that you
will do the same thing with all of its descendants". 

Thanks,
Yu