[Intel-gfx] [PATCH] UXA: Wait until a pageflip actually completes to report it.

[Chris Wilson]

On Thu, May 08, 2014 at 08:54:52AM -0700, Eric Anholt wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > On Wed, May 07, 2014 at 10:44:23PM -0700, Jamey Sharp wrote:
> >> On Wed, May 07, 2014 at 04:55:18PM +0100, Chris Wilson wrote:
> >> > On Mon, May 05, 2014 at 11:05:07PM -0700, Jamey Sharp wrote:
> >> > > UXA was reporting page-flip completion as soon as the flip was scheduled
> >> > > with the kernel, instead of waiting for the kernel to indicate that the
> >> > > flip had actually completed.
> >> > > 
> >> > > Moving the DRI2SwapComplete call to the right place fixes all of our
> >> > > Piglit tests for OML_sync_control when run on xf86-video-intel/UXA,
> >> > > aside from a bit of difficult-to-reproduce flakiness when using a
> >> > > divisor > 1.
> >> > 
> >> > The violation is intentional, as it gives us triple buffering by
> >> > default. It can be disabled.
> >> 
> >> As far as I can tell, this patch has no effect on triple-buffering. I
> >> verified that by logging new_front->handle in intel_do_pageflip: It
> >> rotates through three different BO's on successive flips.
> >
> > The patch blocks clients in GetBuffers until the swap completes,
> > preventing them from rendering to the third buffer.
> 
> I don't see where the client gets blocked in a swapbuffers.  Can you
> point that out?

DRI2ThrottleClient() called by GetBuffers and SwapBuffers will call
IgnoreClient() if swapsPending >= swap_limit and will only wake the
client on calling DRI2SwapComplete(). The premature SwapComplete is to
allow the clients next call to GetBuffers to succeed promptly returning
the third buffer prior to the page flip completing.

(gdb) bt
#0  IgnoreClient (client=client at entry=0x19de6c0) at connection.c:1156
#1  0x00000000005a1bd4 in DRI2ThrottleClient (client=client at entry=0x19de6c0, pDraw=<optimized out>) at dri2.c:713
#2  0x00000000005a4c3b in ProcDRI2GetBuffersWithFormat (client=0x19de6c0) at dri2ext.c:302
#3  ProcDRI2Dispatch (client=0x19de6c0) at dri2ext.c:608
#4  0x000000000043a70e in Dispatch () at dispatch.c:432
#5  0x000000000043f95d in dix_main (argc=6, argv=0x7fffe4302078, envp=<optimized out>) at main.c:296
#6  0x00007f14dd0c6b45 in __libc_start_main (main=0x426680 <main>, argc=6, argv=0x7fffe4302078, init=<optimized out>, fini=<optimized out>, 
    rtld_fini=<optimized out>, stack_end=0x7fffe4302068) at libc-start.c:287
#7  0x00000000004266b3 in _start ()

-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre