til_fb: return queue delay from til_fb_page_get()

til_fb is triple buffered, but when rendering lags behind the
queue is empty making present-to-delay intervals smaller than
when rendering is keeping up and the queue is kept full.

This returns the duration of the returned pages last
submit-to-present delay, effectively measuring how long a page
can expect to wait to be flipped to/presented currently.

A subsequent commit will apply this delay value to the ticks
supplied to rendering.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/til_fb.c b/src/til_fb.c
index 02259e8..8760b84 100644
--- a/src/til_fb.c
+++ b/src/til_fb.c
@@ -420,7 +420,7 @@ static inline _til_fb_page_t * _til_fb_page_get(til_fb_t *fb)
 
 
 /* public interface */
-til_fb_fragment_t * til_fb_page_get(til_fb_t *fb)
+til_fb_fragment_t * til_fb_page_get(til_fb_t *fb, unsigned *res_delay_ticks)
 {
 	_til_fb_page_t	*page;
 
@@ -428,6 +428,12 @@ til_fb_fragment_t * til_fb_page_get(til_fb_t *fb)
 	if (!page)
 		return NULL;
 
+	if (res_delay_ticks) {
+		/* TODO: handle overflows, just asserting for now until it rears its head */
+		assert(page->presented_ticks >= page->submitted_ticks);
+		*res_delay_ticks = page->presented_ticks - page->submitted_ticks;
+	}
+
 	return &page->fragment.public;
 }