Marek Makes MSAA R300 Performance Optimizations

Recently there was finally MSAA support added to the R300 Gallium3D driver. While multi-sample anti-aliasing finally made it to this open-source GPU driver that supports the old ATI R300 through R500 GPUs, benchmarks I did last week showed the R300 MSAA performance was a mess. Fortunately, Marek Olšák has made some significant performance optimizations to the R300g MSAA support.

Benchmarks published today on Phoronix looked at R300 Morphological Anti-Aliasing performance, but as luck would have it, as soon as that testing wrapped up and the article went out, there were some new interesting commits in Mesa Git master. The latest work today by Marek Olšák, likely the most prolific independent contributor to the open-source Radeon Linux graphics driver, is about MSAA performance.

The most interesting Mesa Git commit today by Marek was entitled r300g: implement MSAA compression and fast MSAA color clear. With MSAA compression and fast MSAA color clear, using multi-sample anti-aliasing should now be a hell of a lot faster for those using this Gallium3D driver in conjunction with older Radeon hardware. This is really good since last week's MSAA benchmarks showed the driver in its state just days ago was basically useless with this common anti-aliasing mode.

Today's MSAA R300g commit contains the following description:

These are optimizations which make MSAA a lot faster.

The MSAA work is complete with this commit. (except for enablement of AA optimizations for RGBA16F, for which a patch is ready and waiting until the kernel CS checker fix lands)

MSAA can't be made any faster as far as hw programming is concerned.

The catch is only one process and one colorbuffer can use the optimizations at a time. There usually is only one MSAA colorbuffer, so it shouldn't be an issue.

Also, there is a limit on the size of MSAA colorbuffer resolution in terms of megapixels. If the limit is surpassed, the AA optimizations are disabled.

The limit is:
- 1 Mpix on low-end and some mid-level chipsets (1024x768 and 1280x720)
- 2 Mpix on some mid-level chipsets (1600x1200 and 1920x1080)
- 3 or 4 Mpix on high-end chipsets (2048x1536 or 2560x1600, respectively)

It corresponds to the number of raster pipes (= GB pipes) available, each pipe can hold 1 Mpix of AA compression data.

If it's enabled, the driver prints to stdout:
radeon: Acquired access to AA optimizations.

As soon as the anti-aliasing optimizations for RGBA16F land following the Radeon DRM CS checker fix is in place for the Linux kernel, new MSAA R300g benchmarks will be carried out from Radeon X1000 class hardware. These improvements will be part of Mesa 9.1, to be released in February.