Re: Visual artifacts in Firefox after bus_space changes

[Taylor R Campbell <campbell+netbsd-port-amd64%mumble.net@localhost>]

> Date: Thu, 26 Dec 2019 22:39:58 +0000
> From: David Brownlee <abs%absd.org@localhost>
> 
> I'm seeing visual artifacts in Firefox - I believe after
> http://releng.netbsd.org/cgi-bin/req-9.cgi?show=566 was pulled up to
> netbsd-9, and from a quick test with a current kernel its as if a
> partial screen update can happen, when drawing a new character when
> typing (eg into gmail or slack), highlighting some text or even
> updating a screen element on hover. Its quickly replaced with the
> correct content.

Interesting.  I would be curious to see a stack trace for when
bus_space_barrier gets called during this event -- perhaps with

dtrace -n 'fbt::bus_space_barrier:entry { @[stack()] = count() }'

and running the test (then hit ^C when done), or by modifying
bus_space_barrier to print a stack trace and consulting dmesg.

I don't see any obvious way for it to be called by any of the drmkms
drivers themselves, but perhaps it's coming from another driver.
Either way, my best guess so far is that the impact on timing of
issuing a fence is large enough that it makes drawing activity miss
the deadline for a vblank event.