This was specifically special logic for drm. Before present_sync, it
would also clear out all of its vsync values like this. The old drm code
would save a bunch of samples which would confuse vo.c when unpausing
since it got old, bogus values. Since we make sure to match successive
vsync samples with the swapchain depth and that present sync samples
also match the swapchain depth, this is unneeded.
A ton of code and drm-specific abstractions can be dropped with this.
One important thing to note is that the usage of sbc is completely
dropped here. The utility of that is not particularly clear since the
sbc value was manually incremented before the flip and it's not as if
the drm page flip event gives it to us like it does with the msec and
ust. It can be reintroduced later if there is a need. For drm, we also
add a present_sync_clear_values helper since all presentation feedback
needs to be cleared on pause/resume for it.
When this was originally written, the queuing/list approach was
deliberately removed since it adds more complication and xorg/wayland
don't really use it anyway. In practice, you only really have one frame
in flight with presentation timestamps. However, one slight annoyance is
that the drm code has its own thing which is almost exactly the same and
does its own calculations. Ideally, we'd port drm to this instead, but
the implementation there takes into account N-frames in flight which
probably does actually work. So we need to make present_sync smarter and
be able to handle this.
mpv does actually have its own linked list implementation already which
is a good fit for this. mp_present becomes the list and each
mp_present_entry has its own set of timestamps. During initialization,
we create all the entries we need and then simply treat it like a queue
during the lifecycle of the VO. When an entry is fully used
(present_sync_get_info), then we remove it from the list, zero it out,
and append it to the end for future use. This avoids needing to allocate
memory on every frame (which is what drm currently does) and allows for
a reasonable number of in flight frames at the same time as this should
never grow to some obscene number. The nice thing is that current users
of present_sync don't need to change anything besides the initialization
step.
Wayland had some specific code that it used for implementing the
presentation time protocol. It turns out that xorg's present extension
is extremely similar, so it would be silly to duplicate this whole mess
again. Factor this out to separate, independent code and introduce the
mp_present struct which is used for handling the ust/msc values and some
other associated values. Also, add in some helper functions so all the
dirty details live specifically in present_sync. The only
wayland-specific part is actually obtaining ust/msc values. Since only
wayland or xorg are expected to use this, add a conditional to the build
that only adds this file when either one of those are present.
You may observe that sbc is completely omitted. This field existed in
wayland, but was completely unused (presentation time doesn't return
this). Xorg's present extension also doesn't use this so just get rid of
it all together. The actual calculation is slightly altered so it is
correct for our purposes. We want to get the presentation event of the
last frame that was just occured (this function executes right after the
buffer swap). The adjustment is to just remove the vsync_duration
subtraction. Also, The overly-complicated queue approach is removed.
This has no actual use in practice (on wayland or xorg). Presentation
statistics are only ever used after the immediate preceding swap to
update vsync timings or thrown away.
