FAILED: patch "[PATCH] drm/i915/gt: Flush xcs before tgl breadcrumbs" failed to apply to 5.9-stable tree

The patch below does not apply to the 5.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e67d01d8494640018b08cd767aeb2824a8e11983 Mon Sep 17 00:00:00 2001
From: Chris Wilson <[email protected]>
Date: Mon, 2 Nov 2020 22:10:57 +0000
Subject: [PATCH] drm/i915/gt: Flush xcs before tgl breadcrumbs

In a simple test case that writes to scratch and then busy-waits for the batch to be signaled, we observe that the signal is before the write is posted. That is bad news.

Splitting the flush + write_dword into two separate flush_dw prevents the issue from being reproduced, we can presume the post-sync op is not so post-sync.

Testcase: igt/gem_exec_fence/parallel
Testcase: igt/i915_selftest/live/gt_timelines

index d12861cf0753..f82c6dd1de18 100644
@@ -4994,7 +4994,9 @@ gen12_emit_fini_breadcrumb_tail(struct i915_request *request, u32 *cs)
 
 static u32 *gen12_emit_fini_breadcrumb(struct i915_request *rq, u32 *cs)
 {
-	return gen12_emit_fini_breadcrumb_tail(rq, emit_xcs_breadcrumb(rq, cs));
+	/* XXX Stalling flush before seqno write; post-sync not */
+	cs = emit_xcs_breadcrumb(rq, __gen8_emit_flush_dw(cs, 0, 0, 0));
+	return gen12_emit_fini_breadcrumb_tail(rq, cs);
 }
 
 static u32 *