[Intel-gfx] Tear-free textured video port causes other drawing on the screen to tear

[Simon Farnsworth]

Hello,

We've been trying to work out why textured video causes an otherwise
smooth text scroller (using XCopyArea and VBlank interrupts) to glitch,
and found that setting the port attribute XV_SYNC_TO_VBLANK to 0 fixes
the glitches.

Looking at the code, the driver uses MI_LOAD_SCAN_LINES_INCL and
MI_WAIT_FOR_EVENT to stall the GPU until it's safe to draw when
XV_SYNC_TO_VBLANK is not 0. This is correct for the video, but has the
unfortunate side effect of stalling the scroller's rendering, too. We
would like to stall *just* the Xv blit, not the rest of the access to
the GPU.

The documentation suggests, if I'm reading it properly, that the driver
could use a separate GPU context to continue executing commands while
the Xv batch buffer is stalled - the batch buffer would then resume when
the event had completed and the GPU was otherwise idle.

Am I misunderstanding something? Is there a deeper reason for not using
multiple contexts to enable different bits of rendering to move past
each other in the GPU schedule?
-- 
Simon Farnsworth