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

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Date:   Thu, 25 Jan 2018 14:28:54 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Zhang Rui <rui.zhang@...el.com>, linux-kernel@...r.kernel.org
Cc:     mika.westerberg@...ux.intel.com, lee.jones@...aro.org
Subject: Re: [RFC] intel-lpss: remove .prepare() callback

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


-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy