[Intel-gfx] [RFC] Reduce idle vblank wakeups

[Andrew Lutomirski]

On Wed, Nov 16, 2011 at 6:19 PM, Matthew Garrett <mjg59 at srcf.ucam.org> wrote:
> On Thu, Nov 17, 2011 at 01:26:37AM +0100, Mario Kleiner wrote:
>> On Nov 16, 2011, at 7:48 PM, Matthew Garrett wrote:
>> >For Radeon, I'd have thought you could handle this by scheduling
>> >an irq
>> >for the beginning of scanout (avivo has a register for that) and
>> >delaying the vblank disable until you hit it?
>>
>> For Radeon there is such an irq, but iirc we had some discussions on
>> this, also with Alex Deucher, a while ago and some irq's weren't
>> considered very reliable, or already used for other stuff. The idea
>> i had goes like this:
>>
>> Use the crtc scanout position queries together with the vblank
>> counter queries inside some calibration loop, maybe executed after
>> each modeset, to find out the scanline range in which the hardware
>> vblank counter increments -- basically a forbidden range of scanline
>> positions where the race would happen. Then at each vblank off/on,
>> query scanout position before and after the hw vblank counter query.
>> If according to the scanout positions the vblank counter query
>> happened within the forbidden time window, retry the query. With a
>> well working calibration that should add no delay in most cases and
>> a delay to the on/off code of a few dozen microseconds (=duration of
>> a few scanlines) worst case.
>
> Assuming we're sleeping rather than busy-looping, that's certainly ok.
> My previous experiments with radeon indicated that the scanout irq was
> certainly not entirely reliable - on the other hand, I was trying to use
> it for completing memory reclocking within the vblank interval. It was
> typically still within a few scanlines, so a sanity check there wouldn't
> pose too much of a problem.

I think there's a simpler fix: just keep the hardware and software
counts in sync -- if everything is working correctly (even with all
these crazy races), the difference should be constant.  Patch coming
momentarily.

--Andy