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

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Date:	Wed, 12 Sep 2012 00:23:11 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Dave Airlie <airlied@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"dri-devel" <dri-devel@...ts.freedesktop.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-pci@...r.kernel.org
Subject: Re: runtime PM and special power switches

On Tuesday, September 11, 2012, Alan Stern wrote:
> On Tue, 11 Sep 2012, Rafael J. Wysocki wrote:
> 
> > 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.
> 
> This sounds more like a job for a power domain.  Unless the power
> switch is already in the device hierarchy as a parent to the PCI
> device.

Good idea. :-)

Thanks,
Rafael
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