[Mesa-dev] [PATCH 6/9] mesa: Add a new GetTransformFeedbackVertexCount() driver hook.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On 10/21/2013 05:55 PM, Eric Anholt wrote:
> [snip]
>> This interface means synchronizing with the GPU, which sucks when we
>> have the ability to actually do DTFB in the hardware pipeline (Indirect
>> Parameter Enable of 3DPRIMITIVE).
>
> It's not that simple.
>
> The 3DPRIMITIVE indirect registers require you to specify a vertex count
> (which should be the number of vertices actually written to the SO
> buffer, which may be less than you asked for due to overflow).
>
> As far as I can tell, the Gen7 SOL stage has no mechanism to give you
> the number of vertices written to the SOL buffer.  There is
> SO_NUM_PRIMS_WRITTEN(0-3), which gives you the number of primitives
> actually written.

*headdesk*

OK, so it looks like our hardware is just really not cut out for this
job, and the SW fallback's the way to go.
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