Reviewed-by: Jason Ekstrand <ja...@jlekstrand.net> On Thu, May 26, 2016 at 12:31 AM, Kenneth Graunke <kenn...@whitecape.org> wrote:
> commit 7c8dfa78b98a12c1c5 (i965/draw: Use the real size for vertex > buffers) changed how we programmed the VERTEX_BUFFER_STATE size field. > > Previously, we programmed it to the size of the actual underlying BO, > which is page-aligned, and potentially much larger than the GL buffer > object. This violated the ARB_robust_buffer_access spec. > > With that change, we started programming it based on the range of data > we expect the draw call to actually access - which is based on the > min_index and max_index information provided to glDrawRangeElements(). > > Unfortunately, applications often provide inaccurate range information > to glDrawRangeElements(). For example, all the Unreal demos appear to > draw using a range of [0, 3] when the index buffer's actual index range > is [0, 5]. Such results are undefined, and we are absolutely allowed > to restrict access to the range they specified. However, the failure > mode is usually that nothing draws, or misrendering with wild geometry, > which is kind of bad for a common mistake. And people tend to assume > the range information isn't that important when data is in VBOs. > > There's no real advantage, either. ARB_robust_buffer_access only > requires us to restrict access to the GL buffer object size, not > the range of data we think they should access. Doing that allows > buggy applications to still function. (Note that we still use this > information for busy-tracking, so if they try to overwrite the data > with glBufferSubData, they'll still hit a bug.) This seems to be > safer. > > We may want to provide the more strict range as a debug option, > or scan the VBO and warn against bogus glDrawRangeElements in > debug contexts. That can be done as a later patch, though. > > Makes Unreal demos draw again. > > Signed-off-by: Kenneth Graunke <kenn...@whitecape.org> > --- > src/mesa/drivers/dri/i965/brw_draw_upload.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/src/mesa/drivers/dri/i965/brw_draw_upload.c > b/src/mesa/drivers/dri/i965/brw_draw_upload.c > index f4d1b2c..bfe20c5 100644 > --- a/src/mesa/drivers/dri/i965/brw_draw_upload.c > +++ b/src/mesa/drivers/dri/i965/brw_draw_upload.c > @@ -543,6 +543,7 @@ brw_prepare_vertices(struct brw_context *brw) > buffer->offset = offset; > buffer->stride = glarray->StrideB; > buffer->step_rate = glarray->InstanceDivisor; > + buffer->size = glarray->BufferObj->Size - offset; > > enabled_buffer[j] = intel_buffer; > buffer_range_start[j] = start; > @@ -610,8 +611,6 @@ brw_prepare_vertices(struct brw_context *brw) > > buffer->bo = intel_bufferobj_buffer(brw, enabled_buffer[i], start, > range); > drm_intel_bo_reference(buffer->bo); > - > - buffer->size = start + range; > } > > /* If we need to upload all the arrays, then we can trim those arrays > to > -- > 2.8.2 > >
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