nouveau page_flip function implement not wait vblank, which cause screen garbage

[Ben Skeggs]

On Tue, 2011-10-25 at 14:15 +0200, Francisco Jerez wrote:
> Maarten Maathuis <madman2003 at gmail.com> writes:
> 
> > 2011/10/25 chris <wwzbwwzb at 163.com>:
> >> Can anyone give a suggestion, is wait-vblank fully implemented in
> >> page_flip() for nouveau drm driver?
> >>
> 
> It's intentionally not implemented.  The reason is that I wanted to
> support non-vsync'ed vblank as well, and for vsync'ed blits we had to
> think about a different mechanism for vblank synchronization anyway, so
> I figured it didn't make that much sense to force vblank synchronization
> directly from the pageflip ioctl.
+1 I deliberately didn't flip 1 bit in the NV50/NVC0 page flipping code
for this as well.  The interface IMO is flawed.  Though, that said, we
really should look at doing something properly for this, a lot of people
do want tear-free goodness.

Ben.
> 
> >>
> >> At 2011-10-24 14:30:55,chris <wwzbwwzb at 163.com> wrote:
> >>
> >> Dear,
> >>
> >> I use NVidia Geforce 7300GT graphics card in my PC, and Linux 3.1rc4 kernel
> >> code, git drm 2.4.36.
> >>   When I run the vbltest program, it prints  "60HZ"  which indicated the
> >> implementation of drmWaitVBlank() and drm_vblank_wait()  is correct.
> >>   But when I run modetest with option " -v -s 12:1280x1024" , it prints high
> >> fresh rate up to "150 HZ" .  I examing the code , and found that no waiting
> >> vblank operation is processed in nouveau_crtc_ page_flip() function. The
> >> screen  produced lots of garbage and blink very much.
> 
> That's fine if by "garbage" you just mean it's tearing like crazy.
> 
> >>[...]
> >
> > It seems to be, the actual page flipping is done by software method
> > (see nv04_graph_mthd_page_flip). There is one thing i'm unsure about
> > and that is that we wait for the rendering to be done to the current
> > frontbuffer and not the current backbuffer (this is only done if the
> > page flip channel is different than the rendering channel). Maybe
> > someone else can comment on that.
> 
> There's no need to wait for the backbuffer rendering to end because the
> pageflip is always pushed through the last channel that has queued
> rendering to it, so, the "waiting" is actually done by the GPU. The
> waiting to the current frontbuffer (which in most cases is going to be a
> cross-channel barrier instead of actual CPU waiting) is necessary for
> the (rare) case where you have several channels trying to render to the
> same pageflipped drawable, to make sure that the flips are properly
> synchronized with respect each other.
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