AMD's Marek Has A Patch Helping To Reduce Gallium3D Input Lag

In addition to the work this week on exposing EXT_gpu_shader4 / EXT_texture_buffer_object, well known AMD open-source developer Marek Olšák has also been working a patch to help lower the input lag with Mesa.

Marek posted a new patch on Friday that aims to decrease input lag via syncing sooner in the swap buffers code. The patch decreases the number of active frames in flight by one and flushes before throttling in the "SwapBuffers" code.

Marek noted:

The improvement is apparent with Unigine Heaven.

Previously:
draw frame 2
wait frame 0
flush frame 2
present frame 2

The input lag is 2 frames.

Now:
draw frame 2
flush frame 2
wait frame 1
present frame 2

The input lag is 1 frame. Flushing is done before waiting, because otherwise the device would be idle after waiting.

The patch affects the common Gallium3D code as opposed to being RadeonSI-specific code. As of writing it hasn't been merged to Mesa Git but we'll see if it makes it in before next week's Mesa 19.1.0 code branching / feature freeze.