The same here, previous kernel 3.5.0-25-generic works without problems, 3.5.0-26.42 hanged just now:
$ dmesg|grep i915
[ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.5.0-26-generic root=/dev/mapper/System-root ro quiet splash i915.i915_enable_rc6=1 vt.handoff=7
[ 0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.5.0-26-generic root=/dev/mapper/System-root ro quiet splash i915.i915_enable_rc6=1 vt.handoff=7
[ 1.667363] i915 0000:00:02.0: power state changed by ACPI to D0
[ 1.667367] i915 0000:00:02.0: power state changed by ACPI to D0
[ 1.667652] i915 0000:00:02.0: setting latency timer to 64
[ 1.687950] i915 0000:00:02.0: irq 44 for MSI/MSI-X
[ 2.429882] [drm] Initialized i915 1.6.0 20080730 for 0000:00:02.0 on minor 0
[ 330.684154] i915 0000:00:02.0: power state changed by ACPI to D3
[ 331.826825] i915 0000:00:02.0: power state changed by ACPI to D0
[ 331.826829] i915 0000:00:02.0: power state changed by ACPI to D0
[ 331.826830] i915 0000:00:02.0: setting latency timer to 64
[ 1677.075872] [drm:i915_hangcheck_hung] *ERROR* Hangcheck timer elapsed... GPU hung
[ 1677.075876] [drm] capturing error event; look for more information in /debug/dri/0/i915_error_state
The same here, previous kernel 3.5.0-25-generic works without problems, 3.5.0-26.42 hanged just now:
$ dmesg|grep i915 /vmlinuz- 3.5.0-26- generic root=/dev/ mapper/ System- root ro quiet splash i915.i915_ enable_ rc6=1 vt.handoff=7 /vmlinuz- 3.5.0-26- generic root=/dev/ mapper/ System- root ro quiet splash i915.i915_ enable_ rc6=1 vt.handoff=7 hangcheck_ hung] *ERROR* Hangcheck timer elapsed... GPU hung dri/0/i915_ error_state
[ 0.000000] Command line: BOOT_IMAGE=
[ 0.000000] Kernel command line: BOOT_IMAGE=
[ 1.667363] i915 0000:00:02.0: power state changed by ACPI to D0
[ 1.667367] i915 0000:00:02.0: power state changed by ACPI to D0
[ 1.667652] i915 0000:00:02.0: setting latency timer to 64
[ 1.687950] i915 0000:00:02.0: irq 44 for MSI/MSI-X
[ 2.429882] [drm] Initialized i915 1.6.0 20080730 for 0000:00:02.0 on minor 0
[ 330.684154] i915 0000:00:02.0: power state changed by ACPI to D3
[ 331.826825] i915 0000:00:02.0: power state changed by ACPI to D0
[ 331.826829] i915 0000:00:02.0: power state changed by ACPI to D0
[ 331.826830] i915 0000:00:02.0: setting latency timer to 64
[ 1677.075872] [drm:i915_
[ 1677.075876] [drm] capturing error event; look for more information in /debug/
Content of i915_error_state attached.
Will disable rc6 and test what happens then.