drm/i915/gt: Sanitize GT first

We see that if the HW doesn't actually sleep, the HW may eat the poison
we set in its write-only HWSP during sanitize:

  intel_gt_resume.part.8: 0000:00:02.0
  __gt_unpark: 0000:00:02.0
  gt_sanitize: 0000:00:02.0 force:yes
  process_csb: 0000:00:02.0 vcs0: cs-irq head=5, tail=90
  process_csb: 0000:00:02.0 vcs0: csb[0]: status=0x5a5a5a5a:0x5a5a5a5a
  assert_pending_valid: Nothing pending for promotion!

The CS TAIL pointer should have been reset by reset_csb_pointers(), so
in this case it is likely that we have read back from the CPU cache and
so we must clflush our control over that page. In doing so, push the
sanitisation to the start of the GT sequence so that our poisoning is
assuredly before we start talking to the HW.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1794
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200427084000.10999-1-chris@chris-wilson.co.uk

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

@@ -198,11 +198,12 @@ int intel_gt_resume(struct intel_gt *gt)
 	 * Only the kernel contexts should remain pinned over suspend,
 	 * allowing us to fixup the user contexts on their first pin.
 	 */
+	gt_sanitize(gt, true);
+
 	intel_gt_pm_get(gt);
 
 	intel_uncore_forcewake_get(gt->uncore, FORCEWAKE_ALL);
 	intel_rc6_sanitize(&gt->rc6);
-	gt_sanitize(gt, true);
 	if (intel_gt_is_wedged(gt)) {
 		err = -EIO;
 		goto out_fw;

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

@@ -3931,6 +3931,9 @@ static void execlists_sanitize(struct intel_engine_cs *engine)
 	 * reset the value in the HWSP.
 	 */
 	intel_timeline_reset_seqno(engine->kernel_context->timeline);
+
+	/* And scrub the dirty cachelines for the HWSP */
+	clflush_cache_range(engine->status_page.addr, PAGE_SIZE);
 }
 
 static void enable_error_interrupt(struct intel_engine_cs *engine)