drm/xe/ct: hold fast_lock when reserving space for g2h

Reserving and checking for space on the g2h side relies on the
fast_lock, and not the CT lock since we need to release space from the
fast CT path. Make sure we hold it when checking for space and reserving
it. The main concern is calling __g2h_release_space() as we are reserving
something and since the info.space and info.g2h_outstanding operations
are not atomic we can get some nonsense values back.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_guc_ct.c

@@ -346,7 +346,10 @@ static bool h2g_has_room(struct xe_guc_ct *ct, u32 cmd_len)
 
 static bool g2h_has_room(struct xe_guc_ct *ct, u32 g2h_len)
 {
-	lockdep_assert_held(&ct->lock);
+	if (!g2h_len)
+		return true;
+
+	lockdep_assert_held(&ct->fast_lock);
 
 	return ct->ctbs.g2h.info.space > g2h_len;
 }
@@ -367,15 +370,15 @@ static void h2g_reserve_space(struct xe_guc_ct *ct, u32 cmd_len)
 	ct->ctbs.h2g.info.space -= cmd_len;
 }
 
-static void g2h_reserve_space(struct xe_guc_ct *ct, u32 g2h_len, u32 num_g2h)
+static void __g2h_reserve_space(struct xe_guc_ct *ct, u32 g2h_len, u32 num_g2h)
 {
 	XE_BUG_ON(g2h_len > ct->ctbs.g2h.info.space);
 
 	if (g2h_len) {
-		spin_lock_irq(&ct->fast_lock);
+		lockdep_assert_held(&ct->fast_lock);
+
 		ct->ctbs.g2h.info.space -= g2h_len;
 		ct->g2h_outstanding += num_g2h;
-		spin_unlock_irq(&ct->fast_lock);
 	}
 }
 
@@ -505,21 +508,26 @@ static int __guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action,
 		}
 	}
 
+	if (g2h_len)
+		spin_lock_irq(&ct->fast_lock);
 retry:
 	ret = has_room(ct, len + GUC_CTB_HDR_LEN, g2h_len);
 	if (unlikely(ret))
-		goto out;
+		goto out_unlock;
 
 	ret = h2g_write(ct, action, len, g2h_fence ? g2h_fence->seqno : 0,
 			!!g2h_fence);
 	if (unlikely(ret)) {
 		if (ret == -EAGAIN)
 			goto retry;
-		goto out;
+		goto out_unlock;
 	}
 
-	g2h_reserve_space(ct, g2h_len, num_g2h);
+	__g2h_reserve_space(ct, g2h_len, num_g2h);
 	xe_guc_notify(ct_to_guc(ct));
+out_unlock:
+	if (g2h_len)
+		spin_unlock_irq(&ct->fast_lock);
 out:
 	return ret;
 }