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

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Date:   Tue, 13 Sep 2016 09:24:22 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH][RFC] mfd: intel-lpss: Avoid resuming runtime-suspended
 lpss unnecessarily

On Mon, 12 Sep 2016, Chen Yu wrote:
> 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@...ux.intel.com>
> > > Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> > > ---
> > >  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". 

Are there no defines for this?

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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