[Mesa-dev] [PATCH 10/18] i965: Speculatively flush the batch after transform feedback

[Kristian Høgsberg]

On Mon, Jul 6, 2015 at 12:36 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Monday, July 06, 2015 11:33:15 AM Chris Wilson wrote:
>> Since the purpose of transform feedback tends to be for the client to
>> act upon the results to change the geometry in the scene, it is likely
>> that the client will soon be waiting upon the results. Flush the batch
>> early so that we don't build up a long queue of commands afterwards that
>> could delay the readback.
>> ---
>>  src/mesa/drivers/dri/i965/gen7_sol_state.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/src/mesa/drivers/dri/i965/gen7_sol_state.c b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> index 857ebe5..13dbe5b 100644
>> --- a/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> +++ b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> @@ -494,6 +494,12 @@ gen7_end_transform_feedback(struct gl_context *ctx,
>>
>>     brw_batch_end(&brw->batch);
>>
>> +   /* We will likely want to read the results in the very near future, so
>> +    * push this primitive to hardware if it is currently idle.
>> +    */
>> +   if (!brw_batch_busy(&brw->batch))
>> +      brw_batch_flush(&brw->batch);
>> +
>>     /* EndTransformFeedback() means that we need to update the number of
>>      * vertices written.  Since it's only necessary if DrawTransformFeedback()
>>      * is called and it means mapping a buffer object, we delay computing it
>>
>
> We need some data to justify this change.

I think even the theory is not correct - transform feedback is
typically fed back into the GPU (as new geometry, eg) rather than
consumed by the CPU, and in that case the flush is not helpful. But at
the end of the day, data will tell.

Kristian

> --Ken
>
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