drm/i915/gt: Reset twice

After applying an engine reset, on some platforms like Jasperlake, we
occasionally detect that the engine state is not cleared until shortly
after the resume. As we try to resume the engine with volatile internal
state, the first request fails with a spurious CS event (it looks like
it reports a lite-restore to the hung context, instead of the expected
idle->active context switch).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221212161338.1007659-1-andi.shyti@linux.intel.com

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

@@ -268,6 +268,7 @@ static int ilk_do_reset(struct intel_gt *gt, intel_engine_mask_t engine_mask,
 static int gen6_hw_domain_reset(struct intel_gt *gt, u32 hw_domain_mask)
 {
 	struct intel_uncore *uncore = gt->uncore;
+	int loops = 2;
 	int err;
 
 	/*
@@ -275,18 +276,39 @@ static int gen6_hw_domain_reset(struct intel_gt *gt, u32 hw_domain_mask)
 	 * for fifo space for the write or forcewake the chip for
 	 * the read
 	 */
-	intel_uncore_write_fw(uncore, GEN6_GDRST, hw_domain_mask);
+	do {
+		intel_uncore_write_fw(uncore, GEN6_GDRST, hw_domain_mask);
 
-	/* Wait for the device to ack the reset requests */
-	err = __intel_wait_for_register_fw(uncore,
-					   GEN6_GDRST, hw_domain_mask, 0,
-					   500, 0,
-					   NULL);
+		/*
+		 * Wait for the device to ack the reset requests.
+		 *
+		 * On some platforms, e.g. Jasperlake, we see that the
+		 * engine register state is not cleared until shortly after
+		 * GDRST reports completion, causing a failure as we try
+		 * to immediately resume while the internal state is still
+		 * in flux. If we immediately repeat the reset, the second
+		 * reset appears to serialise with the first, and since
+		 * it is a no-op, the registers should retain their reset
+		 * value. However, there is still a concern that upon
+		 * leaving the second reset, the internal engine state
+		 * is still in flux and not ready for resuming.
+		 */
+		err = __intel_wait_for_register_fw(uncore, GEN6_GDRST,
+						   hw_domain_mask, 0,
+						   2000, 0,
+						   NULL);
+	} while (err == 0 && --loops);
 	if (err)
 		GT_TRACE(gt,
 			 "Wait for 0x%08x engines reset failed\n",
 			 hw_domain_mask);
 
+	/*
+	 * As we have observed that the engine state is still volatile
+	 * after GDRST is acked, impose a small delay to let everything settle.
+	 */
+	udelay(50);
+
 	return err;
 }