[Intel-gfx] [PATCH v8 3/9] drm/i915: Make sprite updates atomic

[Murthy, Arun R]

> Add a mechanism by which we can evade the leading edge of vblank. This
> guarantees that no two sprite register writes will straddle on either
> side of the vblank start, and that means all the writes will be latched
> together in one atomic operation.

Here only one sprite update followed by the primary enable/disable can be
achieved atomically. But I feel update of all planes are to be 
considered, i.e
update of planes per pipe basis to achieve atomicity.
>
> We do the vblank evade by checking the scanline counter, and if it's too
> close to the start of vblank (too close has been hardcoded to 100usec
> for now), we will wait for the vblank start to pass. In order to
> eliminate random delayes from the rest of the system, we operate with
> interrupts disabled, except when waiting for the vblank obviously.

This can be achieved easily by checking the previous vblank time in
drm_get_last_vblanktimestamp(), using this the next vblank time cab be
predicted. Using these with the hardcoded value 100usec, a check can be
made to continue or to wait for a vblank. For waiting for a vblank instead
of adding new function just use the available intel_wait_for_vblank().

last_vblank = drm_get_last_vblanktimestamp();
curr_time = do_getttimeofday();
if (curr_time.tv_usec - (last_vblank +  VBLANK_TIME_INTERVAL) > 100)
     /* acquire lock and proceed*/
else
     /* wait for one vblank, acquire lock and proceed */

Thanks and Regards,
Arun R Murthy
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