From 21e8860ef467001330368eceeb855fde46ab1dec Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Winiarski?= <michal.winiarski@intel.com>
Date: Wed, 25 Oct 2017 22:00:09 +0200
Subject: [PATCH] drm/i915/guc: Do not use 0 for GuC doorbell cookie
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Git-commit: 21e8860ef467001330368eceeb855fde46ab1dec
Patch-mainline: v4.16-rc1
References: FATE#322643 bsc#1055900

Apparently, this value is reserved and may be interpreted as changing
doorbell ownership. Even though we're not observing any side effects
now, let's skip over it to be consistent with the spec.

V2: Apply checkpatch (Sagar)

Suggested-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171025200020.16636-2-michal.winiarski@intel.com
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/gpu/drm/i915/i915_guc_submission.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/i915/i915_guc_submission.c
+++ b/drivers/gpu/drm/i915/i915_guc_submission.c
@@ -475,9 +475,12 @@ static void guc_ring_doorbell(struct i91
 	/* pointer of current doorbell cacheline */
 	db = __get_doorbell(client);
 
-	/* we're not expecting the doorbell cookie to change behind our back */
+	/*
+	 * We're not expecting the doorbell cookie to change behind our back,
+	 * we also need to treat 0 as a reserved value.
+	 */
 	cookie = READ_ONCE(db->cookie);
-	WARN_ON_ONCE(xchg(&db->cookie, cookie + 1) != cookie);
+	WARN_ON_ONCE(xchg(&db->cookie, cookie + 1 ?: cookie + 2) != cookie);
 
 	/* XXX: doorbell was lost and need to acquire it again */
 	GEM_BUG_ON(db->db_status != GUC_DOORBELL_ENABLED);