port-arm/56596: Pinebook Pro graphics noticeably slower since DRM update

[bsiegert%gmail.com@localhost]

>Number:         56596
>Category:       port-arm
>Synopsis:       Pinebook Pro graphics noticeably slower since DRM update
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-arm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jan 01 11:30:00 +0000 2022
>Originator:     Benny Siegert
>Release:        NetBSD current with 9.2 userland
>Organization:
The NetBSD Foundation
>Environment:
NetBSD meguro.bentsukun.ch 9.99.92 NetBSD 9.99.92 (PBP64) #5: Sat Nov 13 16:06:47 UTC 2021  bsiegert%meguro.bentsukun.ch@localhost:/usr/obj/sys/arch/evbarm/compile/PBP64 evbarm

@(#)NetBSD 9.99.92 (PBP64) #1: Mon Dec 20 08:32:05 UTC 2021

>Description:
I updated the NetBSD-current kernel on the Pinebook Pro on December 20,
after the big merge of the new DRM code.

I noticed that the graphics are *much* slower compared to a pre-DRM merge
kernel (November 15). In particular, in a full-screen Firefox window, you
can watch the screen redraw inside the browser window, taking about a
second. In comparison, in the older kernel, this is nearly instantaneous.

I noticed that there is occasionally a kernel message about atomic modeset userland being broken (?), which leads to disabling of atomic. I assume this is related?

>How-To-Repeat:
Run Firefox on a Pinebook Pro with recent -current kernel.
>Fix: