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

[Arun Murthy]

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

Thanks and Regards,
Arun R Murthy
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