When using PBOs to upload texture data, which call triggers the actual DMA operation?

[Clemens Eisserer]

Hi,

I am trying to better understand /optimize texture upload on r600 and
GCN based GPUs.

Currently I use PBOs to upload data generated by a worker thread to
textures, using the following steps:

1. Unmap buffer n (from worker)
2. glTexSubImage2D n-1 to texture n-1
3. bin texture n-2 & draw & glutSwapBuffers
4. map buffer n-3 again and pass it to worker thread

For each buffer only one step is executed per frame to avoid GPU stalls.

However, after I had a look at radeon_gem_objects I am not sure this
approach makes a lot of sence.
All PBOs are located in system memory (GTT), so as far as I understand
it, unmapping a PBO is actually a no-on and doesn't trigger any
transfer?
However, where is the actual DMA transfer triggerd - by
glTexSubImage2D? And at which point the driver checks for DMA
completion - at glutSwapBuffers?

Furthermore, is it possible to perform async upload and rendering in
parallel in case there are no data-dependencies?

Some insights would be really great to better optimize the code.

Thank you in advance & best regards, Clemens