[PATCH 1/2] drm/amd/display: Fix race between vblank irq and pageflip irq.

[Michel Dänzer]

On 22/04/17 01:23 AM, Mario Kleiner wrote:
> Since DC now uses CRTC_VERTICAL_INTERRUPT0 as VBLANK irq trigger
> and vblank interrupts actually happen earliest at start of vblank,
> instead of a bit before vblank, we no longer need some of the
> fudging logic to deal with too early vblank irq handling (grep for
> lb_vblank_lead_lines). This itself fixes a pageflip scheduling
> bug in DC, caused by uninitialized  use of lb_vblank_lead_lines,
> with a wrong startup value of 0. Thanks to the new vblank irq
> trigger this value of zero is now actually correct for DC :).
> 
> A new problem is that vblank irq's race against pflip irq's,
> and as both can fire at first line of vblank, it is no longer
> guaranteed that vblank irq handling (therefore -> drm_handle_vblank()
> -> drm_update_vblank_count()) executes before pflip irq handling
> for a given vblank interval when a pageflip completes. Therefore
> the vblank count and timestamps emitted to user-space as part of
> the pageflip completion event will be often stale and cause new
> timestamping and swap scheduling errors in user-space.
> 
> This was observed with large frequency on R9 380 Tonga Pro.
> 
> Fix this by enforcing a vblank count+timestamp update right
> before emitting the pageflip completion event from the pflip
> irq handler. The logic in core drm_update_vblank_count() makes
> sure that no redundant or conflicting updates happen, iow. the
> call turns into a no-op if it wasn't needed for that vblank,
> burning a few microseconds of cpu time though.
> 
> Successfully tested on AMD R9 380 "Tonga Pro" (VI/DCE 10)
> with DC enabled on the current DC staging branch. Independent
> measurement of pageflip completion timing with special hardware
> measurement equipment now confirms correct pageflip timestamps
> and counts in the pageflip completion events.
> 
> Note that there is another unresolved pageflip bug present in current
> dc staging, which causes pageflips to complete one vblank too early
> when the pageflip ioctl gets called while in vblank. Something seems
> to be amiss in the way amdgpu_dm_do_flip() handles 'target_vblank',
> or how amdgpu_dm_atomic_commit_tail() computes 'target' for calling
> amdgpu_dm_do_flip().

If the last paragraph refers to the problem fixed by patch 2, I'd drop
that paragraph. With that,

Reviewed-by: Michel Dänzer <michel.daenzer at amd.com>


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Earthling Michel Dänzer               |               http://www.amd.com
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