compositor-drm: Don't page flip before a mode is set

The function drm_output_start_repaint_loop() unconditionally issues a
page flip, even if the crtc for that output has not been enabled yet.
That causes the page flip to fail, and drm_output_repaint() is never
called.

Solve this by bypassing the initial page flip if the output needs a
mode set.

This has the caveat of affecting latency predictability for that first
frame and when a "driver" mode fullscreen surface causes a mode set.
However, on both cases the mode set would take an unpredictable amount
of time anyway.

https://bugs.freedesktop.org/show_bug.cgi?id=63812
https://bugs.freedesktop.org/show_bug.cgi?id=64183

src/Makefile.am

@@ -133,7 +133,7 @@ if ENABLE_DRM_COMPOSITOR
 drm_backend = drm-backend.la
 drm_backend_la_LDFLAGS = -module -avoid-version
 drm_backend_la_LIBADD = $(COMPOSITOR_LIBS) $(DRM_COMPOSITOR_LIBS) \
-	../shared/libshared.la
+	../shared/libshared.la -lrt
 drm_backend_la_CFLAGS =				\
 	$(COMPOSITOR_CFLAGS)			\
 	$(DRM_COMPOSITOR_CFLAGS)		\

src/compositor-drm.c

@@ -37,6 +37,7 @@
 #include <linux/input.h>
 #include <assert.h>
 #include <sys/mman.h>
+#include <time.h>
 
 #include <xf86drm.h>
 #include <xf86drmMode.h>
@@ -52,6 +53,10 @@
 #include "udev-seat.h"
 #include "launcher-util.h"
 
+#ifndef DRM_CAP_TIMESTAMP_MONOTONIC
+#define DRM_CAP_TIMESTAMP_MONOTONIC 0x6
+#endif
+
 static int option_current_mode = 0;
 static char *output_name;
 static char *output_mode;
@@ -114,6 +119,8 @@ struct drm_compositor {
 	int use_pixman;
 
 	uint32_t prev_state;
+
+	clockid_t clock;
 };
 
 struct drm_mode {
@@ -663,10 +670,19 @@ drm_output_start_repaint_loop(struct weston_output *output_base)
 		output_base->compositor;
 	uint32_t fb_id;
 
-	if (output->current)
-		fb_id = output->current->fb_id;
-	else
-		fb_id = output->original_crtc->buffer_id;
+	struct timespec ts;
+
+	if (!output->current) {
+		/* We can't page flip if there's no mode set */
+		uint32_t msec;
+
+		clock_gettime(compositor->clock, &ts);
+		msec = ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
+		weston_output_finish_frame(output_base, msec);
+		return;
+	}
+
+	fb_id = output->current->fb_id;
 
 	if (drmModePageFlip(compositor->drm.fd, output->crtc_id, fb_id,
 			    DRM_MODE_PAGE_FLIP_EVENT, output) < 0) {
@@ -1184,7 +1200,8 @@ static int
 init_drm(struct drm_compositor *ec, struct udev_device *device)
 {
 	const char *filename, *sysnum;
-	int fd;
+	uint64_t cap;
+	int fd, ret;
 
 	sysnum = udev_device_get_sysnum(device);
 	if (sysnum)
@@ -1207,6 +1224,11 @@ init_drm(struct drm_compositor *ec, struct udev_device *device)
 
 	ec->drm.fd = fd;
 
+	ret = drmGetCap(fd, DRM_CAP_TIMESTAMP_MONOTONIC, &cap);
+	if (ret == 0 && cap == 1)
+		ec->clock = CLOCK_MONOTONIC;
+	else
+		ec->clock = CLOCK_REALTIME;
 
 	return 0;
 }