vo_gpu: glx: use GLX_OML_sync_control for better vsync reporting

Use the extension to compute the (hopefully correct) video delay and
vsync phase.

This is very fuzzy, because the latency will suddenly be applied after
some frames have already been shown. This means there _will_ be "jumps"
in the time accounting, which can lead to strange effects at start of
playback (such as making initial "dropped" etc. frames worse). The only
reasonable way to fix this would be running a few dummy frame swaps at
start of playback until the latency is known. The same happens when
unpausing.

This only affects display-sync mode.

Correct function was not confirmed. It only "looks right". I don't have
the equipment to make scientifically correct measurements.

A potentially bad thing is that we trust the timestamps we're receiving.
Out of bounds timestamps could wreak havoc. On the other hand, this will
probably cause the higher level code to panic and just disable DS.

As a further caveat, this makes a bunch of assumptions about UST
timestamps. If there are delayed frames (i.e. we skipped one or more
vsyncs), the latency logic is mostly reset. There is no attempt to make
the vo.c skipped vsync logic to use this. Also, the latency computation
determines a vsync duration, and there's no effort to reconcile or share
the vo.c logic for determining vsync duration.

3 files changed, 111 insertions, 0 deletions

diff --git a/video/out/opengl/context.c b/video/out/opengl/context.c
index 43b57aa4ed..44e64b7d9a 100644
--- a/video/out/opengl/context.c
+++ b/video/out/opengl/context.c
@@ -313,9 +313,16 @@ void ra_gl_ctx_swap_buffers(struct ra_swapchain *sw)
     }
 }
 
+static double ra_gl_ctx_get_latency(struct ra_swapchain *sw)
+{
+    struct priv *p = sw->priv;
+    return p->params.get_latency ? p->params.get_latency(sw->ctx) : -1;
+}
+
 static const struct ra_swapchain_fns ra_gl_swapchain_fns = {
     .color_depth   = ra_gl_ctx_color_depth,
     .start_frame   = ra_gl_ctx_start_frame,
     .submit_frame  = ra_gl_ctx_submit_frame,
     .swap_buffers  = ra_gl_ctx_swap_buffers,
+    .get_latency   = ra_gl_ctx_get_latency,
 };
diff --git a/video/out/opengl/context.h b/video/out/opengl/context.h
index 5fccc70033..feaf8e1ab6 100644
--- a/video/out/opengl/context.h
+++ b/video/out/opengl/context.h
@@ -23,6 +23,9 @@ struct ra_gl_ctx_params {
     // function or if you override it yourself.
     void (*swap_buffers)(struct ra_ctx *ctx);
 
+    // See ra_swapchain_fns.get_latency.
+    double (*get_latency)(struct ra_ctx *ctx);
+
     // Set to false if the implementation follows normal GL semantics, which is
     // upside down. Set to true if it does *not*, i.e. if rendering is right
     // side up
diff --git a/video/out/opengl/context_glx.c b/video/out/opengl/context_glx.c
index 462f2cf592..fe210c5f7d 100644
--- a/video/out/opengl/context_glx.c
+++ b/video/out/opengl/context_glx.c
@@ -41,11 +41,20 @@
 #include "context.h"
 #include "utils.h"
 
+// Must be >= max. assumed and supported display latency in frames.
+#define SYNC_SAMPLES 16
+
 struct priv {
     GL gl;
     XVisualInfo *vinfo;
     GLXContext context;
     GLXFBConfig fbc;
+
+    Bool (*XGetSyncValues)(Display*, GLXDrawable, int64_t*, int64_t*, int64_t*);
+    uint64_t ust[SYNC_SAMPLES];
+    uint64_t last_sbc;
+    uint64_t last_msc;
+    double latency;
 };
 
 static void glx_uninit(struct ra_ctx *ctx)
@@ -161,6 +170,13 @@ static bool create_context_x11_gl3(struct ra_ctx *ctx, GL *gl, int gl_version,
 
     mpgl_load_functions(gl, (void *)glXGetProcAddressARB, glxstr, vo->log);
 
