[Mesa-dev] [PATCH] mesa: Avoid format converversions for TexImage with unsized internalFormat

[Chris Wilson]

When selecting a mesa_format to use for a texture, we are constrained by
the user's requested internalFormat. If the requested internalFormat is
generic (e.g. GL_RGBA) it imposes no preferred type or layout and so we
are free to choose any compatible mesa_format. In this case, we can
inspect the incoming pixelformat and see if the hardware natively
supports it, and if so use that for the texture format as then
uploads/downloads are conversion free (implying no loss of fidelity and
should be fastest), utilizing the hardware samplers to do the conversion
and filtering as required (which again should be faster and both higher
and controllable accuracy).

A trivial demonstration using mesa-demos/teximage on the puny byt:

Before:
  GetTexImage(RGB/ubyte 256 x 256): 269.2 images/sec, 50.5 MB/sec
  GetTexImage(RGB/565 256 x 256): 99.5 images/sec, 12.4 MB/sec
  GetTexImage(BGRA/ubyte 256 x 256): 252.0 images/sec, 63.0 MB/sec
  GetTexImage(BGRA/8888r 256 x 256): 229.2 images/sec, 57.3 MB/sec
  GetTexImage(RGBA/float 256 x 256): 1059.5 images/sec, 1059.5 MB/sec

After:
  GetTexImage(RGB/ubyte 256 x 256): 279.6 images/sec, 52.4 MB/sec
  GetTexImage(RGB/565 256 x 256): 6681.9 images/sec, 835.2 MB/sec
  GetTexImage(BGRA/ubyte 256 x 256): 4262.2 images/sec, 1065.6 MB/sec
  GetTexImage(BGRA/8888r 256 x 256): 4262.2 images/sec, 1065.6 MB/sec
  GetTexImage(RGBA/float 256 x 256): 1061.7 images/sec, 1061.7 MB/sec

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ilia Mirkin <imirkin at alum.mit.edu>
Cc: Ian Romanick <ian.d.romanick at intel.com>
---
 src/mesa/main/texformat.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/mesa/main/texformat.c b/src/mesa/main/texformat.c
index 866c7b3..0882763 100644
--- a/src/mesa/main/texformat.c
+++ b/src/mesa/main/texformat.c
@@ -37,6 +37,7 @@
 #include "enums.h"
 #include "mtypes.h"
 #include "texcompress.h"
+#include "teximage.h"
 #include "texformat.h"
 #include "glformats.h"
 
@@ -64,8 +65,26 @@ mesa_format
 _mesa_choose_tex_format(struct gl_context *ctx, GLenum target,
                         GLint internalFormat, GLenum format, GLenum type)
 {
-   (void) format;
+   /* We use the user's pixel format and type as a guide for the texture
+    * format preferrring to store the texels in a matching format to the
+    * incoming pixels. This ideally allows for a conversion free upload
+    * and download path - but only if the requested internalFormat
+    * requires no conversion.
+    */
+   if (_mesa_is_enum_format_unsized(internalFormat)) {
+      /* unsized also implies no preferred internal datatype */
+      mesa_format tex_format = _mesa_format_from_format_and_type(format, type);
+      if (_mesa_format_is_mesa_array_format(tex_format))
+         tex_format = _mesa_format_from_array_format(tex_format);
+      if (_mesa_base_tex_format(ctx, internalFormat) ==
+          _mesa_get_format_base_format(tex_format)) {
+         RETURN_IF_SUPPORTED(tex_format);
+      }
+   }
 
+   /* Could not find an exacting match for a generic internalFormat, pick
+    * from a list of preferred texture formats for each type.
+    */
    switch (internalFormat) {
    /* shallow RGBA formats */
    case 4:
-- 
2.5.1