[RFC PATCH] drm/vblanks: Deal with HW vblank counter resets.

[Michel Dänzer]

On 07/11/17 07:26 AM, Dhinakaran Pandiyan wrote:
> Some HW vblank counters reset due to power management events, which messes
> up the vblank counting logic. This leads to screen freezes with user space
> waiting on vblank events that may not occur if the counter keeps resetting.
> 
> For e.g., After the HW vblank counter resets
> [    9.007359] [drm:drm_update_vblank_count [drm]] updating vblank count
> on crtc 0: current=297, diff=4294965389, hw=5 hw_last=1912
> 
> So, fall back to the SW counter, computed using  vblank timestamps
> and frame duration, when the HW counter value deviates by 50% of the SW
> computed value.
> 
> I have tested this patch on my SKL laptop with i915.enable_psr=1 and it
> *seems* to solve the screen freeze issue seen with PSR when DMC is loaded.
> 
> Known issues:
> 1) The 50% deviation margin is arbitrary.
> 2) "Redundant vblirq ignored" messages are more frequent.
> 
> I am sending this as an RFC to get feedback on whether the fall back
> approach is sane and if it should be implemented in the core.

Is there no way for the driver to know under which circumstances the
reset to 0 might happen? If there is, maybe it could be solved by
calling drm_crtc_vblank_off() before it might happen and
drm_crtc_vblank_on() after it might have happened.

Otherwise, might it be better not to use the HW counter at all when it's
known not to be reliable?


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