+    if (gl_check_extension(glxstr, "GLX_OML_sync_control")) {
+        p->XGetSyncValues =
+            (void *)glXGetProcAddressARB((const GLubyte *)"glXGetSyncValuesOML");
+    }
+    if (p->XGetSyncValues)
+        MP_VERBOSE(vo, "Using GLX_OML_sync_control.\n");
+
     return true;
 }
 
@@ -208,9 +224,91 @@ static void set_glx_attrib(int *attribs, int name, int value)
     }
 }
 
+static double update_latency_oml(struct ra_ctx *ctx)
+{
+    struct priv *p = ctx->priv;
+
+    assert(p->XGetSyncValues);
+
+    p->last_sbc += 1;
+
+    memmove(&p->ust[1], &p->ust[0], (SYNC_SAMPLES - 1) * sizeof(p->ust[0]));
+    p->ust[0] = 0;
+
+    int64_t ust, msc, sbc;
+    if (!p->XGetSyncValues(ctx->vo->x11->display, ctx->vo->x11->window,
+                           &ust, &msc, &sbc))
+        return -1;
+
+    p->ust[0] = ust;
+
+    uint64_t last_msc = p->last_msc;
+    p->last_msc = msc;
+
+    // There was a driver-level discontinuity.
+    if (msc != last_msc + 1)
+        return -1;
+
+    // No frame displayed yet.
+    if (!ust || !sbc || !msc)
+        return -1;
+
+    // We really need to know the time since the vsync happened. There is no way
+    // to get the UST at the time which the frame was queued. So we have to make
+    // assumptions about the UST. The extension spec doesn't define what the UST
+    // is (not even its unit).
+    // Simply assume UST is a simple CLOCK_MONOTONIC usec value. The swap buffer
+    // call happened "some" but very small time ago, so we can get away with
+    // querying the current time. There is also the implicit assumption that
+    // mpv's timer and the UST use the same clock (which it does on POSIX).
+    struct timespec ts;
+    if (clock_gettime(CLOCK_MONOTONIC, &ts))
+        return -1;
+    uint64_t now_monotonic = ts.tv_sec * 1000000LL + ts.tv_nsec / 1000;
+
+    // Actually we need two consecutive displays before we can accurately
+    // measure the latency (because we need to compute vsync_duration).
+    if (!p->ust[1])
+        return -1;
+
+    // Display frame duration.
+    int64_t vsync_duration = p->ust[0] - p->ust[1];
+
+    // Display latency in frames.
+    int64_t n_frames = p->last_sbc - sbc;
+
+    // Too high latency, or other nonsense.
+    if (n_frames < 0 || n_frames >= SYNC_SAMPLES)
+        return -1;
+
+    // Values were not recorded? (Temporary failures etc.)
+    if (!p->ust[n_frames])
+        return -1;
+
+    // Time since last frame display event.
+    int64_t latency_us = now_monotonic - p->ust[n_frames];
+
+    // The frame display event probably happened very recently (about within one
+    // vsync), but the corresponding video frame can be much older.
+    latency_us = (n_frames + 1) * vsync_duration - latency_us;
+
+    return latency_us / (1000.0 * 1000.0);
+}
+
 static void glx_swap_buffers(struct ra_ctx *ctx)
 {
+    struct priv *p = ctx->priv;
+
     glXSwapBuffers(ctx->vo->x11->display, ctx->vo->x11->window);
+
+    if (p->XGetSyncValues)
+        p->latency = update_latency_oml(ctx);
+}
+
+static double glx_get_latency(struct ra_ctx *ctx)
+{
+    struct priv *p = ctx->priv;
+    return p->latency;
 }
 
 static bool glx_init(struct ra_ctx *ctx)
@@ -296,11 +394,14 @@ static bool glx_init(struct ra_ctx *ctx)
 
     struct ra_gl_ctx_params params = {
         .swap_buffers = glx_swap_buffers,
+        .get_latency  = glx_get_latency,
     };
 
     if (!ra_gl_ctx_init(ctx, gl, params))
         goto uninit;
 
+    p->latency = -1;
+
     return true;
 
 uninit: