port-amd64/54808: Visual artifacts in Firefox after bus_space changes

[abs%absd.org@localhost]

>Number:         54808
>Category:       port-amd64
>Synopsis:       Visual artifacts in Firefox after bus_space changes
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Dec 29 15:55:00 +0000 2019
>Originator:     abs%absd.org@localhost
>Release:        NetBSD 9.0_RC1
>Organization:
	
>Environment:
System: NetBSD forsaken.absd.org 9.0_RC1 NetBSD 9.0_RC1 (GENERIC_DIAGNOSTIC) #2: Sat Dec 28 15:13:18 GMT 2019 abs%iris.absd.org@localhost:/opt/netbsd/9/sys/arch/amd64/compile/GENERIC_DIAGNOSTIC amd64
Architecture: x86_64
Machine: amd64
>Description:
        After pullup of http://releng.netbsd.org/cgi-bin/req-9.cgi?show=566 
        ( src/sys/arch/x86/x86/bus_space.c 1.42 & 1.43)
        visual artifacts are observed in Firefox. 

        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.

        Its sometimes quite subtle, and occasionally very unsubtle.

        Two quick videos indicating the issue

        https://photos.google.com/share/AF1QipNaEbA8nrfrK1EpGqvOVjt_hLMtThJQTV_z7XWqlOExJGHsaktPi8V9qxEOW0zi5Q?key=VkV1YkVabHh3TVFwLUJsQTlGMUIxWWNBSndUb01R

        This is on a Thinkpad T480
        i915drmkms0 at pci0 dev 2 function 0: Intel UHD Graphics 620 (GT2) (rev. 0x07)
        intel(0): Using Kernel Mode Setting driver: i915, version 1.6.0 20151010

>How-To-Repeat:
         All tests with GENERIC+DIAGNOSTIC:
         - latest netbsd-9 as of 2019-09-27 - issue seen
         - revert pullup of sys/arch/x86/x86/bus_space.c r1.43 - issue seen
         - also revert pullup of sys/arch/x86/x86/bus_space.c r1.42 - issue no
         longer seen

	Its *possible* that I may have seen it once with r1.43 & r1.42
        reverted - which could imply its a x11/drm issue which the better
        barrier enforcement is making more visible. Will keep running with
        GENERIC+DIAGNOSTIC plus the reversion to see if I see it again
>Fix: