linux-kernel - Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

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Message-ID: <8981266.1iKleIVcUj@vostro.rjw.lan>
Date:	Mon, 07 Sep 2015 23:20:01 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Irina Tirdea <irina.tirdea@...el.com>
Cc:	Alan Stern <stern@...land.harvard.edu>, linux-pm@...r.kernel.org,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Octavian Purdila <octavian.purdila@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

On Monday, September 07, 2015 11:42:41 PM Irina Tirdea wrote:
> Add new option to sysfs control interface, allowing the user to force
> suspend the device.

Had we thought this had been a good idea, we'd have added that thing to
the interface from the start.

The problem with it is that user space generally doesn't know when it is
safe to suspend a device, so it cannot force anything into runtime suspend.

> This is useful for devices that need to be
> suspended when closing the lid of a laptop or the screen of a mobile
> device, while userspace still holds open handles to it and the
> system does not enter system suspend.
> 
> Add the "off" option to the sysfs control power interface, along with
> the already present "on" and "auto". When this attribute is set to
> "off", the device will be force suspended by calling its runtime
> suspend callback and disabling runtime power management so that
> further acceses to the device will not change the actual state.

And how is user space supposed to know that it doesn't break things
this way?

> The device can be resumed by setting the attribute to "on" or "auto".
> The behaviour of the interface when switching only between "on"
> and "auto" states remains unchanged.
> 
> Signed-off-by: Irina Tirdea <irina.tirdea@...el.com>
> ---
> 
> Hi,
> 
> This is a proposal for suspending devices when the closing the lid of
> a laptop or the screen of a mobile device.
> 
> I am testing this with a Goodix touchscreen [1] for an Android mobile
> device. Android has an userspace layer (power HAL) that would normally
> close the touchscreen when the screen is closed. Android touchscreen
> drivers usually provide a custom sysfs interface to allow this.
> This would be better implemented in a common place, to avoid code
> duplication and to simplify the driver code (as previosly discussed
> in [1]).
> 
> I know there are more ways to implement this, so I would appreciate
> your feedback.

So the feedback is that this is not going to work in general.  Please
use a different approach.

Thanks,
Rafael

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