[Intel-gfx] [PATCH 5/5] drm/i915: Allow vblank interrupts during modeset and eliminate some vblank races

[Michel Dänzer]

On 29.05.2014 19:56, Daniel Vetter wrote:
> On Thu, May 29, 2014 at 6:11 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>
>>> We could rename the off/on to pre/post_modeset if you like, but then
>>> someone gets to audit all the other drivers. That someone isn't
>>> going to be me.
>>
>> I appreciate your caution wrt other drivers, but I'm worried that having
>> a second mechanism for keeping the DRM vblank counter consistent might
>> cause subtle problems for drivers using the existing mechanism anyway.
> 
> I think that risk is rather low. Only i915 has been stupid enough to
> use both drm_vblank_off + pre/post_modeset in the normal crtc
> enable/disable functions. Everyone else uses either or the other,
> except for tegra which uses pre/post in the ->prepare/commit hooks and
> off in the crtc->disable callback. So should still get consistent
> results (since after ->disable vblank consistency stops mattering).
> 
> The reason we do all this is that we want to kill the delayed vblank
> put for i915, which is possible if you have a hw vblank counter.

That should also be possible with pre/post_modeset. (Not sure
eliminating it completely is a good idea for the reasons you mentioned
before, but at least decreasing it significantly should be no problem. I
think even dropping the default a lot for all drivers should be rather
low risk)


> There's apparently more stuff wrong here still, so while we wrestle
> around probably best to leave other drivers out. I guess once this is
> all done I have to roll it out across all drivers so that we have
> consistent behaviour again ...

Since AFAICT radeon isn't affected by the problem drm_vblank_off() was
supposed to solve originally, I'd prefer if it didn't start using that
and potentially suffer from all the trouble it seems to cause.


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