[Mesa-dev] [PATCH 0/7] st/mesa: glReadPixels cache

[Nicolai Hähnle]

Hi,

some applications use successive calls to glReadPixels to read data back.
This typically involves a GPU-based blit for each call for de-tiling or
format conversions (e.g. BGRA -> RGBA). Even when the _mesa_readpixels path
is used, such a blit tends to be hidden behind the transfer operations. The
overhead is rather bad for performance, since we have to wait for GPU idle
each time.

This patch series implements a cache which heuristically does a blit of the
entire framebuffer into a temporary texture once, which is then re-used by
immediately following calls to glReadPixels. The cache remains disabled for
drivers that do not prefer blit based texture transfers, i.e. softpipe/llvmpipe.

Aside from a client's application, this also affects ~1400 piglit tests,
which tend to see speedups of 5-10% with this cache in my tests.

While looking for places that invalidate the cache, I also noticed a few
additional spots where I believe the bitmap cache needs to be flushed. I put
those first in this series.

Please review!

Thanks,
Nicolai
--
 src/mesa/state_tracker/st_atom_framebuffer.c |   1 +
 src/mesa/state_tracker/st_cb_bitmap.c        |   3 +
 src/mesa/state_tracker/st_cb_blit.c          |   1 +
 src/mesa/state_tracker/st_cb_clear.c         |   1 +
 src/mesa/state_tracker/st_cb_compute.c       |   4 +
 src/mesa/state_tracker/st_cb_copyimage.c     |   4 +
 src/mesa/state_tracker/st_cb_drawpixels.c    |   2 +
 src/mesa/state_tracker/st_cb_drawtex.c       |   1 +
 src/mesa/state_tracker/st_cb_fbo.h           |   2 +
 src/mesa/state_tracker/st_cb_readpixels.c    | 244 +++++++++++++----
 src/mesa/state_tracker/st_cb_texture.c       |  12 +
 src/mesa/state_tracker/st_context.c          |   3 +
 src/mesa/state_tracker/st_context.h          |  11 +
 src/mesa/state_tracker/st_draw.c             |   1 +
 src/mesa/state_tracker/st_draw_feedback.c    |   1 +
 src/mesa/state_tracker/st_gen_mipmap.c       |   4 +
 16 files changed, 237 insertions(+), 58 deletions(-)