[Mesa-dev] [PATCH v2 7/8] i965: Implement glDrawTransformFeedback().
Kenneth Graunke
kenneth at whitecape.org
Mon Oct 28 22:55:36 CET 2013
On 10/28/2013 04:52 AM, Pohjolainen, Topi wrote:
> On Mon, Oct 28, 2013 at 01:14:55PM +0200, Pohjolainen, Topi wrote:
>> On Fri, Oct 25, 2013 at 10:35:48PM -0700, Kenneth Graunke wrote:
>>> Implementing the GetTransformFeedbackVertexCount() driver hook allows
>>> the VBO module to call us with the right number of vertices.
>>>
>>> The hardware doesn't directly count the number of vertices written by
>>> SOL, so we instead use the SO_NUM_PRIMS_WRITTEN(n) counters and multiply
>>> by the number of vertices per primitive.
>>>
>>> Unfortunately, counting the number of primitives generated is tricky:
>>> a program might pause a transform feedback operation, start a second one
>>> with a different object, then switch back and resume. Both transform
>>> feedback operations share the SO_NUM_PRIMS_WRITTEN counters.
>>>
>>> To work around this, we save the counter values at Begin, Pause, Resume,
>>> and End. This "bookends" each section where transform feedback is
>>> active for the current object. Adding up differences of pairs gives
>>> us the number of primitives generated. (This is similar to what we
>>> do for occlusion queries on platforms without hardware contexts.)
>>>
>>> v2: Fix missing parenthesis in assertion (caught by Eric Anholt).
>>>
>>> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>>> Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>
>>> Reviewed-by: Eric Anholt <eric at anholt.net>
>>> ---
>>> src/mesa/drivers/dri/i965/brw_context.c | 2 +
>>> src/mesa/drivers/dri/i965/brw_context.h | 26 ++++
>>> src/mesa/drivers/dri/i965/gen6_sol.c | 1 +
>>> src/mesa/drivers/dri/i965/gen7_sol_state.c | 190 ++++++++++++++++++++++++++++-
>>> 4 files changed, 218 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
>>> index 623273c..f4e04b6 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_context.c
>>> +++ b/src/mesa/drivers/dri/i965/brw_context.c
>>> @@ -252,6 +252,8 @@ brw_init_driver_functions(struct brw_context *brw,
>>>
>>> functions->NewTransformFeedback = brw_new_transform_feedback;
>>> functions->DeleteTransformFeedback = brw_delete_transform_feedback;
>>> + functions->GetTransformFeedbackVertexCount =
>>> + brw_get_transform_feedback_vertex_count;
>>> if (brw->gen >= 7) {
>>> functions->BeginTransformFeedback = gen7_begin_transform_feedback;
>>> functions->EndTransformFeedback = gen7_end_transform_feedback;
>>> diff --git a/src/mesa/drivers/dri/i965/brw_context.h b/src/mesa/drivers/dri/i965/brw_context.h
>>> index 48aa4c1..c72bad1 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_context.h
>>> +++ b/src/mesa/drivers/dri/i965/brw_context.h
>>> @@ -880,11 +880,33 @@ struct intel_batchbuffer {
>>> } saved;
>>> };
>>>
>>> +#define BRW_MAX_XFB_STREAMS 4
>>> +
>>> struct brw_transform_feedback_object {
>>> struct gl_transform_feedback_object base;
>>>
>>> /** A buffer to hold SO_WRITE_OFFSET(n) values while paused. */
>>> drm_intel_bo *offset_bo;
>>> +
>>> + /** The most recent primitive mode (GL_TRIANGLES/GL_POINTS/GL_LINES). */
>>> + GLenum primitive_mode;
>>> +
>>> + /**
>>> + * Count of primitives generated during this transform feedback operation.
>>> + * @{
>>> + */
>>> + uint64_t prims_generated[BRW_MAX_XFB_STREAMS];
>>> + drm_intel_bo *prim_count_bo;
>>> + unsigned prim_count_buffer_index; /**< in number of uint64_t units */
>>> + /** @} */
>>> +
>>> + /**
>>> + * Number of vertices written between last Begin/EndTransformFeedback().
>>> + *
>>> + * Used to implement DrawTransformFeedback().
>>> + */
>>> + uint64_t vertices_written[BRW_MAX_XFB_STREAMS];
>>> + bool vertices_written_valid;
>>> };
>>>
>>> /**
>>> @@ -1574,6 +1596,10 @@ brw_begin_transform_feedback(struct gl_context *ctx, GLenum mode,
>>> void
>>> brw_end_transform_feedback(struct gl_context *ctx,
>>> struct gl_transform_feedback_object *obj);
>>> +GLsizei
>>> +brw_get_transform_feedback_vertex_count(struct gl_context *ctx,
>>> + struct gl_transform_feedback_object *obj,
>>> + GLuint stream);
>>>
>>> /* gen7_sol_state.c */
>>> void
>>> diff --git a/src/mesa/drivers/dri/i965/gen6_sol.c b/src/mesa/drivers/dri/i965/gen6_sol.c
>>> index 2e6c86a..af5bed9 100644
>>> --- a/src/mesa/drivers/dri/i965/gen6_sol.c
>>> +++ b/src/mesa/drivers/dri/i965/gen6_sol.c
>>> @@ -162,6 +162,7 @@ brw_delete_transform_feedback(struct gl_context *ctx,
>>> }
>>>
>>> drm_intel_bo_unreference(brw_obj->offset_bo);
>>> + drm_intel_bo_unreference(brw_obj->prim_count_bo);
>>>
>>> free(brw_obj);
>>> }
>>> diff --git a/src/mesa/drivers/dri/i965/gen7_sol_state.c b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>>> index 27421da..7cac8fe 100644
>>> --- a/src/mesa/drivers/dri/i965/gen7_sol_state.c
>>> +++ b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>>> @@ -249,14 +249,179 @@ const struct brw_tracked_state gen7_sol_state = {
>>> .emit = upload_sol_state,
>>> };
>>>
>>> +/**
>>> + * Tally the number of primitives generated so far.
