[Mesa-dev] [PATCH 2/2] mesa: _mesa_format_convert should be endian agnostic

Oded Gabbay oded.gabbay at gmail.com
Sun Aug 2 03:37:56 PDT 2015

This patch fixes a bug that is manifested in the read path of mesa when
running on big-endian machines. The effects can be seen when running
piglit sanity test and/or taking a screen capture.

The bug is caused when _mesa_format_convert receives src_format as
mesa_format, which it thens changes to mesa_array_format. During this
change, it checks for endianness and swaps the bytes accordingly.
However, because the bytes are _already_ swapped in the memory itself
(being written there by llvmpipe), and src_format value matches the
_actual_ contents of the memory, the result of the read is wrong.

Therefore, because other layers/functions, such as llvm or
read_rgba_pixels() takes care whether we are dealing with big-endian or
little-endian, _mesa_format_convert should be endian agnostic to avoid
duplicate swapping of bytes.

btw, it is also mentioned in the comment of the function that it doesn't
handle byte-swapping, so the original code contradicts the documentation.

Signed-off-by: Oded Gabbay <oded.gabbay at gmail.com>
CC: "10.5 10.6" <mesa-stable at lists.freedesktop.org>
 src/mesa/main/formats.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/mesa/main/formats.c b/src/mesa/main/formats.c
index baeb1bf..7a41cb0 100644
--- a/src/mesa/main/formats.c
+++ b/src/mesa/main/formats.c
@@ -372,10 +372,8 @@ uint32_t
 _mesa_format_to_array_format(mesa_format format)
    const struct gl_format_info *info = _mesa_get_format_info(format);
-   if (_mesa_little_endian())
-      return info->ArrayFormat;
-   else
-      return _mesa_array_format_flip_channels(info->ArrayFormat);
+   return info->ArrayFormat;
 static struct hash_table *format_array_format_table;

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