[Mesa-dev] [PATCH 3/4] dri3: Prefer the last chosen back when finding a new one.

[Eric Anholt]

With the buffer_age code, I need to be able to potentially call this more
than once per frame, and it would be bad if a new special event showing up
meant I chose a different back mid-frame.  Now, once we've chosen a back
for the frame, another find_back will choose it again since we know that
it won't have ->busy set until swap.

Note that this makes find_back return a buffer id instead of a backbuffer
index.  That's kind of a silly distinction anyway, since it's an identity
mapping between the two (it's the front buffer that is at an offset).
---
 src/glx/dri3_glx.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/src/glx/dri3_glx.c b/src/glx/dri3_glx.c
index 72ec8ec..e5862f3 100644
--- a/src/glx/dri3_glx.c
+++ b/src/glx/dri3_glx.c
@@ -1093,13 +1093,13 @@ dri3_find_back(xcb_connection_t *c, struct dri3_drawable *priv)
 
    for (;;) {
       for (b = 0; b < priv->num_back; b++) {
-         int id = DRI3_BACK_ID(b);
+         int id = DRI3_BACK_ID((b + priv->cur_back) % priv->num_back);
          struct dri3_buffer *buffer = priv->buffers[id];
 
-         if (!buffer)
-            return b;
-         if (!buffer->busy)
-            return b;
+         if (!buffer || !buffer->busy) {
+            priv->cur_back = id;
+            return id;
+         }
       }
       xcb_flush(c);
       ev = xcb_wait_for_special_event(c, priv->special_event);
@@ -1126,13 +1126,10 @@ dri3_get_buffer(__DRIdrawable *driDrawable,
    int                  buf_id;
 
    if (buffer_type == dri3_buffer_back) {
-      int back = dri3_find_back(c, priv);
+      buf_id = dri3_find_back(c, priv);
 
-      if (back < 0)
+      if (buf_id < 0)
          return NULL;
-
-      priv->cur_back = back;
-      buf_id = DRI3_BACK_ID(priv->cur_back);
    } else {
       buf_id = DRI3_FRONT_ID;
    }
-- 
1.9.0