Bluetooth: btnxpuart: Resolve TX timeout error in power save stress test

This fixes the tx timeout issue seen while running a stress test on
btnxpuart for couple of hours, such that the interval between two HCI
commands coincide with the power save timeout value of 2 seconds.

Test procedure using bash script:
<load btnxpuart.ko>
hciconfig hci0 up
//Enable Power Save feature
hcitool -i hci0 cmd 3f 23 02 00 00
while (true)
do
    hciconfig hci0 leadv
    sleep 2
    hciconfig hci0 noleadv
    sleep 2
done

Error log, after adding few more debug prints:
Bluetooth: btnxpuart_queue_skb(): 01 0A 20 01 00
Bluetooth: hci0: Set UART break: on, status=0
Bluetooth: hci0: btnxpuart_tx_wakeup() tx_work scheduled
Bluetooth: hci0: btnxpuart_tx_work() dequeue: 01 0A 20 01 00
Can't set advertise mode on hci0: Connection timed out (110)
Bluetooth: hci0: command 0x200a tx timeout

When the power save mechanism turns on UART break, and btnxpuart_tx_work()
is scheduled simultaneously, psdata->ps_state is read as PS_STATE_AWAKE,
which prevents the psdata->work from being scheduled, which is responsible
to turn OFF UART break.

This issue is fixed by adding a ps_lock mutex around UART break on/off as
well as around ps_state read/write.
btnxpuart_tx_wakeup() will now read updated ps_state value. If ps_state is
PS_STATE_SLEEP, it will first schedule psdata->work, and then it will
reschedule itself once UART break has been turned off and ps_state is
PS_STATE_AWAKE.

Tested above script for 50,000 iterations and TX timeout error was not
observed anymore.
Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

drivers/bluetooth/btnxpuart.c

@@ -126,6 +126,7 @@ struct ps_data {
 	struct hci_dev *hdev;
 	struct work_struct work;
 	struct timer_list ps_timer;
+	struct mutex ps_lock;
 };
 
 struct wakeup_cmd_payload {
@@ -317,6 +318,9 @@ static void ps_start_timer(struct btnxpuart_dev *nxpdev)
 
 	if (psdata->cur_psmode == PS_MODE_ENABLE)
 		mod_timer(&psdata->ps_timer, jiffies + msecs_to_jiffies(psdata->h2c_ps_interval));
+
+	if (psdata->ps_state == PS_STATE_AWAKE && psdata->ps_cmd == PS_CMD_ENTER_PS)
+		cancel_work_sync(&psdata->work);
 }
 
 static void ps_cancel_timer(struct btnxpuart_dev *nxpdev)
@@ -337,6 +341,7 @@ static void ps_control(struct hci_dev *hdev, u8 ps_state)
 	    !test_bit(BTNXPUART_SERDEV_OPEN, &nxpdev->tx_state))
 		return;
 
+	mutex_lock(&psdata->ps_lock);
 	switch (psdata->cur_h2c_wakeupmode) {
 	case WAKEUP_METHOD_DTR:
 		if (ps_state == PS_STATE_AWAKE)
@@ -350,12 +355,15 @@ static void ps_control(struct hci_dev *hdev, u8 ps_state)
 			status = serdev_device_break_ctl(nxpdev->serdev, 0);
 		else
 			status = serdev_device_break_ctl(nxpdev->serdev, -1);
+		msleep(20); /* Allow chip to detect UART-break and enter sleep */
 		bt_dev_dbg(hdev, "Set UART break: %s, status=%d",
 			   str_on_off(ps_state == PS_STATE_SLEEP), status);
 		break;
 	}
 	if (!status)
 		psdata->ps_state = ps_state;
+	mutex_unlock(&psdata->ps_lock);
+
 	if (ps_state == PS_STATE_AWAKE)
 		btnxpuart_tx_wakeup(nxpdev);
 }
@@ -391,17 +399,25 @@ static void ps_setup(struct hci_dev *hdev)
 
 	psdata->hdev = hdev;
 	INIT_WORK(&psdata->work, ps_work_func);
+	mutex_init(&psdata->ps_lock);
 	timer_setup(&psdata->ps_timer, ps_timeout_func, 0);
 }
 
-static void ps_wakeup(struct btnxpuart_dev *nxpdev)
+static bool ps_wakeup(struct btnxpuart_dev *nxpdev)
 {
 	struct ps_data *psdata = &nxpdev->psdata;
+	u8 ps_state;
 
-	if (psdata->ps_state != PS_STATE_AWAKE) {
+	mutex_lock(&psdata->ps_lock);
+	ps_state = psdata->ps_state;
+	mutex_unlock(&psdata->ps_lock);
+
+	if (ps_state != PS_STATE_AWAKE) {
 		psdata->ps_cmd = PS_CMD_EXIT_PS;
 		schedule_work(&psdata->work);
+		return true;
 	}
+	return false;
 }
 
 static int send_ps_cmd(struct hci_dev *hdev, void *data)
@@ -1171,7 +1187,6 @@ static struct sk_buff *nxp_dequeue(void *data)
 {
 	struct btnxpuart_dev *nxpdev = (struct btnxpuart_dev *)data;
 
-	ps_wakeup(nxpdev);
 	ps_start_timer(nxpdev);
 	return skb_dequeue(&nxpdev->txq);
 }
@@ -1186,6 +1201,9 @@ static void btnxpuart_tx_work(struct work_struct *work)
 	struct sk_buff *skb;
 	int len;
 
+	if (ps_wakeup(nxpdev))
+		return;
+
 	while ((skb = nxp_dequeue(nxpdev))) {
 		len = serdev_device_write_buf(serdev, skb->data, skb->len);
 		hdev->stat.byte_tx += len;