staging: vchiq_arm: Add missing memory barrier comments

One of points on the TODO list was to comment the memory barriers.
This patch addresses the missing ones in order to help reviewers.

Link: https://github.com/raspberrypi/linux/pull/5066Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/20220624163902.6913-1-stefan.wahren@i2se.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c

@@ -602,6 +602,10 @@ static struct vchiq_arm_state *vchiq_platform_get_arm_state(struct vchiq_state *
 void
 remote_event_signal(struct remote_event *event)
 {
+	/*
+	 * Ensure that all writes to shared data structures have completed
+	 * before signalling the peer.
+	 */
 	wmb();
 
 	event->fired = 1;

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c

@@ -528,6 +528,7 @@ remote_event_wait(wait_queue_head_t *wq, struct remote_event *event)
 			return 0;
 		}
 		event->armed = 0;
+		/* Ensure that the peer sees that we are not waiting (armed == 0). */
 		wmb();
 	}
 
@@ -650,6 +651,7 @@ request_poll(struct vchiq_state *state, struct vchiq_service *service,
 
 skip_service:
 	state->poll_needed = 1;
+	/* Ensure the slot handler thread sees the poll_needed flag. */
 	wmb();
 
 	/* ... and ensure the slot handler runs. */
@@ -1156,6 +1158,7 @@ queue_message_sync(struct vchiq_state *state, struct vchiq_service *service,
 
 	remote_event_wait(&state->sync_release_event, &local->sync_release);
 
+	/* Ensure that reads don't overtake the remote_event_wait. */
 	rmb();
 
 	header = (struct vchiq_header *)SLOT_DATA_FROM_INDEX(state,
@@ -1959,6 +1962,7 @@ slot_handler_func(void *v)
 		DEBUG_TRACE(SLOT_HANDLER_LINE);
 		remote_event_wait(&state->trigger_event, &local->trigger);
 
+		/* Ensure that reads don't overtake the remote_event_wait. */
 		rmb();
 
 		DEBUG_TRACE(SLOT_HANDLER_LINE);
@@ -2021,6 +2025,7 @@ sync_func(void *v)
 
 		remote_event_wait(&state->sync_trigger_event, &local->sync_trigger);
 
+		/* Ensure that reads don't overtake the remote_event_wait. */
 		rmb();
 
 		msgid = header->msgid;
@@ -3081,6 +3086,10 @@ enum vchiq_status vchiq_bulk_transfer(struct vchiq_instance *instance, unsigned
 	if (vchiq_prepare_bulk_data(instance, bulk, offset, uoffset, size, dir))
 		goto unlock_error_exit;
 
+	/*
+	 * Ensure that the bulk data record is visible to the peer
+	 * before proceeding.
+	 */
 	wmb();
 
 	vchiq_log_info(vchiq_core_log_level, "%d: bt (%d->%d) %cx %x@%pad %pK",