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            <b><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - [BDW]igt/drv_missed_irq_hang sporadically causes *ERROR* Unclaimed register detected"
   href="https://bugs.freedesktop.org/show_bug.cgi?id=85890#c4">Comment # 4</a>
              on <a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - [BDW]igt/drv_missed_irq_hang sporadically causes *ERROR* Unclaimed register detected"
   href="https://bugs.freedesktop.org/show_bug.cgi?id=85890">bug 85890</a>
              from <span class="vcard"><a class="email" href="mailto:huax.lu@intel.com" title="lu hua <huax.lu@intel.com>"> <span class="fn">lu hua</span></a>
</span></b>
        <pre>Created <span class=""><a href="attachment.cgi?id=110109" name="attach_110109" title="dmesg">attachment 110109</a> <a href="attachment.cgi?id=110109&action=edit" title="dmesg">[details]</a></span>
dmesg

I think I reproduce it. Add i915.mmio_debug=1, run ./tools_test then run
./drv_missed_irq_hang, system reports warning. 

root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests#
./drv_missed_irq_hang
Interrupts masked
Interrupts unmasked
Cleared missed interrupts
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# dmesg -r|egrep
"<[1-4]>"|grep drm
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# ./tools_test
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# dmesg -r|egrep
"<[1-4]>"|grep drm
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests#
./drv_missed_irq_hang
Interrupts masked
Interrupts unmasked
Cleared missed interrupts
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# dmesg -r|egrep
"<[1-4]>"|grep drm
<4>[   51.940478] WARNING: CPU: 2 PID: 4735 at
drivers/gpu/drm/i915/intel_uncore.c:726
hsw_unclaimed_reg_debug.isra.12+0x62/0x72 [i915]()
<4>[   51.940480] Modules linked in: dm_mod iTCO_wdt iTCO_vendor_support ppdev
snd_hda_codec_hdmi pcspkr i2c_i801 snd_hda_intel snd_hda_controller
snd_hda_codec snd_hwdep snd_pcm snd_timer lpc_ich mfd_core snd soundcore
battery parport_pc parport ac acpi_cpufreq i915 button video drm_kms_helper drm
cfbfillrect cfbimgblt cfbcopyarea
<4>[   51.940496] CPU: 2 PID: 4735 Comm: gem_exec_blt Not tainted
3.18.0-rc6_drm-intel-nightly_904b85_20141127+ #1990
<4>[   51.940628]  [<ffffffffa0010738>] ? drm_ioctl+0x279/0x3bc [drm]

Call trace: 
[   51.940478] WARNING: CPU: 2 PID: 4735 at
drivers/gpu/drm/i915/intel_uncore.c:726
hsw_unclaimed_reg_debug.isra.12+0x62/0x72 [i915]()
[   51.940479] Unclaimed register detected before writing to register 0x101008
[   51.940480] Modules linked in: dm_mod iTCO_wdt iTCO_vendor_support ppdev
snd_hda_codec_hdmi pcspkr i2c_i801 snd_hda_intel snd_hda_controller
snd_hda_codec snd_hwdep snd_pcm s
nd_timer lpc_ich mfd_core snd soundcore battery parport_pc parport ac
acpi_cpufreq i915 button video drm_kms_helper drm cfbfillrect cfbimgblt
cfbcopyarea
[   51.940496] CPU: 2 PID: 4735 Comm: gem_exec_blt Not tainted
3.18.0-rc6_drm-intel-nightly_904b85_20141127+ #1990
[   51.940498]  0000000000000000 0000000000000009 ffffffff8178d582
ffff880148a5fa08
[   51.940500]  ffffffff8103a8cc ffff8800a3e954c0 ffffffffa00c0850
ffff8800a7e2de90
[   51.940502]  ffff880002d10060 0000000000101008 0000000000101008
0000000000000001
[   51.940504] Call Trace:
[   51.940510]  [<ffffffff8178d582>] ? dump_stack+0x41/0x51
[   51.940514]  [<ffffffff8103a8cc>] ? warn_slowpath_common+0x78/0x90
[   51.940524]  [<ffffffffa00c0850>] ?
hsw_unclaimed_reg_debug.isra.12+0x62/0x72 [i915]
[   51.940527]  [<ffffffff8103a97c>] ? warn_slowpath_fmt+0x45/0x4a
[   51.940536]  [<ffffffffa00c0850>] ?
hsw_unclaimed_reg_debug.isra.12+0x62/0x72 [i915]
[   51.940545]  [<ffffffffa00c1239>] ? gen8_write32+0x8e/0x11a [i915]
[   51.940554]  [<ffffffffa009bffd>] ? gen8_ggtt_insert_entries+0xf7/0x116
[i915]
[   51.940562]  [<ffffffffa009b66c>] ? ggtt_bind_vma+0x75/0xc8 [i915]
[   51.940571]  [<ffffffffa00a319a>] ? i915_gem_object_pin+0x4bb/0x6d1 [i915]
[   51.940580]  [<ffffffffa0097e36>] ? i915_switch_context+0x180/0x556 [i915]
[   51.940589]  [<ffffffffa00bc7e0>] ?
intel_ring_invalidate_all_caches+0x25/0x7b [i915]
[   51.940597]  [<ffffffffa009a94d>] ?
i915_gem_ringbuffer_submission+0x21b/0x7ea [i915]
[   51.940605]  [<ffffffffa009a3bd>] ?
i915_gem_do_execbuffer.isra.12+0xaf0/0xbcf [i915]
[   51.940614]  [<ffffffffa009f2da>] ? i915_gem_object_get_pages+0x5d/0x99
[i915]
[   51.940622]  [<ffffffffa009b3df>] ? i915_gem_execbuffer2+0x173/0x208 [i915]
[   51.940628]  [<ffffffffa0010738>] ? drm_ioctl+0x279/0x3bc [drm]
[   51.940635]  [<ffffffffa009b26c>] ? i915_gem_execbuffer+0x350/0x350 [i915]
[   51.940639]  [<ffffffff8111e6e3>] ? do_vfs_ioctl+0x3ec/0x435
[   51.940642]  [<ffffffff8104e839>] ? task_work_run+0x78/0x87
[   51.940644]  [<ffffffff8111e775>] ? SyS_ioctl+0x49/0x78
[   51.940647]  [<ffffffff81792e7f>] ? int_signal+0x12/0x17
[   51.940649]  [<ffffffff81792c12>] ? system_call_fastpath+0x12/0x17
[   51.940651] ---[ end trace fbd493ba277c3c86 ]---</pre>
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