mailbox: add API to query available TX queue slots

Clients sometimes need to know whether the mailbox TX queue has room
before posting a new message. Rather than exposing internal queue state
through a struct field, provide a proper accessor function that returns
the number of available slots for a given channel.

This lets clients choose to back off when the queue is full instead of
hitting the -ENOBUFS error path and the misleading "Try increasing
MBOX_TX_QUEUE_LEN" warning.

Tested-by: Tanmay Shah <tanmay.shah@amd.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

drivers/mailbox/mailbox.c

@@ -218,6 +218,29 @@ bool mbox_client_peek_data(struct mbox_chan *chan)
 }
 EXPORT_SYMBOL_GPL(mbox_client_peek_data);
 
+/**
+ * mbox_chan_tx_slots_available - Query the number of available TX queue slots.
+ * @chan: Mailbox channel to query.
+ *
+ * Clients may call this to check how many messages can be queued via
+ * mbox_send_message() before the channel's TX queue is full. This helps
+ * clients avoid the -ENOBUFS error without needing to increase
+ * MBOX_TX_QUEUE_LEN.
+ * This can be called from atomic context.
+ *
+ * Return: Number of available slots in the channel's TX queue.
+ */
+unsigned int mbox_chan_tx_slots_available(struct mbox_chan *chan)
+{
+	unsigned int ret;
+
+	guard(spinlock_irqsave)(&chan->lock);
+	ret = MBOX_TX_QUEUE_LEN - chan->msg_count;
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mbox_chan_tx_slots_available);
+
 /**
  * mbox_send_message -	For client to submit a message to be
  *				sent to the remote.

include/linux/mailbox_client.h

@@ -45,6 +45,7 @@ int mbox_send_message(struct mbox_chan *chan, void *mssg);
 int mbox_flush(struct mbox_chan *chan, unsigned long timeout);
 void mbox_client_txdone(struct mbox_chan *chan, int r); /* atomic */
 bool mbox_client_peek_data(struct mbox_chan *chan); /* atomic */
+unsigned int mbox_chan_tx_slots_available(struct mbox_chan *chan); /* atomic */
 void mbox_free_channel(struct mbox_chan *chan); /* may sleep */
 
 #endif /* __MAILBOX_CLIENT_H */