Midgard has no hardware support for transform feedback, so we simulate it in software. Lucky us.
What Midgard does do is write out vertex shader outputs to main memory unconditonally. Fragment shaders read varyings back from main memory; there's no on-chip storage for varyings. Whether this was a reasonable design is a question I will not be engaging in this commit message.
What that does mean is that, in some sense, Midgard *always* does transform feedback uncondtionally, and there's no way to turn off transform feedback. Normally, we would allocate some scratch memory every frame to store the varyings in an arbitrary format (interleaved for simplicity), and then feed that scratch to the fragment shader and discard when the rendering completes.
The only difference now is that sometimes, for some buffers, we use a BO provided to us by Gallium and a format provided by Gallium, instead of allocating the memory and choosing the format ourselves. This has some limitations -- in particular, it only works at vec4 granularity, so a corresponding GLSL linkage patch is needed to correctly implement transform feedback for non-vec4 types. Nevertheless, given the hardware already works in this admittedly-bizarre fashion, transform feedback is "free". Or, at least, it's no more expensive than any other rendering.
Specifically not implemented is dynamically-sized transform feedback (i.e. with geometry/tesselation shaders).
Spoiler alert: Midgard has no support for geometry *or* tessellation shaders, despite advertising support. They get compiled to *massive* compute shaders. How's that for checkbox compliance?
29cfd154e38 panfrost: Implement transform feedback
src/gallium/drivers/panfrost/pan_varyings.c | 254 ++++++++++++++++++++++++----
1 file changed, 217 insertions(+), 37 deletions(-)