[Mesa-dev] [mesa 9/9] glx/dri2: Implement getBufferAge

[Chris Wilson]

Within the DRI2GetBuffers return packet there is a 4-byte field that is
currently unused by any driver, i.e. flags. With the co-operation of a
suitably modified X server, we can pass the last SBC on which the buffer
was defined (i.e. the last SwapBuffers for which it was used) and 0 if
it is fresh (with a slight loss of precision). We can then compare the
flags field of the DRIBuffer against the current swap buffers count and
so compute the age of the back buffer (thus satisfying
GLX_EXT_buffer_age).

As we reuse a driver specific field within the DRI2GetBuffers packet, we
first query whether the X/DDX are ready to supply the new value using a
DRI2GetParam request.

Another caveat is that we need to complete the SwapBuffers/GetBuffers
roundtrip before reporting the back buffer age so that it tallies
against the buffer used for rendering. As with all things X, there is a
race between the query and the rendering where the buffer may be
invalidated by the server. However, for the primary usecase (that of a
compositing manager), the DRI2Drawable is only accessible to a single
client mitigating the impact of the issue.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 configure.ac       |  2 +-
 src/glx/dri2_glx.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 870435c..ca1da86 100644
--- a/configure.ac
+++ b/configure.ac
@@ -65,7 +65,7 @@ LIBDRM_INTEL_REQUIRED=2.4.52
 LIBDRM_NVVIEUX_REQUIRED=2.4.33
 LIBDRM_NOUVEAU_REQUIRED="2.4.33 libdrm >= 2.4.41"
 LIBDRM_FREEDRENO_REQUIRED=2.4.57
-DRI2PROTO_REQUIRED=2.6
+DRI2PROTO_REQUIRED=2.9
 DRI3PROTO_REQUIRED=1.0
 PRESENTPROTO_REQUIRED=1.0
 LIBUDEV_REQUIRED=151
diff --git a/src/glx/dri2_glx.c b/src/glx/dri2_glx.c
index 0577804..b43f115 100644
--- a/src/glx/dri2_glx.c
+++ b/src/glx/dri2_glx.c
@@ -917,6 +917,67 @@ dri2GetBuffersWithFormat(__DRIdrawable * driDrawable,
 }
 
 static int
+dri2HasBufferAge(int screen, struct glx_display * priv)
+{
+   const struct dri2_display *const pdp =
+	   (struct dri2_display *)priv->dri2Display;
+   CARD64 value;
+
+   if (pdp->driMajor <= 1 && pdp->driMinor < 4)
+	   return 0;
+
+   value = 0;
+   if (!DRI2GetParam(priv->dpy, RootWindow(priv->dpy, screen),
+                     DRI2ParamXHasBufferAge, &value))
+      return 0;
+
+   return value;
+}
+
+static int
+dri2GetBufferAge(__GLXDRIdrawable *pdraw)
+{
+   struct dri2_drawable *priv = (struct dri2_drawable *) pdraw;
+   int i, age = 0;
+
+   if (priv->swap_pending) {
+	   unsigned int attachments[5];
+	   DRI2Buffer *buffers;
+
+	   for (i = 0; i < priv->bufferCount; i++)
+		   attachments[i] = priv->buffers[i].attachment;
+
+	   buffers = DRI2GetBuffers(priv->base.psc->dpy, priv->base.xDrawable,
+				    &priv->width, &priv->height,
+				    attachments, i, &i);
+	   if (buffers == NULL)
+		   return 0;
+
+	   process_buffers(priv, buffers, i);
+	   free(buffers);
+
+	   dri2XcbSwapBuffersComplete(priv);
+   }
+
+   if (!priv->have_back)
+      return 0;
+
+   for (i = 0; i < priv->bufferCount; i++) {
+	   if (priv->buffers[i].attachment != __DRI_BUFFER_BACK_LEFT)
+		   continue;
+
+	   if (priv->buffers[i].flags == 0)
+		   continue;
+
+	   age = priv->last_swap_sbc - priv->buffers[i].flags + 1;
+	   if (age < 0)
+		   age = 0;
+   }
+
+   return age;
+}
+
+static int
 dri2SetSwapInterval(__GLXDRIdrawable *pdraw, int interval)
 {
    xcb_connection_t *c = XGetXCBConnection(pdraw->psc->dpy);
@@ -1290,6 +1351,10 @@ dri2CreateScreen(int screen, struct glx_display * priv)
    psp->setSwapInterval = NULL;
    psp->getSwapInterval = NULL;
    psp->getBufferAge = NULL;
+   if (dri2HasBufferAge(screen, priv)) {
+	   psp->getBufferAge = dri2GetBufferAge;
+	   __glXEnableDirectExtension(&psc->base, "GLX_EXT_buffer_age");
+   }
 
    if (pdp->driMinor >= 2) {
       psp->getDrawableMSC = dri2DrawableGetMSC;
-- 
2.1.4