Occasionally gpu hangs occur on my system, but the specified gpu crash dump file is always empty.

Linux version: 4.1.2-2, SMP PREEMPT, x86_64 GNU/Linux

$ dmesg | grep drm
[  +0,010497] [drm] Initialized drm 1.1.0 20060810
[  +0,005761] [drm] Memory usable by graphics device = 2048M
[  +0,000001] fb: switching to inteldrmfb from EFI VGA
[  +0,000070] [drm] Replacing VGA console driver
[  +0,006563] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[  +0,000003] [drm] Driver supports precise vblank timestamp query.
[  +0,000039] [drm] Initialized i915 1.6.0 20150327 for 0000:00:02.0 on minor 0
[  +0,010486] fbcon: inteldrmfb (fb0) is primary device
[  +0,004627] i915 0000:00:02.0: fb0: inteldrmfb frame buffer device
[Jul21 18:13] [drm] stuck on blitter ring
[  +0,000633] [drm] GPU HANG: ecode 6:2:0x000097f7, in Xorg [417], reason: Ring hung, action: reset
[  +0,000001] [drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
[  +0,000001] [drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
[  +0,000001] [drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
[  +0,000001] [drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
[  +0,000001] [drm] GPU crash dump saved to /sys/class/drm/card0/error
[  +0,502197] drm/i915: Resetting chip after gpu hang
[  +0,000014] [drm:i915_reset [i915]] *ERROR* Failed to reset chip: -110