kern/50618: [netbsd-7] Massive interrupt storm when DPMS active

[joel.bertrand%systella.fr@localhost]

>Number:         50618
>Category:       kern
>Synopsis:       [netbsd-7] Massive interrupt storm when DPMS active
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jan 04 09:30:00 +0000 2016
>Originator:     BERTRAND Joël
>Release:        netbsd-7 Dec 28 2015 (present in netbsd 7.0 stable)
>Organization:
>Environment:
NetBSD legendre.systella.fr 7.0_STABLE NetBSD 7.0_STABLE (CUSTOM) #0: Mon Dec 28 19:24:33 CET 2015  root%legendre.systella.fr@localhost:/usr/src/sys/arch/amd64/compile/CUSTOM amd64
>Description:
On a 64 bits server containing :
cpu0 at mainbus0 apid 0: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu1 at mainbus0 apid 2: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu2 at mainbus0 apid 4: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu3 at mainbus0 apid 6: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu4 at mainbus0 apid 1: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu5 at mainbus0 apid 3: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu6 at mainbus0 apid 5: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
cpu7 at mainbus0 apid 7: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz, id 0x306c3
...
i915drmkms0 at pci0 dev 2 function 0: vendor 0x8086 product 0x0412 (rev. 0x06)
drm: Memory usable by graphics device = 2048M
drm: Supports vblank timestamp caching Rev 2 (21.10.2013).
drm: Driver supports precise vblank timestamp query.
i915drmkms0: interrupting at ioapic0 pin 16 (i915)
intelfb0 at i915drmkms0
i915drmkms0: info: registered panic notifier
i915drmkms0: More than 8 outputs detected via ACPI
intelfb0: framebuffer at 0xffff80011c7d6000, size 1280x1024, depth 32, stride 5120
...

I notice system gets an interrupt storm when running Xorg and when DPMS has powered off the screen. When the screen is blank, interrupt rate is around 40k interrupts by second (!). In dmesg, I obtain following messages :

DRM error in intel_uncore_check_errors: Unclaimed register before interrupt
DRM error in hsw_unclaimed_reg_clear: Unknown unclaimed register before writing to c5010
DRM error in hsw_unclaimed_reg_clear: Unknown unclaimed register before writing to c5010

If I stop Xorg or if I deactivate DPMS, trouble disappears.

For information, graphic adapter is known by Xorg as :
[    43.540] (II) intel(0): Using Kernel Mode Setting driver: i915, version 1.6.0 20080730
[    43.565] (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
[    43.565] (--) intel(0): Integrated Graphics Chipset: Intel(R) HD Graphics 4600
[    43.565] (--) intel(0): CPU: x86-64, sse2, sse3, ssse3, sse4.1, sse4.2, avx, avx2

>How-To-Repeat:
Only install NetBSD 7.0 (or current netbsd-7), configure Xorg and wait that DPMS turn screen off.
>Fix:
Turn DPMS off if it is possible.