vo_null: add framerate emulation

DOCS/man/vo.rst

@@ -824,6 +824,12 @@ Available video output drivers are:
 ``null``
     Produces no video output. Useful for benchmarking.
 
+    Usually, it's better to disable video with ``--no-video`` instead.
+
+    ``fps=<value>``
+        Simulate display FPS. This artificially limits how many frames the
+        VO accepts per second.
+
 ``caca``
     Color ASCII art video output driver that works on a text console.
 

video/out/vo_null.c

@@ -19,14 +19,18 @@
  * with mpv.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
-#include <errno.h>
-#include "config.h"
 #include "common/msg.h"
 #include "vo.h"
 #include "video/mp_image.h"
+#include "osdep/timer.h"
+#include "options/m_option.h"
+
+struct priv {
+    int64_t last_vsync;
+
+    double cfg_fps;
+};
 
 static void draw_image(struct vo *vo, mp_image_t *mpi)
 {
@@ -35,6 +39,18 @@ static void draw_image(struct vo *vo, mp_image_t *mpi)
 
 static void flip_page(struct vo *vo)
 {
+    struct priv *p = vo->priv;
+    if (p->cfg_fps) {
+        int64_t ft = 1e6 / p->cfg_fps;
+        int64_t prev_vsync = mp_time_us() / ft;
+        int64_t target_time = (prev_vsync + 1) * ft;
+        for (;;) {
+            int64_t now = mp_time_us();
+            if (now >= target_time)
+                break;
+            mp_sleep_us(target_time - now);
+        }
+    }
 }
 
 static int query_format(struct vo *vo, int format)
@@ -58,9 +74,18 @@ static int preinit(struct vo *vo)
 
 static int control(struct vo *vo, uint32_t request, void *data)
 {
+    struct priv *p = vo->priv;
+    switch (request) {
+    case VOCTRL_GET_DISPLAY_FPS:
+        if (!p->cfg_fps)
+            break;
+        *(double *)data = p->cfg_fps;
+        return VO_TRUE;
+    }
     return VO_NOTIMPL;
 }
 
+#define OPT_BASE_STRUCT struct priv
 const struct vo_driver video_out_null = {
     .description = "Null video output",
     .name = "null",
@@ -71,4 +96,9 @@ const struct vo_driver video_out_null = {
     .draw_image = draw_image,
     .flip_page = flip_page,
     .uninit = uninit,
+    .priv_size = sizeof(struct priv),
+    .options = (const struct m_option[]) {
+        OPT_DOUBLE("fps", cfg_fps, M_OPT_RANGE, .min = 0, .max = 10000),
+        {0},
+    },
 };