glamor: Add a note about the state of GL_ARB_map_buffer_range.

GLES2 Xephyr is failing due to lack of glMapBuffer() with the read
bits set, and I decided to see if we can just switch everything to
glMapBufferRange().  I'm undecided, and it largely depends on whether
we find people are interested in using glamor for the windows X server.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Markus Wick <markus at selfnet.de>

1 files changed, 13 insertions, 0 deletions

diff --git a/glamor/glamor.c b/glamor/glamor.c
index fa753bb1a..fe9f76179 100644
--- a/glamor/glamor.c
+++ b/glamor/glamor.c
@@ -319,6 +319,19 @@ glamor_init(ScreenPtr screen, unsigned int flags)
 
     gl_version = glamor_gl_get_version();
 
+    /* We'd like to require GL_ARB_map_buffer_range or
+     * GL_OES_map_buffer_range, since it offers more information to
+     * the driver than plain old glMapBuffer() or glBufferSubData().
+     * It's been supported on Mesa on the desktop since 2009 and on
+     * GLES2 since October 2012.  It's supported on Apple's iOS
+     * drivers for SGX535 and A7, but apparently not on most Android
+     * devices (the OES extension spec wasn't released until June
+     * 2012).
+     *
+     * 82% of 0 A.D. players (desktop GL) submitting hardware reports
+     * have support for it, with most of the ones lacking it being on
+     * Windows with Intel 4-series (G45) graphics or older.
+     */
     if (glamor_priv->gl_flavor == GLAMOR_GL_DESKTOP) {
         if (gl_version < GLAMOR_GL_VERSION_ENCODE(1, 3)) {
             ErrorF("Require OpenGL version 1.3 or later.\n");