linux-kernel - Re: runtime PM and special power switches

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Message-Id: <201209112255.10486.rjw@sisk.pl>
Date:	Tue, 11 Sep 2012 22:55:10 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Dave Airlie <airlied@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"dri-devel" <dri-devel@...ts.freedesktop.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-pci@...r.kernel.org
Subject: Re: runtime PM and special power switches

Hi,

On Tuesday, September 11, 2012, Dave Airlie wrote:
> Hi Rafael,
> 
> I've been investigating runtime PM support for some use-cases on GPUs.
> 
> In some laptops we have a secondary GPU (optimus) that can be powered
> up for certain 3D tasks and then turned off when finished with. Now I
> did an initial pass on supporting it without using the kernel runtime
> PM stuff, but Alan said I should take a look so here I am.

Alan Stern or Alan Cox? :-)

> While I've started to get a handle on things, we have a bit of an
> extra that I'm not sure we cater for.
> 
> Currently we get called from the PCI layer which after we are finished
> with our runtime suspend callback, will go put the device into the
> correct state etc, however on these optimus/powerxpress laptops we
> have a separate ACPI or platform driver controlled power switch that
> we need to call once the PCI layer is finished the job. This switch
> effectively turns the power to the card completely off leaving it
> drawing no power.
> 
> No we can't hit the switch from the driver callback as the PCI layer
> will get lost, so I'm wondering how you'd envisage we could plug this
> in.

Hmm.  In principle we might modify pci_pm_runtime_suspend() so that it
doesn't call pci_finish_runtime_suspend() if pci_dev->state_saved is
set.  That would actually make it work in analogy with pci_pm_suspend_noirq(),
so perhaps it's not even too dangerous.

Thanks,
Rafael
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