Issues adding driver support for present extension

[Keith Packard]

Frank Binns <frank.binns at imgtec.com> writes:

> Hi,
> I've been working on adding support for the present extension in the PVR 
> video driver and I've hit a couple of issues related to flipping:

Awesome. As you may know, this has only been implemented in the Intel
driver and hasn't been accepted upstream yet. The server API closely
matches the internals of that driver, and while I spent a week trying to
rewrite that this spring, I didn't succeed. I'd love to clean it up and
make it actually usable.

> 1) When the display driver supports async flipping this mode of 
> operation is always used. This means that, when the swap interval is 
> none 0, the present extension relies on vblank events to get 
> synchronised flips. However, by the time we get the event, wait for any 
> outstanding rendering and do the flip we have probably missed the vblank 
> period meaning we get screen artefacts.

Right, this is fast enough on my laptop GPU to work. I wonder why you're
waiting for the GPU to flush all of the rendering though? You should
only need to flush rendering targeting the new back buffer and then flip
the scanout engine to that.

> Is there a good reason (other 
> than simplifying the code) why present isn't relying on the default 
> behaviour of drmModePageFlip() to get synchronised flips?

Yes, there's a really good reason -- the drm interface doesn't provide
for any way to replace the pending buffer. Once you've queued it, you're
stuck until that buffer becomes active, tying down *two* buffers in the
kernel. And, you don't get the benefit of providing a new buffer after
the swap is first queued.

> 2) When the display driver doesn't support async flipping and the swap 
> interval is 0 we see swaps alternating between flipping and blitting. 
> This results in a back buffer continually being created and destroyed on 
> the client side, which is obviously highly undesirable.

That's a bug. I know I've fixed similar things several times; which X
server version are you running?

> My understanding is that, in the async case, a vblank event gets 
> requested for the next vblank. Any swaps that come in before this 
> event's received never get presented to the screen. In addition to this, 
> it's fairly unlikely that we catch things in the middle of a flip, i.e. 
> we've called drmModePageFlip() but not got back the flip event, meaning 
> that any queued vblank events, i.e. queued swaps, are going to be 
> completed via a flip. However, in the non-async case we're very likely 
> to catch things in the middle of a flip as a flip can only be completed 
> during a vblank period. The result is this ping-ponging between flipping 
> and blitting.

>
> My current thoughts are that two things probably need to be done to fix 
> this:
> a) If present gets a swap for the same MSC as a pending flip then it 
> should never try to do the presentation and send back 
> PresentCompleteModeSkip in the reply.

No, this is wrong -- you want to *replace* the old presentation with the
new one. Leaving the old one in place is not what the application wants
at all.

> b) If present gets a swap for an MSC in the future then it shouldn't be 
> prevented from completing the swap via a flip just because there is 
> currently a pending flip.

Agreed. I think this is fixed in the current code.

> Does this sound reasonable, is there a better way of fixing this or am I 
> missing something?

As I said, the current driver API was designed with the Intel driver in
mind, and using the current DRM kernel interface's limitations. It may
be that the current DRM kernel API just isn't suitable for async
updates.

-- 
keith.packard at intel.com
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