On Wednesday, November 07, 2012 10:49:04 AM Alan Stern wrote:

On Wed, 7 Nov 2012, Huang Ying wrote:

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Devices will be disabled if the PCI driver is unbound from the PCI
device.

Yes.  But without a PCI driver, nothing will call 
pm_runtime_set_suspended.

pm_runtime_set_suspended will be called in pci_device_remove or error
path of local_pci_probe.

Yes, okay.  But that's what we want.  Unused, driverless PCI devices
shouldn't force their parents to remain at full power.

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  And even if something does call 
pm_runtime_set_suspended, it's still not a problem -- the device can't 
be used without a driver.

The VGA device can be used without a driver.

Ah, right, that's your _real_ problem.  You should have mentioned this 
in the original Changelog for the patch.

Rafael, this does need to be fixed.

Yup.

The PCI subsystem assumes that 
driverless devices are not in use, so they are disabled for runtime PM 
and marked as suspended.  This is not appropriate for VGA devices, 
which can indeed be used without a driver.

I'm not sure what the best solution is.  Maybe we should Ying's 
proposal a step farther:

	Make pm_runtime_set_suspended() fail if runtime PM is 
	forbidden.

	Make pm_runtime_forbid() call pm_runtime_set_active()
	(and do a runtime resume of the parent) if disable_depth > 0.

I'd prefer this one.  The callers of pm_runtime_forbid() may actually
reasonably expect something like this to happen.

	Make the PCI runtime-idle routine call 
	pm_runtime_set_suspended() if disable_depth > 0.  Or maybe
	do this for all devices, in the runtime PM core.

That would mean calling it on every call to pm_runtime_put() and friends
which seems to be a bit wasteful.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.