drm/i915: Flush everything on switching to the kernel_context

Even though all rendering should have been flushed at the end of the
previous requests, add an extra flush after switching to the
kernel_context. As the switch to the kernel_context is used when idling
the gpu (e.g. suspend), having an extra layer of paranoia to ensure
everything is flushed to memory seems sensible.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171126214856.23702-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -620,7 +620,14 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv)
 								 GFP_KERNEL);
 		}
 
-		i915_add_request(req);
+		/*
+		 * Force a flush after the switch to ensure that all rendering
+		 * and operations prior to switching to the kernel context hits
+		 * memory. This should be guaranteed by the previous request,
+		 * but an extra layer of paranoia before we declare the system
+		 * idle (on suspend etc) is advisable!
+		 */
+		__i915_add_request(req, true);
 	}
 
 	return 0;