RADV's Vulkan Ray-Tracing LBVH Extended Back To All GCN GPUs

Mesa's Radeon Vulkan "RADV" driver is in the unique position of supporting Vulkan ray-tracing for older AMD GPUs rather than just the latest-generation RDNA2 GPUs with dedicated ray-tracing cores. Though it's slower on these older GPUs, the code is in place for this open-source driver and the latest addition is now supporting LBVH going back to AMD GFX6 hardware -- in other words, all GCN GPUs.

Independent developer Konstantin Seurer has been working a lot on the Linear Bounding Volume Hierarchy (LBVH) support for RADV in using that approach for building acceleration structures. In select workloads like say the GravityMark benchmark, it meant going from ~13 FPS to ~250 FPS for modern AMD GPUs on this unofficial AMD Vulkan driver.

RADV LBVH merged last month into Mesa 22.2. The initial implementation was limited to AMD RDNA graphics processors, which isn't that surprising considering RDNA2 in particular is the primary focus given the dedicated RT hardware.

Mesa developer Rhys Perry this week merged some minor ray-tracing optimizations for RADV while notably now allows LBVH for building acceleration structures going back to GFX6 hardware. GFX6 is GCN 1.0 hardware that can be used with RADV when switching from the Radeon DRM to AMDGPU kernel driver. So now LBVH can be used for all GCN and RDNA GPUs with RADV to improve the ray-tracing performance at least for cases like GravityMark, but on the older Radeon hardware lacking RT cores still don't expect much out of the performance.

See this MR for the RADV LBVH for Gen6+ and the other minor ray-tracing optimizations that were merged this week for Mesa 22.2.