drm/i915: Flush the ring stop bit after clearing RING_HEAD in reset

drm/i915: Flush the ring stop bit after clearing RING_HEAD in reset

Inside the live_hangcheck (reset) selftests, we occasionally see
failures like

<7>[  239.094840] i915_gem_set_wedged rcs0
<7>[  239.094843] i915_gem_set_wedged 	current seqno 19a98, last 19a9a, hangcheck 0 [5158 ms]
<7>[  239.094846] i915_gem_set_wedged 	Reset count: 6239 (global 1)
<7>[  239.094848] i915_gem_set_wedged 	Requests:
<7>[  239.095052] i915_gem_set_wedged 		first  19a99 [e8c:5f] prio=1024 @ 5159ms: (null)
<7>[  239.095056] i915_gem_set_wedged 		last   19a9a [e81:1a] prio=139 @ 5159ms: igt/rcs0[5977]/1
<7>[  239.095059] i915_gem_set_wedged 		active 19a99 [e8c:5f] prio=1024 @ 5159ms: (null)
<7>[  239.095062] i915_gem_set_wedged 		[head 0220, postfix 0280, tail 02a8, batch 0xffffffff_ffffffff]
<7>[  239.100050] i915_gem_set_wedged 		ring->start:  0x00283000
<7>[  239.100053] i915_gem_set_wedged 		ring->head:   0x000001f8
<7>[  239.100055] i915_gem_set_wedged 		ring->tail:   0x000002a8
<7>[  239.100057] i915_gem_set_wedged 		ring->emit:   0x000002a8
<7>[  239.100059] i915_gem_set_wedged 		ring->space:  0x00000f10
<7>[  239.100085] i915_gem_set_wedged 	RING_START: 0x00283000
<7>[  239.100088] i915_gem_set_wedged 	RING_HEAD:  0x00000260
<7>[  239.100091] i915_gem_set_wedged 	RING_TAIL:  0x000002a8
<7>[  239.100094] i915_gem_set_wedged 	RING_CTL:   0x00000001
<7>[  239.100097] i915_gem_set_wedged 	RING_MODE:  0x00000300 [idle]
<7>[  239.100100] i915_gem_set_wedged 	RING_IMR: fffffefe
<7>[  239.100104] i915_gem_set_wedged 	ACTHD:  0x00000000_0000609c
<7>[  239.100108] i915_gem_set_wedged 	BBADDR: 0x00000000_0000609d
<7>[  239.100111] i915_gem_set_wedged 	DMA_FADDR: 0x00000000_00283260
<7>[  239.100114] i915_gem_set_wedged 	IPEIR: 0x00000000
<7>[  239.100117] i915_gem_set_wedged 	IPEHR: 0x02800000
<7>[  239.100120] i915_gem_set_wedged 	Execlist status: 0x00044052 00000002
<7>[  239.100124] i915_gem_set_wedged 	Execlist CSB read 5 [5 cached], write 5 [5 from hws], interrupt posted? no, tasklet queued? no (enabled)
<7>[  239.100128] i915_gem_set_wedged 		ELSP[0] count=1, ring->start=00283000, rq: 19a99 [e8c:5f] prio=1024 @ 5164ms: (null)
<7>[  239.100132] i915_gem_set_wedged 		ELSP[1] count=1, ring->start=00257000, rq: 19a9a [e81:1a] prio=139 @ 5164ms: igt/rcs0[5977]/1
<7>[  239.100135] i915_gem_set_wedged 		HW active? 0x5
<7>[  239.100250] i915_gem_set_wedged 		E 19a99 [e8c:5f] prio=1024 @ 5164ms: (null)
<7>[  239.100338] i915_gem_set_wedged 		E 19a9a [e81:1a] prio=139 @ 5164ms: igt/rcs0[5977]/1
<7>[  239.100340] i915_gem_set_wedged 		Queue priority: 139
<7>[  239.100343] i915_gem_set_wedged 		Q 0 [e98:19] prio=132 @ 5164ms: igt/rcs0[5977]/8
<7>[  239.100346] i915_gem_set_wedged 		Q 0 [e84:19] prio=121 @ 5165ms: igt/rcs0[5977]/2
<7>[  239.100349] i915_gem_set_wedged 		Q 0 [e87:19] prio=82 @ 5165ms: igt/rcs0[5977]/3
<7>[  239.100352] i915_gem_set_wedged 		Q 0 [e84:1a] prio=44 @ 5164ms: igt/rcs0[5977]/2
<7>[  239.100356] i915_gem_set_wedged 		Q 0 [e8b:19] prio=20 @ 5165ms: igt/rcs0[5977]/4
<7>[  239.100362] i915_gem_set_wedged 	drv_selftest [5894] waiting for 19a99

where the GPU saw an arbitration point and idles; AND HAS NOT BEEN RESET!
The RING_MODE indicates that is idle and has the STOP_RING bit set, so
try clearing it.

v2: Only clear the bit on restarting the ring, as we want to be sure the
STOP_RING bit is kept if reset fails on wedging.
v3: Spot when the ring state doesn't make sense when re-initialising the
engine and dump it to the logs so that we don't have to wait for an
error later and try to guess what happened earlier.
v4: Prepare to print all the unexpected state, not just the first.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518100933.2239-1-chris@chris-wilson.co.uk