From 110f9efa858f584c6bed177cd48d0c0f526940e1 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Mon, 13 Jul 2020 17:05:49 +0100
Subject: drm/i915/gt: Only swap to a random sibling once upon creation
Git-commit: 110f9efa858f584c6bed177cd48d0c0f526940e1
Patch-mainline: v5.8-rc6
References: bsc#1152489

The danger in switching at random upon intel_context_pin is that the
context may still actually be inflight, as it will not be scheduled out
until a context switch after it is complete -- that may be a long time
after we do a final intel_context_unpin.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2118
Fixes: 6d06779e8672 ("drm/i915: Load balancing across a virtual engine")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.3+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713160549.17344-1-chris@chris-wilson.co.uk
(cherry picked from commit 90a987205c6cf74116a102ed446d22d92cdaf915)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c |   17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -3479,26 +3479,16 @@ static void virtual_engine_initial_hint(
 	 * typically be the first we inspect for submission.
 	 */
 	swp = prandom_u32_max(ve->num_siblings);
-	if (!swp)
-		return;
-
-	swap(ve->siblings[swp], ve->siblings[0]);
-	virtual_update_register_offsets(ve->context.lrc_reg_state,
-					ve->siblings[0]);
+	if (swp)
+		swap(ve->siblings[swp], ve->siblings[0]);
 }
 
 static int virtual_context_pin(struct intel_context *ce)
 {
 	struct virtual_engine *ve = container_of(ce, typeof(*ve), context);
-	int err;
 
 	/* Note: we must use a real engine class for setting up reg state */
-	err = __execlists_context_pin(ce, ve->siblings[0]);
-	if (err)
-		return err;
-
-	virtual_engine_initial_hint(ve);
-	return 0;
+	return __execlists_context_pin(ce, ve->siblings[0]);
 }
 
 static void virtual_context_enter(struct intel_context *ce)
@@ -3851,6 +3841,7 @@ intel_execlists_create_virtual(struct i9
 
 	__set_bit(CONTEXT_ALLOC_BIT, &ve->context.flags);
 
+	virtual_engine_initial_hint(ve);
 	return &ve->context;
 
 err_put: