Bug 91373 - Nouveau fills kern.log with gigabytes of data when molecule screensaver is run
Summary: Nouveau fills kern.log with gigabytes of data when molecule screensaver is run
Status: RESOLVED INVALID
Alias: None
Product: xorg
Classification: Unclassified
Component: Driver/nouveau (show other bugs)
Version: unspecified
Hardware: Other All
: medium normal
Assignee: Nouveau Project
QA Contact: Xorg Project Team
URL:
Whiteboard:
Keywords:
Depends on:
Blocks:
 
Reported: 2015-07-17 14:46 UTC by Olivier Diotte
Modified: 2016-02-23 07:48 UTC (History)
0 users

See Also:
i915 platform:
i915 features:

Attachments

Description Olivier Diotte 2015-07-17 14:46:08 UTC 
On Ubuntu 14.04.2, with both the stock kernel and 3.19.0-22, nouveau fills kern.log with messages like these:

====================8< CUT HERE 8<=================================================================
$ tail -n 3000 kern.log.1 | head -n 50
Jul 10 18:02:35 dioo-XPS kernel: [114893.357149] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.406261] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.406269] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.420442] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.420447] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.479210] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.479217] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.568349] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.568356] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.622806] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.622813] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.637217] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.637221] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.695836] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.695843] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.784309] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.784316] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.838180] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.838187] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.853754] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.853759] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.913521] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.913528] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114893.977242] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114893.977249] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.068780] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.068788] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.122599] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.122606] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.137025] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.137030] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:35 dioo-XPS kernel: [114894.195675] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:35 dioo-XPS kernel: [114894.195682] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.284069] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.284078] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.339830] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.339837] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.353411] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.353416] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.412188] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.412204] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.501906] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.501913] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.558826] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.558831] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.570001] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.570006] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.628831] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
Jul 10 18:02:36 dioo-XPS kernel: [114894.628839] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[16558]] subc 5 mthd 0x0000 data 0x0000906e
Jul 10 18:02:36 dioo-XPS kernel: [114894.717193] nouveau E[ PFIFO][0000:01:00.0] PBDMA0: LBREQ
====================8< CUT HERE 8<==================================================================

On Kernel 4.2.0-rc2 (compiled based on the stock kernel's config), the computer just locks up:

I xscreensaver-command -lock, then logged back in (no issue there), then I xscreensaver-command -lock again and the computer just locked up. Caps lock light was unresponsive on the keyboard, Ctrl+Alt+Delete didn't work, RESIUB magic keys either.

After hard rebooting the computer, here is what what in kern.log and syslog:

Jul 16 17:19:45 dioo-XPS rtkit-daemon[2678]: Supervising 4 threads of 1 processes of 1 users.
Jul 16 17:27:15 dioo-XPS kernel: [ 619.825758] nouveau E[ PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
Jul 16 17:27:15 dioo-XPS kernel: [ 619.825764] nouveau E[ PFIFO][0000:01:00.0] PGRAPH engine fault on channel 2, recovering...
Jul 16 17:28:35 dioo-XPS dbus[894]: [system] Activating service name='org.freedesktop.systemd1' (using servicehelper)

The downstream bug is https://bugs.launchpad.net/ubuntu/+source/linux-lts-vivid/+bug/1474538
Comment 1 Olivier Diotte 2015-07-17 14:54:34 UTC 
Looks much better on v3.19.

The computer didn't lock up after 3 xscreensaver-command -lock.

Interestingly though, on the HDMI screen, the first time, the molecule wasn't rotating and error messages appeared. This was in dmesg:

[  131.177645] nouveau E[   PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
[  131.177651] nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 7, recovering...
[  160.403618] nouveau E[molecule[2408]] failed to idle channel 0xcccc0000 [molecule[2408]]
[  175.398186] nouveau E[molecule[2408]] failed to idle channel 0xcccc0000 [molecule[2408]]

Didn't get any error since, still testing.
Comment 2 Olivier Diotte 2015-07-17 16:27:04 UTC 
v3.19 is no go.

============8< CUT HERE 8<======================
$ grep 'nouveau' /var/log/syslog | head -n 100
Jul 17 10:48:09 dioo-XPS kernel: [    5.463520] fb: switching to nouveaufb from VESA VGA
Jul 17 10:48:09 dioo-XPS kernel: [    5.465859] nouveau  [  DEVICE][0000:01:00.0] BOOT0  : 0x108060a1
Jul 17 10:48:09 dioo-XPS kernel: [    5.465862] nouveau  [  DEVICE][0000:01:00.0] Chipset: GK208 (NV108)
Jul 17 10:48:09 dioo-XPS kernel: [    5.465863] nouveau  [  DEVICE][0000:01:00.0] Family : NVE0
Jul 17 10:48:09 dioo-XPS kernel: [    5.578886] nouveau  [   VBIOS][0000:01:00.0] using image from PROM
Jul 17 10:48:09 dioo-XPS kernel: [    5.579066] nouveau  [   VBIOS][0000:01:00.0] BIT signature found
Jul 17 10:48:09 dioo-XPS kernel: [    5.579067] nouveau  [   VBIOS][0000:01:00.0] version 80.28.56.00.01
Jul 17 10:48:09 dioo-XPS kernel: [    5.579752] nouveau  [     PMC][0000:01:00.0] MSI interrupts enabled
Jul 17 10:48:09 dioo-XPS kernel: [    5.579792] nouveau  [     PFB][0000:01:00.0] RAM type: DDR3
Jul 17 10:48:09 dioo-XPS kernel: [    5.579793] nouveau  [     PFB][0000:01:00.0] RAM size: 1024 MiB
Jul 17 10:48:09 dioo-XPS kernel: [    5.579794] nouveau  [     PFB][0000:01:00.0]    ZCOMP: 0 tags
Jul 17 10:48:09 dioo-XPS kernel: [    5.581061] nouveau  [    VOLT][0000:01:00.0] GPU voltage: 850000uv
Jul 17 10:48:10 dioo-XPS kernel: [    6.879231] nouveau  [  PTHERM][0000:01:00.0] FAN control: toggle
Jul 17 10:48:10 dioo-XPS kernel: [    6.879242] nouveau  [  PTHERM][0000:01:00.0] fan management: automatic
Jul 17 10:48:10 dioo-XPS kernel: [    6.879280] nouveau  [  PTHERM][0000:01:00.0] internal sensor: yes
Jul 17 10:48:10 dioo-XPS kernel: [    6.879321] nouveau  [     CLK][0000:01:00.0] 07: core 405 MHz memory 810 MHz 
Jul 17 10:48:10 dioo-XPS kernel: [    6.879354] nouveau  [     CLK][0000:01:00.0] 0f: core 967 MHz memory 2002 MHz 
Jul 17 10:48:10 dioo-XPS kernel: [    6.879414] nouveau  [     CLK][0000:01:00.0] --: core 405 MHz memory 810 MHz 
Jul 17 10:48:10 dioo-XPS kernel: [    6.943833] nouveau  [     DRM] VRAM: 1024 MiB
Jul 17 10:48:10 dioo-XPS kernel: [    6.943834] nouveau  [     DRM] GART: 1048576 MiB
Jul 17 10:48:10 dioo-XPS kernel: [    6.943837] nouveau  [     DRM] TMDS table version 2.0
Jul 17 10:48:10 dioo-XPS kernel: [    6.943838] nouveau  [     DRM] DCB version 4.0
Jul 17 10:48:10 dioo-XPS kernel: [    6.943839] nouveau  [     DRM] DCB outp 00: 01000f02 00020030
Jul 17 10:48:10 dioo-XPS kernel: [    6.943840] nouveau  [     DRM] DCB outp 01: 02011f62 00020010
Jul 17 10:48:10 dioo-XPS kernel: [    6.943841] nouveau  [     DRM] DCB outp 02: 02022f10 00000000
Jul 17 10:48:10 dioo-XPS kernel: [    6.943841] nouveau  [     DRM] DCB conn 00: 00001031
Jul 17 10:48:10 dioo-XPS kernel: [    6.943843] nouveau  [     DRM] DCB conn 01: 00002161
Jul 17 10:48:10 dioo-XPS kernel: [    6.943843] nouveau  [     DRM] DCB conn 02: 00000200
Jul 17 10:48:10 dioo-XPS kernel: [    6.949791] nouveau  [     DRM] MM: using COPY for buffer copies
Jul 17 10:48:11 dioo-XPS kernel: [    7.186686] nouveau  [     DRM] allocated 1920x1080 fb: 0x60000, bo ffff8802128c1c00
Jul 17 10:48:11 dioo-XPS kernel: [    7.186814] fbcon: nouveaufb (fb0) is primary device
Jul 17 10:48:11 dioo-XPS kernel: [    7.412257] nouveau 0000:01:00.0: fb0: nouveaufb frame buffer device
Jul 17 10:48:11 dioo-XPS kernel: [    7.412258] nouveau 0000:01:00.0: registered panic notifier
Jul 17 10:48:11 dioo-XPS kernel: [    7.415034] [drm] Initialized nouveau 1.2.2 20120801 for 0000:01:00.0 on minor 0
Jul 17 10:50:16 dioo-XPS kernel: [  131.177645] nouveau E[   PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
Jul 17 10:50:16 dioo-XPS kernel: [  131.177651] nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 7, recovering...
Jul 17 10:50:45 dioo-XPS kernel: [  160.403618] nouveau E[molecule[2408]] failed to idle channel 0xcccc0000 [molecule[2408]]
Jul 17 10:51:00 dioo-XPS kernel: [  175.398186] nouveau E[molecule[2408]] failed to idle channel 0xcccc0000 [molecule[2408]]
Jul 17 11:09:38 dioo-XPS kernel: [ 1293.243951] nouveau E[   PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
Jul 17 11:09:38 dioo-XPS kernel: [ 1293.243957] nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 6, recovering...
Jul 17 11:10:14 dioo-XPS kernel: [ 1328.576624] molecule[2657]: segfault at 38 ip 00007fbb4cdc8770 sp 00007ffd655351b8 error 4 in nouveau_dri.so[7fbb4cabd000+746000]
Jul 17 11:10:14 dioo-XPS kernel: [ 1328.812565] molecule[2658]: segfault at 38 ip 00007f15962d0770 sp 00007ffd1e3a8da8 error 4 in nouveau_dri.so[7f1595fc5000+746000]
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332138] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332145] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332160] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332163] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332176] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332178] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332194] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332196] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332211] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332215] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332234] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ACQUIRE
Jul 17 11:47:01 dioo-XPS kernel: [ 3535.332239] nouveau E[   PFIFO][0000:01:00.0] PBDMA0: ch 2 [Xorg[2343]] subc 0 mthd 0x001c data 0x00001004
==================8< CUT HERE 8<============================================
Comment 3 Ilia Mirkin 2015-07-17 16:35:20 UTC 
4.0 received some additional fixes to ctxsw logic. Please try that or 4.1.
Comment 4 Olivier Diotte 2015-07-20 14:09:27 UTC 
(In reply to Ilia Mirkin from comment #3)
> 4.0 received some additional fixes to ctxsw logic. Please try that or 4.1.

