[Mesa-dev] i965 GL_ARB_buffer_storage

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On 02/27/2014 02:52 PM, Eric Anholt wrote:
>> One thing I noticed while working on this was that we only reallocate buffer
>> storage for INVALIDATE_BUFFER_BIT when UNSYNCHRONIZED_BIT is unset.  The
>> ARB_mbr spec says that the contents "may be discarded", not "must be
>> discarded".  However, while writing the glamor code I happened to type this
>> for the wraparound case:
>> 
>>             glamor_priv->vb = glMapBufferRange(GL_ARRAY_BUFFER,
>>                                                0, size,
>>                                                GL_MAP_WRITE_BIT |
>>                                                GL_MAP_INVALIDATE_BUFFER_BIT |
>>                                                GL_MAP_UNSYNCHRONIZED_BIT |
>>                                                GL_MAP_PERSISTENT_BIT |
>>                                                GL_MAP_COHERENT_BIT);
>> 
>> intending that the buffer storage get reallocated, and that we not worry about
>> any synchronization after that.  My code would have been broken on the i965
>> driver.  I'm wondering if this is the intended behavior of the spec, or if we
>> want to treat the "may" as a "must".
>
> I don't understand.  Your code here sets GL_MAP_INVALIDATE_BUFFER_BIT
> and GL_MAP_UNSYNCHRONIZED_BIT, which would have reallocated the storage,
> as you intended.  Was this a mispaste - you originally lacked the
> GL_MAP_UNSYNCHRONIZED_BIT, were surprised, and eventually added it?

Here's the only i965 code referencing MAP_INVALIDATE_BUFFER_BIT:

   if (!(access & GL_MAP_UNSYNCHRONIZED_BIT)) {
      if (drm_intel_bo_references(brw->batch.bo, intel_obj->buffer)) {
	 if (access & GL_MAP_INVALIDATE_BUFFER_BIT) {
	    drm_intel_bo_unreference(intel_obj->buffer);
	    intel_bufferobj_alloc_buffer(brw, intel_obj);
	 } else {
            perf_debug("Stalling on the GPU for mapping a busy buffer "
                       "object\n");
	    intel_batchbuffer_flush(brw);
	 }
      } else if (drm_intel_bo_busy(intel_obj->buffer) &&
		 (access & GL_MAP_INVALIDATE_BUFFER_BIT)) {
	 drm_intel_bo_unreference(intel_obj->buffer);
	 intel_bufferobj_alloc_buffer(brw, intel_obj);
      }
   }

so for that glamor call we wouldn't have thrown out the storage, even
though it was referenced by the batch or the GPU.  I think we should
pull the INVALIDATE_BUFFER test out and just always invalidate when it's
set.
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