Re: runtime PM and special power switches

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday, September 12, 2012, Dave Airlie wrote:
> On Wed, Sep 12, 2012 at 7:32 AM, Alan Stern <stern@rowland.harvard.edu> 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.
> 
> I'll have to investigate power domains then,
> 
> The switch is hidden in many different places, one some laptops its in
> a ACPI _DSM on one GPU, on others its in an ACPI _DSM on the other
> one, in some its in a different ACPI _DSM, then we have it in the ACPI
> ATPX method on others, and finally Apple have it in a piece of hw that
> isn't just on the LPC bus or somewhere like that.
> 
> Currently we just hide it all inside vga_switcheroo and I'd just need
> an interface to call that once the layers have stopped poking
> registers in PCI config space, if we could fix PCI runtime suspend so
> the driver was the last to get called then that would also not suck.

Well, as I said, we may try to change the PCI layer so that it doesn't
access the device any more in pci_pm_runtime_suspend() if it sees that
pci_dev->state_saved has been set by the driver's callback.  Then,
your drivers would only need to set pci_dev->state_saved in their
.runtime_suspend() callbacks.

Alternatively, which may be less hackish but more work, you can set the
pm_domain pointer in the device structure to a struct dev_pm_domain whose
ops will just call the corresponding bus type's ops except for
.runtime_suspend() that will execute the additional ACPI stuff after calling
the bus type's method.

Thanks,
Rafael