I compiled v4.1, but actually booted into 4.2.0-rc2 (derp), yet it didn't crash or write any error to the log files over the whole weekend.

I will keep using it and see if I can still reproduce, but right now, this appears to work for me.

I can still test v4.1 or v4.0 if you feel it would be useful.
Comment 5 Olivier Diotte 2015-07-20 16:02:47 UTC 
4.2 is no go, it just locked up again.

What is interesting is that the keyboard was unresponsive (Caps Lock light didn't turn on, RESIUB wouldn't work), neither would the ACPI power off (hitting the power button), yet the computer was still responsive to the network (apache was still serving Web pages, etc). I enabled SSH so that I can log in (or try to in any case) to the computer if it happens again.

I am now testing v4.1.
Comment 6 Olivier Diotte 2015-07-21 13:53:41 UTC 
v4.1.0 is no go:

Jul 20 12:55:17 dioo-XPS kernel: [ 3734.622246] usb 1-11: USB disconnect, device number 24
Jul 20 18:20:59 dioo-XPS kernel: [23270.087336] nouveau E[   PFIFO][0000:01:00.0] SCHED_ERROR [ CTXSW_TIMEOUT ]
Jul 20 18:20:59 dioo-XPS kernel: [23270.087341] nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 2, recovering...
Jul 21 08:05:11 dioo-XPS kernel: [72704.336625] audit: type=1400 audit(1437480311.763:67): apparmor="STATUS" operation="profile_replace" name="/usr/lib/cups/backend/cups-pdf" pid=4404 comm="apparmor_parser"

I'll try v4.0 (since I don't have much to lose by doing so), but I'm guessing there won't be much difference.
Comment 7 Olivier Diotte 2015-07-21 13:55:59 UTC 
One thing I did notice that I forgot to mention:

This morning, while the computer was locked up, I tried Ctrl+Alt+Delete, RESIUB then the ACPI power off and then tried to SSH into the box.

The computer turned off after several minutes which seems to indicate that the ACPI power off worked, albeit very slowly (and without updating the screen).
Comment 8 Olivier Diotte 2015-07-28 14:02:42 UTC 
For anyone struggling with this bug, I haven't gotten a crash yet with Ubuntu 14.04's linux-image-3.13.0-58-generic kernel (Ubuntu version 3.13.0-58.97) over the last four days.

Maybe it'll reproduce again, but at the very this kernel version looks much more stable.
Comment 9 Olivier Diotte 2015-07-30 19:41:45 UTC 
After a week's worth of usage on kernel 3.13.0, I'm confident enough to assert that that kernel version is usable.
Comment 10 Christopher M. Penalver 2016-02-23 07:48:39 UTC 
Original reporter hasn't responded to most recent comment from https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1474538 .

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