On 12.04.2018 01:30, Cyr, Aric wrote:
At least with VDPAU, video players are already explicitly specifying the
target presentation time, so no changes should be required at that
level. Don't know about other video APIs.

The X11 Present extension protocol is also prepared for specifying the
target presentation time already, the support for it just needs to be
implemented.

I'm perfectly OK with presentation time-based *API*.  I get it from a user 
mode/app perspective, and that's fine.  We need that feedback and would like 
help defining that portions of the stack.
However, I think it doesn't make as much sense as a *DDI* because it doesn't 
correspond to any hardware real or logical (i.e. no one would implement it in 
HW this way) and the industry specs aren't defined that way.
You can have libdrm or some other usermode component translate your presentation time 
into a frame duration and schedule it.  What's the advantage of having this in kernel 
besides the fact we lose the intent of the application and could prevent features and 
optimizations.  When it gets to kernel, I think it is much more elegant for the flip 
structure to contain a simple duration that says "hey, show this frame on the screen 
for this long".  Then we don't need any clocks or timers just some simple math and 
program the hardware.

There isn't necessarily an inherent advantage to having this translation in the kernel. However, we *must* do this translation in a place that is owned by display experts (i.e., you guys), because only you guys know how to actually do that translation reliably and correctly.

Since your work is currently limited to the kernel, it makes sense to do it in the kernel.

If the translation doesn't happen in a place that you feel comfortable working on, we're setting ourselves up for a future where this hypothetical future UMD component will get this wrong, and there'll be a lot of finger-pointing between you guys and whoever writes that UMD, with likely little willingness to actually go into the respective other codebase to fix what's wrong. And that's a pretty sucky future.

Cheers,
Nicolai

P.S.: I'm also a little surprised that you seem to be saying that requesting a target present time is basically impossible (at least, that's kind of implied by your statement about mGPUs), and yet there's precedent for such APIs in both Vulkan and VDPAU.



In short,
  1) We can simplify media players' lives by helping them get really, really 
close to their content rate, so they wouldn't need any frame rate conversion.
      They'll still need A/V syncing though, and variable refresh cannot solve 
this and thus is way out of scope of what we're proposing.

  2) For gaming, don't even try to guess a frame duration, the driver/hardware 
will do a better job every time, just specify duration=0 and flip as fast as 
you can.

Regards,
   Aric

P.S. Thanks for the Croteam link.  Interesting, but basically nullified by 
variable refresh rate displays.  You won't have 
stuttering/microstuttering/juddering/tearing if your display's refresh rate 
matches the render/present rate of the game.  Maybe I should grab The Talos 
Principle to see how well it works with FreeSync display :)

--
ARIC CYR
PMTS Software Engineer | SW – Display Technologies





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