[RFC] Reduce idle vblank wakeups

[Michel Dänzer]

On Mit, 2011-11-16 at 09:20 -0500, Matthew Garrett wrote: 
> The drm core currently waits 5 seconds from userspace dropping a request
> for vblanks to vblanks actually being disabled. This appears to be a
> workaround for broken hardware, but results in a mostly idle desktop
> generating a huge number of wakeups that are entirely unnecessary but which
> consume measurable amounts of power. This patchset makes the vblank timeout
> per-device rather than global, making it easy for drivers to override the
> behaviour without compromising other graphics hardware in the system. It
> then removes the delay on Intel hardware. I've tested this successfully on
> Sandybridge without any evidence of spurious or missing irqs, but I don't
> know how well it works on older hardware. Feedback not only welcome, but
> positively desired.

Have you discussed this with Mario Kleiner (CC'd)?

I thought the main reason for the delay wasn't broken hardware but to
avoid constantly ping-ponging the vblank IRQ between on and off with
apps which regularly neeed the vblank counter value, as that could make
the counter unreliable. Maybe I'm misremembering though.

Even if I'm not, lowering the delay shouldn't be a problem, so long as
it's long enough that at least apps which need the vblank counter every
or every other frame don't cause the IRQ to ping-pong. But that
shouldn't depend on the hardware.


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer