Fixing the R/B swap the right way

If you’ve ever looked at a GPU render and seen blue where red should be, you’ve met the R/B swap problem. For etnaviv this has been a long-standing source of complexity. We were solving it in the shader, but the proprietary blob driver had a simpler approach all along. As part of my work at Igalia, I finally sat down and did it properly.

The problem

Vivante GPUs have a quirk: the Pixel Engine (PE) always writes pixels in BGRA byte order. When your API says “render to R8G8B8A8_UNORM”, what actually lands in memory is B, G, R, A. Every byte of every pixel, every frame. The hardware just works that way.