> Since the same discussion is happening with etnaviv: Why exactly does mesa
> need to mmap imported buffers?

The golden question!

Seemingly, (one of the) reasons is that the state tracker does
colourspace conversion in software if the winsys wants a format that the
driver doesn't advertise. Unrelated to this, that's almost certainly a
performance issue and might explain quite a bit...

In this case, the winsys seems to want RGB10_A2 but we settle for RGBA8.
I thought that was just changing EGL negotiation stuff.. I had no idea
it was covering up in software... not good...