Gallium historically has treated pipeline statistics queries as a single query, PIPE_QUERY_PIPELINE_STATISTICS, which returns a block of 11
values. This was originally patterned after the D3D1x API. Much later, Brian introduced an OpenGL extension that exposed these counters - but it exposes 11 separate queries, each of which returns a single value.
Today, st/mesa simply queries all 11 values, and returns a single value. While pipeline statistics counters aren't typically performance critical, this is still not a great fit. A D3D1x->GL translator might request all 11 counters by creating 11 separate GL queries...which Gallium would map to reads of all 11 values each time, resulting in a total 121 counter reads. That's not ideal.
This patch adds a new cap, PIPE_CAP_QUERY_PIPELINE_STATISTICS_SINGLE, and corresponding query type PIPE_QUERY_PIPELINE_STATISTICS_SINGLE. When calling create_query(), q->index should be set to one of the PIPE_STAT_QUERY_* enums to select a counter. Unlike the block query, this returns the value in pipe_query_result::u64 (as it's a single
value) instead of the pipe_query_data_pipeline_statistics group.
We update st/mesa to expose ARB_pipeline_statistics_query if either capability is set, preferring the new SINGLE variant when available.
Thanks to Roland, Ilia, and Marek for helping me sort this out.
d644698b44 gallium: Add the ability to query a single pipeline statistics counter
src/gallium/auxiliary/util/u_screen.c | 1 +
src/gallium/docs/source/context.rst | 5 ++++
src/gallium/include/pipe/p_defines.h | 2 ++
src/mesa/state_tracker/st_cb_queryobj.c | 43 +++++++++++++++++++++++++++++++--
src/mesa/state_tracker/st_context.c | 2 ++
src/mesa/state_tracker/st_context.h | 1 +
src/mesa/state_tracker/st_extensions.c | 1 +
7 files changed, 53 insertions(+), 2 deletions(-)