linux-kernel - Re: [RFC] intel-lpss: remove .prepare() callback

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Message-ID: <1516979226.2752.3.camel@intel.com>
Date:   Fri, 26 Jan 2018 23:07:06 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     mika.westerberg@...ux.intel.com, lee.jones@...aro.org
Subject: Re: [RFC] intel-lpss: remove .prepare() callback

Hi, Andy,

On Thu, 2018-01-25 at 14:28 +0200, Andy Shevchenko wrote:
> On Thu, 2018-01-25 at 16:12 +0800, Zhang Rui wrote:
> > 
> > The .prepare() callback of intel-lpss driver does nothing but wakes
> > up
> > its
> > children. I don't know if there is any reason to do so, but to me,
> > this is
> > not preferred because it should be the child device driver to do so
> > when
> > necessary, not the parent device driver.
> > Plus, .prepare() does not support asynchronization.
> > 
> > For example, on MS Surface Pro 4, there are 4 intel-lpss devices
> > which
> > are runtime suspended before system suspend, resuming each of them
> > takes
> > more than 100 milliseconds. Thus the .prepare() of intel-lpss
> > driver
> > takes
> > 400ms+ on my surface pro 4, and I've seen platforms with 16 intel
> > lpss
> > devices.
> > 
> > With this patch applied, the child devices are resumed in the
> > .suspend()
> > stage of the child device, and they are done in parallel, thus only
> > 100ms
> > is needed, no matter how many intel lpss devices there are.
> > 
> > I have tested it on three different platforms and didn't find any
> > obvious
> > problem caused by this patch, and it indeed reduces the suspend
> > time a
> > lot.
> > 
> > @@ -482,17 +482,6 @@ static int resume_lpss_device(struct device
> > *dev,
> > -	device_for_each_child_reverse(dev, NULL,
> > resume_lpss_device);
> Besides introduced compiler warning, did you check the latest linux-
> pm
> changes? 
> 
> commit 8425ec7faff005500aad89b9fc00e5ba91ac57b9
> Author: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Date:   Wed Jan 3 01:34:53 2018 +0100
> 
>     PM / mfd: intel-lpss: Use DPM_FLAG_SMART_SUSPEND
> 
No, thanks for the pointer, I will check this.

BTW, is there any reason that we need this .prepare() callback?

thanks,
rui