On Fri, 2 Nov 2012 14:24:12 -0400, Kristian H??gsberg <krh at bitplanet.net> wrote: > On Fri, Nov 2, 2012 at 9:29 AM, Chris Wilson <chris at chris-wilson.co.uk> wrote: > > On Thu, 1 Nov 2012 20:06:03 +0200, ville.syrjala at linux.intel.com wrote: > >> From: Ville Syrj??l?? <ville.syrjala at linux.intel.com> > >> > >> As per Chris Wilson's suggestion make > >> i915_gem_execbuffer_wait_for_flips() go away. > >> > >> This was used to stall the GPU ring while there are pending > >> page flips involving the relevant BO. Ie. while the BO is still > >> being scanned out by the display controller. > >> > >> The recommended alternative is to use the page flip events to > >> wait for the page flips to fully complete before reusing the BO > >> of the old front buffer. Or use more buffers. > >> > >> Signed-off-by: Ville Syrj??l?? <ville.syrjala at linux.intel.com> > > Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk> > > > > Needs an ack from either Jesse or Kristian. > > I wouldn't mind seeing this go; we don't rely on this behavior in > Weston, we use the events. However, this is a user visible change in > behavior of the pageflip ioctl. I don't remember if X needs this for > the case where it's pageflipping to fullscreen client buffers... I > think it might (or at least might have), since we did it this way so > that the client wouldn't have to wait for a protocol event before > starting the next frame. With this the client can start rendering and > submit the first batch before it has to block. It also minimizes the > time from pageflip done to the client can start rendering, since the > kernel now just unblocks the client directly. Of course you can argue > that we can fix that with more buffers, and maybe it's fixed in newer > servers / ddx drivers, but it doesn't change that this is how it used > to work. Without this, the client doesn't know when it's new > backbuffer done scanning out. We never relied on this in userspace, as you provided the flip-completed event at the same time as you introduced pageflipping and that was used in the original ddx to throttle the client. And from experience, I can say that this blocking is not sufficient to prevent userspace from being stupid... -Chris -- Chris Wilson, Intel Open Source Technology Centre
