drm/i915: Clear stop-engine for a pardoned reset

If we pardon a per-engine reset, we may leave the STOP_RING bit asserted
in RING_MI_MODE resulting in the engine hanging. Unconditionally clear
it on the per-engine exit path as we know that either we skipped the
reset and so need the cancellation, or the reset was successful and the
cancellation is a no-op, or there was an error and we will follow up
with a full-reset or wedging (both of which will stop the engines again
as required).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107188
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106560
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180814171857.24673-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.c

@@ -2079,6 +2079,7 @@ int i915_reset_engine(struct intel_engine_cs *engine, const char *msg)
 		goto out;
 
 out:
+	intel_engine_cancel_stop_cs(engine);
 	i915_gem_reset_finish_engine(engine);
 	return ret;
 }

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -788,6 +788,16 @@ int intel_engine_stop_cs(struct intel_engine_cs *engine)
 	return err;
 }
 
+void intel_engine_cancel_stop_cs(struct intel_engine_cs *engine)
+{
+	struct drm_i915_private *dev_priv = engine->i915;
+
+	GEM_TRACE("%s\n", engine->name);
+
+	I915_WRITE_FW(RING_MI_MODE(engine->mmio_base),
+		      _MASKED_BIT_DISABLE(STOP_RING));
+}
+
 const char *i915_cache_level_str(struct drm_i915_private *i915, int type)
 {
 	switch (type) {

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -906,6 +906,7 @@ int intel_init_blt_ring_buffer(struct intel_engine_cs *engine);
 int intel_init_vebox_ring_buffer(struct intel_engine_cs *engine);
 
 int intel_engine_stop_cs(struct intel_engine_cs *engine);
+void intel_engine_cancel_stop_cs(struct intel_engine_cs *engine);
 
 u64 intel_engine_get_active_head(const struct intel_engine_cs *engine);
 u64 intel_engine_get_last_batch_head(const struct intel_engine_cs *engine);