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

[Daniel Vetter]

On Tue, Nov 07, 2017 at 10:47:00AM +0100, Michel Dänzer wrote:
> 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?

We know when it happens, so agreed this isn't a good/workable solution
really. I thought the plan to fix that was to fix up our runtime pm to
make sure the vblank counter doesn't get reset while we need it (pending
flip or vblank). And in-between (when the vblank counter is totally off)
we'd fix any mismatch by adjusting the sw vblank counter with an explicit
call (where we can use the elapsed time to estimate the elapsed vblank
counts well enough). Adding a magic hack like this doesn't sound like a
good plan to me indeed.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch