On 03/13/2014 01:57 AM, Kenneth Graunke wrote:
> This function has three cases:
> 1. Data is in user arrays
> 2. Data is in a buffer object, but misaligned.
> 3. Data is acceptable as is.
>
> Previously, there was a "Turn into a proper VBO" comment above all three
> cases, even though it only applies to case 1, and maybe case 2 (with a
> specific interpretation of "proper".) This patch replaces that with
> more specific comments for each case.
>
> The expanded comments explain where the alignment restrictions come from
> and give an example of how to produce misaligned data in OpenGL.
>
> Signed-off-by: Kenneth Graunke <kenn...@whitecape.org>
> ---
> src/mesa/drivers/dri/i965/brw_draw_upload.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_draw_upload.c
> b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> index f2945c1..98a8277 100644
> --- a/src/mesa/drivers/dri/i965/brw_draw_upload.c
> +++ b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> @@ -838,18 +838,25 @@ static void brw_upload_indices(struct brw_context *brw)
> ib_size = ib_type_size * index_buffer->count;
> bufferobj = index_buffer->obj;
>
> - /* Turn into a proper VBO:
> - */
> if (!_mesa_is_bufferobj(bufferobj)) {
> - /* Get new bufferobj, offset:
> - */
> + /* Copy the index data from user arrays into a buffer object. */
> intel_upload_data(brw, index_buffer->ptr, ib_size, ib_type_size,
> &bo, &offset);
> } else {
> offset = (GLuint) (unsigned long) index_buffer->ptr;
>
> - /* If the index buffer isn't aligned to its element size, we have to
> - * rebase it into a temporary.
> + /* Even though the index data is in a buffer object, it may not meet
> + * the hardware's alignment restrictions. From the
> 3DSTATE_INDEX_BUFFER
> + * documentation, DWord 1:
> + *
> + * "This field contains the size-aligned (as specified by Index Format)
> + * Graphics Address of the first element of interest within the index
> + * buffer."
> + *
> + * OpenGL allows arbitrary byte offsets into the buffer. For example,
> + * glDrawElements with index == 1 and a type of GL_UNSIGNED_SHORT.
> + *
> + * If the data isn't aligned to the element size, copy it to a
> temporary.
Have we actually encountered applications that do that? The GL spec
does say:
"Clients must align data elements consistent with the requirements
of the client platform, with an additional base-level requirement
that an offset within a buffer to a datum comprising N basic
machine units be a multiple of N."
> */
> if ((ib_type_size - 1) & offset) {
> perf_debug("copying index buffer to a temporary to work around "
> @@ -866,6 +873,7 @@ static void brw_upload_indices(struct brw_context *brw)
>
> ctx->Driver.UnmapBuffer(ctx, bufferobj, MAP_INTERNAL);
> } else {
> + /* The index data is already in an acceptable BO. Use as is. */
> bo = intel_bufferobj_buffer(brw, intel_buffer_object(bufferobj),
> offset, ib_size);
> drm_intel_bo_reference(bo);
>
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