Hi Ken,
From what I've gathered the proposed patch is incorrect and is (most
likely) working around a buggy application behaviour. Afaics Ian
suggested that we add a driconf option for such applications.
Should I consider this patch for the stable branch or the above sounds
about right and we can drop it ?
Thanks
Emil
On 14/10/14 23:42, Kenneth Graunke wrote:
> According to INTEL_DEBUG=perf, "Borderlands: The Pre-Sequel" was
> stalling on nearly every glBufferSubData call, with very slightly
> overlapping busy ranges.
>
> It turns out the draw upload code was accidentally including an extra
> stride's worth of data in the vertex buffer size due to a simple
> off-by-one error. We considered this extra bit of buffer space to be
> busy (in use by the GPU), when it was actually idle.
>
> The new diagram should make it easier to understand the formula. It's
> basically what I drew on paper when working through an actual
> glDrawRangeElements call.
>
> Eliminates all glBufferSubData stalls in "Borderlands: The Pre-Sequel."
>
> Signed-off-by: Kenneth Graunke
> Cc: mesa-sta...@lists.freedesktop.org
> ---
> src/mesa/drivers/dri/i965/brw_draw_upload.c | 22 +-
> 1 file changed, 21 insertions(+), 1 deletion(-)
>
> No Piglit regressions on Haswell. This might help Dota 2 and Serious Sam 3
> as well, but I haven't checked.
>
> diff --git a/src/mesa/drivers/dri/i965/brw_draw_upload.c
> b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> index 5a12439..6cb653c 100644
> --- a/src/mesa/drivers/dri/i965/brw_draw_upload.c
> +++ b/src/mesa/drivers/dri/i965/brw_draw_upload.c
> @@ -486,8 +486,28 @@ brw_prepare_vertices(struct brw_context *brw)
>offset = 0;
>size = intel_buffer->Base.Size;
> } else {
> + /* Compute the size/amount of data referenced by the GPU.
> + * If the data is interleaved, StrideB may be larger than
> + * _ElementSize. As an example, assume we have 2
> interleaved
> + * attributes A and B. The data is organized like this:
> + *
> + * StrideEltSize
> + *_,,_,
> + * /\ / \
> + *A: --- --- --- --- --- ---
> + *B:--- --- --- --- --- ---
> + *
> + * |= 4 elts ==| (4-1) * Stride + EltSize
> + *
> + * max_index - min_index gives the number of elements that
> + * will be referenced. Say we're drawing 4 elements. On
> + * the first three, we need the full stride in order to get
> + * to the next element. But on the last, we only want the
> + * element size, since we don't actually read the other
> + * interleaved vertex attributes stored beyond that.
> + */
>offset = buffer->offset + min_index * buffer->stride;
> - size = (buffer->stride * (max_index - min_index) +
> + size = (buffer->stride * MAX2(max_index - min_index - 1,
> 0) +
>glarray->_ElementSize);
> }
> }
>
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