rpmsg: glink: Send READ_NOTIFY command in FIFO full case

The current design sleeps unconditionally in TX FIFO full case and
wakeup only after sleep timer expires which adds random delays in
clients TX path.

Avoid sleep and use READ_NOTIFY command so that writer can be woken up
when remote notifies about read completion by sending IRQ.

Signed-off-by: Deepak Kumar Singh <[e-mail]>
Signed-off-by: Arun Kumar Neelakantam <[e-mail]>
Signed-off-by: Bjorn Andersson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/rpmsg/qcom_glink_native.c

···
  * @rcids:	idr of all channels with a known remote channel id
  * @features:	remote features
  * @intentless:	flag to indicate that there is no intent
+ * @tx_avail_notify: Waitqueue for pending tx tasks
+ * @sent_read_notify: flag to check cmd sent or not
  */
 struct qcom_glink {
 	struct device *dev;
···
 	unsigned long features;
 
 	bool intentless;
+	wait_queue_head_t tx_avail_notify;
+	bool sent_read_notify;
 };
 
 enum {
···
 	glink->tx_pipe->write(glink->tx_pipe, hdr, hlen, data, dlen);
 }
 
+static void qcom_glink_send_read_notify(struct qcom_glink *glink)
+{
+	struct glink_msg msg;
+
+	msg.cmd = cpu_to_le16(RPM_CMD_READ_NOTIF);
+	msg.param1 = 0;
+	msg.param2 = 0;
+
+	qcom_glink_tx_write(glink, &msg, sizeof(msg), NULL, 0);
+
+	mbox_send_message(glink->mbox_chan, NULL);
+	mbox_client_txdone(glink->mbox_chan, 0);
+}
+
 static int qcom_glink_tx(struct qcom_glink *glink,
 			 const void *hdr, size_t hlen,
 			 const void *data, size_t dlen, bool wait)
···
 			goto out;
 		}
 
+		if (!glink->sent_read_notify) {
+			glink->sent_read_notify = true;
+			qcom_glink_send_read_notify(glink);
+		}
+
 		/* Wait without holding the tx_lock */
 		spin_unlock_irqrestore(&glink->tx_lock, flags);
 
-		usleep_range(10000, 15000);
+		wait_event_timeout(glink->tx_avail_notify,
+				   qcom_glink_tx_avail(glink) >= tlen, 10 * HZ);
 
 		spin_lock_irqsave(&glink->tx_lock, flags);
+
+		if (qcom_glink_tx_avail(glink) >= tlen)
+			glink->sent_read_notify = false;
 	}
 
 	qcom_glink_tx_write(glink, hdr, hlen, data, dlen);
···
 	unsigned int cmd;
 	int ret = 0;
 
+	/* To wakeup any blocking writers */
+	wake_up_all(&glink->tx_avail_notify);
+
 	for (;;) {
 		avail = qcom_glink_rx_avail(glink);
 		if (avail < sizeof(msg))
···
 	struct glink_channel *channel;
 	unsigned long flags;
 
+	/* To wakeup any blocking writers */
+	wake_up_all(&glink->tx_avail_notify);
+
 	spin_lock_irqsave(&glink->idr_lock, flags);
 	channel = idr_find(&glink->lcids, lcid);
 	if (WARN(!channel, "close ack on unknown channel\n")) {
···
 	spin_lock_init(&glink->rx_lock);
 	INIT_LIST_HEAD(&glink->rx_queue);
 	INIT_WORK(&glink->rx_work, qcom_glink_work);
+	init_waitqueue_head(&glink->tx_avail_notify);
 
 	spin_lock_init(&glink->idr_lock);
 	idr_init(&glink->lcids);