Use GL_PACK_ALIGNMENT of 1 whenever possible

The Intel driver currently has an optimisation when calling
glReadPixels into a PBO so that it will use a blit instead of the Mesa
fallback path. However this only works if the GL_PACK_ALIGNMENT is
exactly 1, even if this would be equivalent to a higher alignment
value because the bpp*width is already aligned. To make it more likely
to hit this fast path, we now detect this situation and explicitly use
an alignment of 1. To make this work the texture driver needs to be
passed down the bpp*width as well as the rowstride when configuring
the alignment.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/cogl/cogl-texture.c b/cogl/cogl-texture.c
index 26173451..29489838 100644
--- a/cogl/cogl-texture.c
+++ b/cogl/cogl-texture.c
@@ -274,12 +274,29 @@ _cogl_texture_prep_gl_alignment_for_pixels_upload (int pixels_rowstride)
 }
 
 void
-_cogl_texture_prep_gl_alignment_for_pixels_download (int pixels_rowstride)
+_cogl_texture_prep_gl_alignment_for_pixels_download (int bpp,
+                                                     int width,
+                                                     int rowstride)
 {
+  int alignment;
+
   _COGL_GET_CONTEXT (ctx, NO_RETVAL);
 
-  GE( ctx, glPixelStorei (GL_PACK_ALIGNMENT,
-                          calculate_alignment (pixels_rowstride)) );
+  /* If no padding is needed then we can always use an alignment of 1.
+   * We want to do this even though it is equivalent to the alignment
+   * of the rowstride because the Intel driver in Mesa currently has
+   * an optimisation when reading data into a PBO that only works if
+   * the alignment is exactly 1.
+   *
+   * https://bugs.freedesktop.org/show_bug.cgi?id=46632
+   */
+
+  if (rowstride == bpp * width)
+    alignment = 1;
+  else
+    alignment = calculate_alignment (rowstride);
+
+  GE( ctx, glPixelStorei (GL_PACK_ALIGNMENT, alignment) );
 }
 
 /* FIXME: wrap modes should be set on pipelines not textures */