This implements a minimalistic algorithm that can be used to obtain an
approximate solution for the integer programming problem of finding
the optimal tile dimensions based on an estimate of the tile cache
consumption per pixel of the current graphics pipeline -- Including
the TC footprint of render targets, depth and stencil buffers and
their auxiliary surfaces. Considering other (less local) memory
accesses performed by the pipeline (like texturing and shader storage)
would be useful (and could be considered by this algorithm with little
modification), but it would be pretty difficult to estimate the L3
cache consumption per pixel of such accesses based on static analysis
of the pipeline state alone without some sort of dynamic feedback.
The present algorithm returns a config with tile area large enough to
utilize a target fraction of the L3, which can be adjusted to obtain
greater/lower utilization of the L3 at the cost of higher/lower risk
of L3 cache thrashing respectively. The aspect ratio of the tile
layout returned attempts to minimize the number of poorly utilized
tiles around the boundaries of the framebuffer (due to partial
coverage), since having the tile sequencer process additional tiles
comes at a cost due to the latency of the additional passes, even if
they're mostly empty. Finally, among the solutions with satisfactory
cache footprint and tile count, the tile aspect ratio closest to 1 is
returned where possible, since tiles with very high aspect ratios can
have a negative impact on cache locality.
The algorithm is primarily intended for TBIMR, but it could be used
for PTBR as well with little modifications, since the TBIMR-specific
assumptions are few and noted in comments below.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25493>
