AMD's Older Radeon GPUs Get Ray Tracing Support on Linux – Tom's Hardware

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By 23 September 2021
Teaching an old dog new tricks
Sometimes new technologies can work on old hardware. Who needs one of the best graphics cards when you can just use older hardware? This week enthusiasts working on Mesa, an open source implementation of OpenGL and Vulkan APIs for Linux, created a merge request for the RADV ray tracing driver for AMD’s RDNA 2 GPUs, enabling it to work with previous-generation graphics cards. The question is, how well will they work?
“This PR implements ray-tracing for older generations (Navi, Vega, Polaris, etc.),” wrote Joshua Ashton, a developer of DXVK and other Direct3D-on-Vulkan works for Valve, reports Phoronix. “It does this by emulating the AMD bvh intersection instructions in software. Right now this passes CTS the same as on RDNA 2 cards.” 
In recent times software and hardware ray tracing support has been the focus of many industrial discussion as the technology has to be implemented properly to bring significant quality improvements. But that also causes massive performance hits even on modern hardware that supports hardware acceleration for ray tracing, such as AMD’s Radeon RX 6000-series GPUs based on the RDNA 2 architecture.
Ubisoft’s decision not to implement ray tracing into Far Cry 6 for the latest game consoles greatly illustrates this controversy. But apparently Linux enthusiasts want to enable ray tracing support in Vulkan even on outdated architectures like the original RDNA from 2019 as well as GCN 4/5 from 2016/2017.  
There are six levels of hardware ray tracing support by GPUs, according to Jon Peddie, the head of Jon Peddie Research. RDNA 2 GPUs are believed to be at Level 3 as they support Bounding Volume Hierarchy (BVH) processing in hardware. Meanwhile, GPUs based on AMD’s architectures introduced in 2016–2019 do not support any hardware acceleration for ray tracing, but they can emulate BVH processing in software using ray-box and ray-triangle testers via standard shader instructions.
While such emulation is legit and can produce similar results to Level 3 RT hardware, it also requires a huge investment of compute horsepower for real-time gaming, something that outdated GPUs simply do not have. AMD has stated that its ray accelerators are about ten times faster than doing the same work via shaders, for example. Of course, ray tracing can be used to create beautiful images not in real time, but those who need this capability usually can afford proper hardware. Nonetheless, the idea of adding ray tracing support to previous-generation hardware poses some theoretical interest.
Nvidia for example supports DirectX Raytracing (DXR) on its GTX 10- and 16-series GPUs (with 6GB or more VRAM) via drivers that emulate the ray tracing calculations via software. Even the fastest of such GPUs — the GTX 1080 Ti — tends to perform at less than half the non-DXR rate, and that was in ‘lighter’ first-gen DXR games like Battlefield V, Shadow of the Tomb Raider and Metro Exodus. We would expect doing RT calculations in software on AMD hardware to show a similar performance hit.
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