Flickering screen in Neverball on drm-radeon-testing

[Mario Kleiner]

> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue,  1 Jun 2010 12:25:42 -0700 (PDT)
> From: bugzilla-daemon at freedesktop.org
> Subject: [Bug 28341] Flickering screen in Neverball on
> 	drm-radeon-testing
> To: dri-devel at lists.freedesktop.org
> Message-ID: <20100601192542.ACE5B1300E8 at annarchy.freedesktop.org>
> Content-Type: text/plain; charset="UTF-8"
>
> https://bugs.freedesktop.org/show_bug.cgi?id=28341
>
> --- Comment #3 from Magnus Jensen <magnus at jensenligan.se>  
> 2010-06-01 12:25:42 PDT ---
> Sorry. ignore my last attachment. These  errors continue to spawn  
> and i don't
> think they are related to neverball. i'm going to recompile mesa  
> and ddx, and
> post another dmesg soon.
> Just waiting for a kernel compile to finish.
>

I saw a similar flickering with latest Xorg stack (mesa master,  
xserver, ddx etc. master) and the 2.6.34 tree from radeon testing  
with my own toolkit on a R600 gpu. This setup uses the new dri sync &  
swap bits and changes how glXSwapbuffers works.

My best hunch would be that submission of new rendering commands by  
the direct rendering client to the gpu doesn't block after a  
glXSwapbuffers() call until the bufferswap completed, i.e., some  
synchronization is missing there.

This shows flickering, but load and timing dependent...

.... glXXX rendering commands to draw image.
glXSwapBuffers();
glBegin(GL_POINTS);
glVertex2i(10,10);
glEnd();
glFinish();
Take a  swap completion timestamp here.
glClear();
...more rendering commands

-> I use this glVertex2i, .... glFinish() sequence to wait for swap  
completion and get a timestamp. This works on any os/gpu combo ever  
tested, but doesn't seem to work reliably anymore with the new radeon  
sync & swap bits in place. Display flickers, presumably because the  
glClear() executes almost immediately after the glXSwapBuffers is  
scheduled, and before the bufferswap has actually taken place ->  
Clear the backbuffer before swapping.

This however:

.... glXXX rendering commands to draw image.
glXSwapBuffers();
glXWaitForSbcOML(...);
glClear();
...

does work, because the new glXWaitForSbcOML() blocks the client until  
swap completion, so glClear() can only get submitted to the gpu after  
the swap completed.

[Except that glXWaitForSbcOML itself currently has a bug, for which  
i'll send a patch for xorg master/1.8.1 soon].

Didn't do detailed testing, but maybe this points into the right  
direction?

best,
-mario


*********************************************************************
Mario Kleiner
Max Planck Institute for Biological Cybernetics
Spemannstr. 38
72076 Tuebingen
Germany

e-mail: mario.kleiner at tuebingen.mpg.de
office: +49 (0)7071/601-1623
fax:    +49 (0)7071/601-616
www:    http://www.kyb.tuebingen.mpg.de/~kleinerm
*********************************************************************
"For a successful technology, reality must take precedence
over public relations, for Nature cannot be fooled."
(Richard Feynman)