>>> + *
>>> + * The buffer contains a series of pairs:
>>> + * (<start0, start1, start2, start3>, <end0, end1, end2, end3>) ;
>>> + * (<start0, start1, start2, start3>, <end0, end1, end2, end3>) ;
>>> + *
>>> + * For each stream, we subtract the pair of values (end - start) to get the
>>> + * number of primitives generated during one section. We accumulate these
>>> + * values, adding them up to get the total number of primitives generated.
>>> + */
>>> +static void
>>> +gen7_tally_prims_generated(struct brw_context *brw,
>>> + struct brw_transform_feedback_object *obj)
>>> +{
>>> + /* If the current batch is still contributing to the number of primitives
>>> + * generated, flush it now so the results will be present when mapped.
>>> + */
>>> + if (drm_intel_bo_references(brw->batch.bo, obj->prim_count_bo))
>>> + intel_batchbuffer_flush(brw);
>>> +
>>> + if (unlikely(brw->perf_debug && drm_intel_bo_busy(obj->prim_count_bo)))
>>> + perf_debug("Stalling for # of transform feedback primitives written.\n");
>>> +
>>> + drm_intel_bo_map(obj->prim_count_bo, false);
>>> + uint64_t *prim_counts = obj->prim_count_bo->virtual;
>>> +
>>> + assert(obj->prim_count_buffer_index % (2 * BRW_MAX_XFB_STREAMS) == 0);
>>> + int pairs = obj->prim_count_buffer_index / (2 * BRW_MAX_XFB_STREAMS);
>>> +
>>> + for (int i = 0; i < pairs; i++) {
>>> + for (int s = 0; s < BRW_MAX_XFB_STREAMS; s++) {
>>> + obj->prims_generated[s] +=
>>> + prim_counts[BRW_MAX_XFB_STREAMS + s] - prim_counts[s];
>>> + }
>>> + prim_counts += 2 * BRW_MAX_XFB_STREAMS; /* move to the next pair */
>>> + }
>>> +
>>> + drm_intel_bo_unmap(obj->prim_count_bo);
>>> +
>>> + /* Release the BO; we've already tallied all the data it contained. */
>>> + drm_intel_bo_unreference(obj->prim_count_bo);
>>> + obj->prim_count_bo = NULL;
>>> +}
>>> +
>>> +/**
>>> + * Store the SO_NUM_PRIMS_WRITTEN counters for each stream (4 uint64_t values)
>>> + * to prim_count_bo.
>>> + *
>>> + * If prim_count_bo is out of space, gather up the results so far into
>>> + * prims_generated[] and allocate a new buffer with enough space.
>>> + *
>>> + * The number of primitives written is used to compute the number of vertices
>>> + * written to a transform feedback stream, which is required to implement
>>> + * DrawTransformFeedback().
>>> + */
>>> +static void
>>> +gen7_save_primitives_written_counters(struct brw_context *brw,
>>> + struct brw_transform_feedback_object *obj)
>>> +{
>>> + const int streams = BRW_MAX_XFB_STREAMS;
>>> +
>>> + /* Check if there's enough space for a new pair of four values. */
>>> + if (obj->prim_count_bo != NULL &&
>>> + obj->prim_count_buffer_index + 2 * streams >= 4096 / sizeof(uint64_t)) {
>>> + /* Gather up the results so far and release the BO. */
>>> + gen7_tally_prims_generated(brw, obj);
>>> + }
>>> +
>>> + /* Allocate a new buffer if needed. A page should be plenty. */
>>> + if (obj->prim_count_bo == NULL) {
>>> + obj->prim_count_buffer_index = 0;
>>> + obj->prim_count_bo =
>>> + drm_intel_bo_alloc(brw->bufmgr, "xfb primitive counts", 4096, 4096);
>>
>> I was wondering why 'gen7_tally_prims_generated()' needs to dispose the buffer
>> object and this logic here to reallocate another. Couldn't we re-use the old
>> bo and simply let 'gen7_tally_prims_generated()' reset the
>> 'prim_count_buffer_index' to zero?
>>
>> Below 'brw_compute_xfb_vertices_written()' wants to tally also but it guards
>> itself from adding counters multiple times using the flag
>> 'vertices_written_valid'. In fact if it didn't 'gen7_tally_prims_generated()'
>> would call 'drm_intel_bo_map()' against NULL pointer, right? Or did I
>> understand the logic all wrong?
>
> Further to the "20 questions", try to bear with me. I may easily be missing
> things but would this patch work without the register storing and loading
> introduced in patch four if pause() and resume() hooks saved the counters the
> same way as begin() and end()? In other words, one would track the deltas
> between begin()/pause(), pause()/resume() and resume()/end() in addition to the
> begin()/end() (if no pause()-resume() sequence were in between)?
No. Patch 4 saves and restores the SO_WRITE_OFFSET(n) registers, which
is the offset at which the hardware will write data. It needs to be a
legitimate offset for whatever buffer is currently bound, which changes
with the XFB object. Otherwise the GPU will write off the end of a
buffer and hang.
This patch uses the SO_NUM_PRIMS_WRITTEN(n) registers, which just track
the number of primitives generated. Since we're just counting things,
deltas work fine.
--Ken
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