According to mali_kbase, all Bifrost and Valhall GPUs are affected by
issue TSIX_2033. This hardware bug breaks the INTERSECT frame shader
mode when forcing clean_tile_writes. What does that mean?
The hardware considers a tile "clean" if it has been cleared but not
drawn to. Setting clean_tile_write forces the hardware to write back
such "clean" tiles to main memory.
Bifrost hardware supports frame shaders, which insert a rectangle into
every tile according to a configured rule. Frame shaders are used in
Panfrost to implement tile reloads (i.e. LOAD_OP_LOAD). Two modes are
relevant to the current discussion: ALWAYS, which always inserts a frame
shader, and INTERSECT, which tries to only insert where there is
geometry. Normally, we use INTERSECT for tile reloads as it is more
efficient than ALWAYS-- it allows us to skip reloads of tiles that are
discarded and never written back to memory.
From a software perspective, Panfrost's current logic is correct: if we
clear, we set clean_tile_writes, else we use an INTERSECT frame shader.
There is no software interaction between the two.
Unfortunately, there is a hardware interaction. The hardware forces
clean_tile_writes in certain circumstances when AFBC is used.
Ordinarily, this is a hardware implementation detail and invisible to
software. Unfortunately, this implicit clean tile write is enough to
trigger the hardware bug when using INTERSECT. As such, we need to
detect this case and use ALWAYS instead of INTERSECT for correct
results.
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13205>
Alyssa Rosenzweig
342ed4909f