drm/i915/execlists: Add a paranoid flush of the CSB pointers upon reset

After a GPU reset, we need to drain all the CS events so that we have an
accurate picture of the execlists state at the time of the reset. Be
paranoid and force a read of the CSB write pointer from memory.
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/20190912092933.4729-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -2359,6 +2359,10 @@ static void __execlists_reset(struct intel_engine_cs *engine, bool stalled)
 	struct i915_request *rq;
 	u32 *regs;
 
+	mb(); /* paranoia: read the CSB pointers from after the reset */
+	clflush(execlists->csb_write);
+	mb();
+
 	process_csb(engine); /* drain preemption events */
 
 	/* Following the reset, we need to reload the CSB read/write pointers */