[Mesa-dev] [PATCH v2 0/9] Add ARB_query_buffer_object (swrast)

[Kenneth Graunke]

On 03/27/2014 01:59 PM, Rafal Mielniczuk wrote:
> Hello,
> 
> This is the second version of the series implementing 
> ARB_query_buffer_object in mesa.
> 
> Main changes to the first version are:
>  - Enable extension only for software driver
>  - Fix possible segfault in patch #7
>  - Fix typos and comment style issues
>  
> Drivers, which are not able to implement it without
> CPU roundtrip can just mark extension flag as true
> and use software fallback.
> 
> I did not add any ctx.Driver callback for now, as I was 
> not sure what would be the required api there. 
> Perhaps it would be better to add this while implementing 
> gpu acceleration in driver?
> 
> I tested the extension by enabling software fallback in i965 driver.
> This is not included in this series.
> 
> Thanks,
> Rafal Mielniczuk (9):
>   glapi: Add xml infrastructure for ARB_query_buffer_object
>   mesa: Add ARB_query_buffer_object extension flag
>   mesa: Add QueryBuffer to context
>   mesa: Handle QUERY_BUFFER_BINDING in GetIntegerv
>   mesa: Handle QUERY_RESULT_NO_WAIT in GetQueryObject{ui64}v
>   mesa: Fix typos in function names in queryobj
>   mesa: Implement software fallback for ARB_query_buffer_object
>   mesa: Enable GL_ARB_query_buffer_object for software drivers
>   doc: mark GL_ARB_query_buffer_object as done for swrast
> 
>  docs/GL3.txt                                   |   2 +-
>  src/mapi/glapi/gen/ARB_query_buffer_object.xml |  18 ++++
>  src/mapi/glapi/gen/Makefile.am                 |   1 +
>  src/mapi/glapi/gen/gl_API.xml                  |   4 +
>  src/mesa/main/bufferobj.c                      |   5 ++
>  src/mesa/main/extensions.c                     |   2 +
>  src/mesa/main/get.c                            |   5 ++
>  src/mesa/main/get_hash_params.py               |   3 +
>  src/mesa/main/mtypes.h                         |   3 +
>  src/mesa/main/queryobj.c                       | 109 +++++++++++++++++++++----
>  10 files changed, 135 insertions(+), 17 deletions(-)
>  create mode 100644 src/mapi/glapi/gen/ARB_query_buffer_object.xml

Hi Rafal!

Thanks for doing this!  Looking through the spec, I had a few questions.

Are applications required to bind GL_QUERY_BUFFER buffer before starting
a query (and keep it bound for the duration of the query)?  Or, does it
only matter at glGetQueryObject time?

I'm guessing it's the latter - otherwise, it sure seems like the spec
would have had to answer what happens if you change/delete it while the
query is active...i.e.

   glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
   glBeginQuery(GL_SAMPLES_PASSED, queryId);
   ...
   glBindBuffer(GL_QUERY_BUFFER, 0); /* or some other buffer... */
   ...
   glEndQuery(GL_SAMPLES_PASSED);

In i965, we use a buffer object (query->bo - see brw_context.h:846)
during the query.  We take an initial snapshot of some counter (i.e.
PS_DEPTH_COUNT for occlusion queries, TIMESTAMP for timer queries) at
BeginQuery time, and a second snapshot at EndQuery time.  Both get
stored in query->bo at offsets 0 and 8 (1*sizeof(uint64_t)).

When they ask for the results, we map that buffer and use the CPU to
subtract the two values.  So, avoiding a CPU round trip seems tricky.

If we instead zeroed the counters at BeginQuery time, we would have a
single counter that we could possibly blit from query->bo to the
currently bound GL_QUERY_BUFFER via intel_emit_linear_blit.  But I don't
think we can do that for the TIMESTAMP register, and doing so would make
implementing some upcoming GL extensions harder.

So, maybe we could bind query->bo as a UBO or TexBO, bind a shader that
would read from there, subtract the two values, and write that to the
GL_QUERY_BUFFER object.  This could probably be done by drawing a single
point primitive with a vertex shader and rasterizer discard.  This could
work, but it seems pretty ugly...

We might also be able to use
MI_LOAD_REGISTER_MEM/MI_MATH/MI_STORE_REGISTER_MEM on Haswell, but I
don't know how that works yet.

--Ken

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