exa/mixed: Exclude frontbuffer from deferred pixmap handling.

- Apps like xterm can trigger a lot of fallback rendering.
- This can lead to (annoyingly) high latencies, because you
  have to wait for the block handler.
- You need a driver that doesn't directly access the front
  buffer to trigger this (NV50+ nouveau for example).
- Repeatingly doing dmesg on an xterm with a bitmap font
  will reveal that you never see part of the text.
- I have recieved at least one complaint in the past of slow
  terminal performance, which was related to core font
  rendering.
- This does sacrifice some throughput, roughly 33% slower.

Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Maarten Maathuis <madman2003@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/exa/exa_migration_mixed.c b/exa/exa_migration_mixed.c
index fb4715135..4f49905d3 100644
--- a/exa/exa_migration_mixed.c
+++ b/exa/exa_migration_mixed.c
@@ -138,6 +138,7 @@ void
 exaDamageReport_mixed(DamagePtr pDamage, RegionPtr pRegion, void *closure)
 {
     PixmapPtr pPixmap = closure;
+    ScreenPtr pScreen = pPixmap->drawable.pScreen;
     ExaPixmapPriv(pPixmap);
 
     /* Move back results of software rendering on system memory copy of mixed driver
@@ -149,10 +150,18 @@ exaDamageReport_mixed(DamagePtr pDamage, RegionPtr pRegion, void *closure)
     if (!pExaPixmap->use_gpu_copy && exaPixmapHasGpuCopy(pPixmap)) {
 	ExaScreenPriv(pPixmap->drawable.pScreen);
 
-	if (pExaScr->deferred_mixed_pixmap &&
-	    pExaScr->deferred_mixed_pixmap != pPixmap)
-	    exaMoveInPixmap_mixed(pExaScr->deferred_mixed_pixmap);
-	pExaScr->deferred_mixed_pixmap = pPixmap;
+	/* Front buffer: Don't wait for the block handler to copy back the data.
+	 * This avoids annoying latency if you encounter a lot of software rendering.
+	 */
+	if (pPixmap == pScreen->GetScreenPixmap(pScreen))
+		exaMoveInPixmap_mixed(pPixmap);
+	else {
+		if (pExaScr->deferred_mixed_pixmap &&
+		    pExaScr->deferred_mixed_pixmap != pPixmap)
+		    exaMoveInPixmap_mixed(pExaScr->deferred_mixed_pixmap);
+
+		pExaScr->deferred_mixed_pixmap = pPixmap;
+	}
     }
 }