kern/55198: When the video card is Intel Mobile 965GM Express, i915drmkms has error CPU pipe A FIFO underrun, also X freezes/hangs

[joaopaulo1511%hotmail.com@localhost]

>Number:         55198
>Category:       kern
>Synopsis:       When the video card is Intel Mobile 965GM Express, i915drmkms has error CPU pipe A FIFO underrun, also X freezes/hangs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 23 01:15:00 +0000 2020
>Originator:     João Paulo Vinha Bittar
>Release:        9.0
>Organization:
home user
>Environment:
NetBSD localhost 9.0 NetBSD 9.0 (GENERIC) #0: Fri Feb 14 00:06:28 UTC 2020 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/i386/compile/GENERIC i386
>Description:
This problem report is similar to the one reported at "http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=55133";, but I think it is sufficiently different to deserve its own PR#.

On a laptop with Intel Mobile 965GM Express Chipset (i915drmkms driver on kernel), booting NetBSD 9.0 gives the following "dmesg | grep intel" lines:

[     5.467170] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun
[     5.747346] intelfb0 at i915drmkms0
[     5.747346] intelfb0: framebuffer at 0xdb891000, size 1280x800, depth 32, stride 5120
[     6.697919] wsdisplay0 at intelfb0 (kbdmux ignored): console (default, vt100 emulation), using wskbd0
[    15.893517] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun
[     5.398721] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:141)i9xx_set_fifo_underrun_reporting] *ERROR* pipe A underrun
[     5.398721] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun
[     5.678906] intelfb0 at i915drmkms0
[     5.678906] intelfb0: framebuffer at 0xdb88b000, size 1280x800, depth 32, stride 5120
[     6.409366] wsdisplay0 at intelfb0 (kbdmux ignored): console (default, vt100 emulation), using wskbd0
[    15.735309] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun
[     6.368271] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun
[     6.648435] intelfb0 at i915drmkms0
[     6.648435] intelfb0: framebuffer at 0xdb8b0000, size 1280x800, depth 32, stride 5120
[     7.618971] wsdisplay0 at intelfb0 (kbdmux ignored): console (default, vt100 emulation), using wskbd0
[    16.784096] kern error: [drm:(/usr/src/sys/external/bsd/drm2/dist/drm/i915/intel_fifo_underrun.c:363)intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun


Also, running startx doesn't open the default twm environment with three xterm and xclock opened, as X freezes/hangs after showing the first xterm.  It is needed to change back to the virtual terminal (CTRL+ALT+F#) and kill the X server, as even the only xterm that shows up is frozen/hung.
>How-To-Repeat:
Install NetBSD 9.0 on a laptop with Intel Mobile 965GM Express Chipset, boot it and run startx.  You will see the dmsg error messages in green and X server hanging up/freezing.

I did that on an Acer Aspire 5720Z which came with this specific graphics card (some models may come with Intel Mobile 960GM or 965PM).
>Fix:
I don't know how to fix it, but I know that FreeBSD 12.1 (i386) boots without errors and X works just by running startx.