compositor-drm: Introduce fb_last member

Clean up some ambiguity around current/next: current could previously
have referred to a buffer which was being displayed, or the last buffer
being displayed whilst we waited for a configuration we'd requested to
take effect.

Introduce a new variable, fb_last, which exclusively holds the latter
case, thus leaving us with three unambiguous members: fb_pending is used
as a scratch space for a buffer we're about to post, fb_current holds
the buffer we last requested to display (whether active yet or not), and
fb_last is again scratch space for a buffer which is no longer being
displayed after a previous configuration reqeust.

Signed-off-by: Daniel Stone <daniels@collabora.com>

Differential Revision: https://phabricator.freedesktop.org/D1411

1 files changed, 30 insertions, 12 deletions

diff --git a/libweston/compositor-drm.c b/libweston/compositor-drm.c
index 3cbb1513..825e0afe 100644
--- a/libweston/compositor-drm.c
+++ b/libweston/compositor-drm.c
@@ -189,7 +189,7 @@ struct drm_output {
 	int current_cursor;
 
 	struct weston_plane fb_plane;
-	struct drm_fb *fb_current, *fb_pending;
+	struct drm_fb *fb_current, *fb_pending, *fb_last;
 	struct backlight *backlight;
 
 	struct drm_fb *dumb[2];
@@ -210,7 +210,7 @@ struct drm_sprite {
 
 	struct weston_plane plane;
 
-	struct drm_fb *fb_current, *fb_pending;
+	struct drm_fb *fb_current, *fb_pending, *fb_last;
 	struct drm_output *output;
 	struct drm_backend *backend;
 
@@ -755,6 +755,8 @@ drm_output_repaint(struct weston_output *output_base,
 	if (output->disable_pending || output->destroy_pending)
 		return -1;
 
+	assert(!output->fb_last);
+
 	drm_output_render(output, damage);
 	if (!output->fb_pending)
 		return -1;
@@ -780,6 +782,10 @@ drm_output_repaint(struct weston_output *output_base,
 		goto err_pageflip;
 	}
 
+	output->fb_last = output->fb_current;
+	output->fb_current = output->fb_pending;
+	output->fb_pending = NULL;
+
 	output->page_flip_pending = 1;
 
 	drm_output_set_cursor(output);
@@ -794,6 +800,8 @@ drm_output_repaint(struct weston_output *output_base,
 			.request.sequence = 1,
 		};
 
+		/* XXX: Set output much earlier, so we don't attempt to place
+		 *      planes on entirely the wrong output. */
 		if ((!s->fb_current && !s->fb_pending) ||
 		    !drm_sprite_crtc_supported(output, s))
 			continue;
@@ -825,6 +833,9 @@ drm_output_repaint(struct weston_output *output_base,
 		}
 
 		s->output = output;
+		s->fb_last = s->fb_current;
+		s->fb_current = s->fb_pending;
+		s->fb_pending = NULL;
 		output->vblank_pending = 1;
 	}
 
@@ -934,9 +945,9 @@ vblank_handler(int fd, unsigned int frame, unsigned int sec, unsigned int usec,
 	drm_output_update_msc(output, frame);
 	output->vblank_pending = 0;
 
-	drm_fb_unref(s->fb_current);
-	s->fb_current = s->fb_pending;
-	s->fb_pending = NULL;
+	assert(s->fb_last || s->fb_current);
+	drm_fb_unref(s->fb_last);
+	s->fb_last = NULL;
 
 	if (!output->page_flip_pending) {
 		ts.tv_sec = sec;
@@ -964,9 +975,8 @@ page_flip_handler(int fd, unsigned int frame,
 	 * we just want to page flip to the current buffer to get an accurate
 	 * timestamp */
 	if (output->page_flip_pending) {
-		drm_fb_unref(output->fb_current);
-		output->fb_current = output->fb_pending;
-		output->fb_pending = NULL;
+		drm_fb_unref(output->fb_last);
+		output->fb_last = NULL;
 	}
 
 	output->page_flip_pending = 0;
@@ -1496,10 +1506,16 @@ drm_output_switch_mode(struct weston_output *output_base, struct weston_mode *mo
 	output->base.current_mode->flags =
 		WL_OUTPUT_MODE_CURRENT | WL_OUTPUT_MODE_PREFERRED;
 
-	/* reset rendering stuff. */
+	/* XXX: This drops our current buffer too early, before we've started
+	 *      displaying it. Ideally this should be much more atomic and
+	 *      integrated with a full repaint cycle, rather than doing a
+	 *      sledgehammer modeswitch first, and only later showing new
+	 *      content.
+	 */
 	drm_fb_unref(output->fb_current);
-	drm_fb_unref(output->fb_pending);
-	output->fb_current = output->fb_pending = NULL;
+	assert(!output->fb_last);
+	assert(!output->fb_pending);
+	output->fb_last = output->fb_current = NULL;
 
 	if (b->use_pixman) {
 		drm_output_fini_pixman(output);
@@ -2718,6 +2734,7 @@ create_sprites(struct drm_backend *b)
 
 		sprite->possible_crtcs = plane->possible_crtcs;
 		sprite->plane_id = plane->plane_id;
+		sprite->fb_last = NULL;
 		sprite->fb_current = NULL;
 		sprite->fb_pending = NULL;
 		sprite->backend = b;
@@ -2749,8 +2766,9 @@ destroy_sprites(struct drm_backend *backend)
 				sprite->plane_id,
 				output->crtc_id, 0, 0,
 				0, 0, 0, 0, 0, 0, 0, 0);
+		drm_fb_unref(sprite->fb_last);
 		drm_fb_unref(sprite->fb_current);
-		drm_fb_unref(sprite->fb_pending);
+		assert(!sprite->fb_pending);
 		weston_plane_release(&sprite->plane);
 		free(sprite);
 	}