staging: vchiq_core: Add comments to remote event parts

struct remote_event and the related functions remote_event_*() provides
a higher function between ARM core and VPU. It's very helpful for a
reviewer to have explaining comments about these parts.
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/20221222141553.138563-1-stefan.wahren@i2se.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -498,6 +498,7 @@ vchiq_set_conn_state(struct vchiq_state *state, enum vchiq_connstate newstate)
 	vchiq_platform_conn_state_changed(state, oldstate, newstate);
 }
 
+/* This initialises a single remote_event, and the associated wait_queue. */
 static inline void
 remote_event_create(wait_queue_head_t *wq, struct remote_event *event)
 {
@@ -536,6 +537,10 @@ remote_event_wait(wait_queue_head_t *wq, struct remote_event *event)
 	return 1;
 }
 
+/*
+ * Acknowledge that the event has been signalled, and wake any waiters. Usually
+ * called as a result of the doorbell being rung.
+ */
 static inline void
 remote_event_signal_local(wait_queue_head_t *wq, struct remote_event *event)
 {
@@ -544,6 +549,7 @@ remote_event_signal_local(wait_queue_head_t *wq, struct remote_event *event)
 	wake_up_all(wq);
 }
 
+/* Check if a single event has been signalled, waking the waiters if it has. */
 static inline void
 remote_event_poll(wait_queue_head_t *wq, struct remote_event *event)
 {
@@ -551,6 +557,10 @@ remote_event_poll(wait_queue_head_t *wq, struct remote_event *event)
 		remote_event_signal_local(wq, event);
 }
 
+/*
+ * VCHIQ used a small, fixed number of remote events. It is simplest to
+ * enumerate them here for polling.
+ */
 void
 remote_event_pollall(struct vchiq_state *state)
 {

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

@@ -166,6 +166,24 @@ struct vchiq_bulk_queue {
 	struct vchiq_bulk bulks[VCHIQ_NUM_SERVICE_BULKS];
 };
 
+/*
+ * Remote events provide a way of presenting several virtual doorbells to a
+ * peer (ARM host to VPU) using only one physical doorbell. They can be thought
+ * of as a way for the peer to signal a semaphore, in this case implemented as
+ * a workqueue.
+ *
+ * Remote events remain signalled until acknowledged by the receiver, and they
+ * are non-counting. They are designed in such a way as to minimise the number
+ * of interrupts and avoid unnecessary waiting.
+ *
+ * A remote_event is as small data structures that live in shared memory. It
+ * comprises two booleans - armed and fired:
+ *
+ * The sender sets fired when they signal the receiver.
+ * If fired is set, the receiver has been signalled and need not wait.
+ * The receiver sets the armed field before they begin to wait.
+ * If armed is set, the receiver is waiting and wishes to be woken by interrupt.
+ */
 struct remote_event {
 	int armed;
 	int fired;