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SubjectRe: runtime PM and special power switches
On Wed, Sep 12, 2012 at 3:13 PM, Dave Airlie <> wrote:
> On Wed, Sep 12, 2012 at 8:58 AM, Rafael J. Wysocki <> wrote:
>> On Wednesday, September 12, 2012, Dave Airlie wrote:
>>> On Wed, Sep 12, 2012 at 7:32 AM, 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.
>>> 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 g 2et 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.
> Actually it appears I'll need this, I'd forgotten things are a bit
> messier than I thought
> So there are two variants on the _DSM for nvidia dual-gpu machines,
> the older pre-optimus _DSM requires
> an explicit power off call post-D3, however for optimus _DSM the D3
> transition will flick the power switch, however
> the pci code then goes and seem to turn the device back to D0 for some
> reason. So yes after save state,
> I'd really appreciate if it the pci layer would stop poking my device.
>> 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.
> I've mostly written this, and it seems to work, I've jsut set a
> pm_domain in the vga switcheroo code that copies the dev->bus->pm
> into a private structure. I'll need this for the old nvidia and radeon
> poweroffs.
is my current WIP branch
contains what I've done to override the 2 pms ops we need to override
for older GPUs
is my current PCI workaround.


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