[Mesa-dev] What use do swap interval > 1 and OML_sync_control divisor and remainder have?

[Pekka Paalanen]

Hi,

I am investigating what kind of Wayland protocol extensions would be
needed to support proper presentation timing. Looking at existing
works, I am wondering about two things whether they have any real use.

Where is swap interval (e.g. eglSwapInterval) greater than one useful?
Intervals 0 and 1 I understand, and Mesa EGL Wayland already supports
those. But when would you like to make your framerate a fraction of the
display's?

When are the target-MSC related remainder and divisor parameters as
defined in the GLX_OML_sync_control useful? Why does also X11 Present
protocol include remainder and divisor?

GLX_OML_sync_control defines that for interlaced displays MSC is
incremented for each field. With divisor and remainder you could then
target only top or bottom fields. Is that useful, and do we care about
interlaced displays anymore?

I am contemplating on not supporting these, because I am going to
propose using an UST-like clock as the "standard clock language" in
Wayland present extension. Supporting MSC-based timings would add
complexity. Therefore I would like to know where and how the above
mentioned are useful, because I cannot imagine it myself.

Please, let me know of real actual use cases and existing software,
where these features offer a visible benefit and what that benefit is
exactly. I am not interested in what one might do in theory, I am
interested in real-world examples where they have proved useful. Well,
maybe also theories if they allow taking advantage of some new cool
technology.

Btw. if you think that using UST for presentation timing and feedback
is nonsense, and MSC is the only right way, let me know and I can start
another email thread about that detail after preparing my material.


Thanks,
pq