Patch "serial: 8250_port: fix runtime PM use in __do_stop_tx_rs485()" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

    serial: 8250_port: fix runtime PM use in __do_stop_tx_rs485()

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     serial-8250_port-fix-runtime-pm-use-in-__do_stop_tx_rs485.patch

and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Shevchenko <[email protected]>
Date: Wed, 31 Aug 2016 19:46:55 +0300
Subject: serial: 8250_port: fix runtime PM use in __do_stop_tx_rs485()

From: Andy Shevchenko <[email protected]>

commit b3965767d86cf4534dfe1affbde0453d3224ed7f upstream.

There are calls to serial8250_rpm_{get|put}() in __do_stop_tx_rs485() that are certainly placed in a wrong location. I dunno how it had been tested with runtime PM enabled because it is obvious "sleep in atomic context" error.

Besides that serial8250_rpm_get() is called immediately after an IO just happened. It implies that the device is already powered on, see implementation of serial8250_em485_rts_after_send() and serial8250_clear_fifos() for the details.

There is no bug have been seen due to, as I can guess, use of auto suspend mode when scheduled transaction to suspend is invoked quite lately than it's needed for a few writes to the port. It might be possible to trigger a warning if stop_tx_timer fires when device is suspended.

Refactor the code to use runtime PM only in case of timer function.

---
 drivers/tty/serial/8250/8250_port.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

@@ -1415,12 +1415,8 @@ static void __do_stop_tx_rs485(struct ua
 	if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX)) {
 		serial8250_clear_fifos(p);
 
-		serial8250_rpm_get(p);
-
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
 		serial_port_out(&p->port, UART_IER, p->ier);
-
-		serial8250_rpm_put(p);
 	}
 }
 

@@ -1430,6 +1426,7 @@ static void serial8250_em485_handle_stop
 	struct uart_8250_em485 *em485 = p->em485;
 	unsigned long flags;
 
+	serial8250_rpm_get(p);
 	spin_lock_irqsave(&p->port.lock, flags);
 	if (em485 &&
 	    em485->active_timer == &em485->stop_tx_timer) {

@@ -1437,6 +1434,7 @@ static void serial8250_em485_handle_stop
 		em485->active_timer = NULL;
 	}
 	spin_unlock_irqrestore(&p->port.lock, flags);
+	serial8250_rpm_put(p);
 }
 
 static void __stop_tx_rs485(struct uart_8250_port *p)

@@ -1476,7 +1474,7 @@ static inline void __stop_tx(struct uart
 		unsigned char lsr = serial_in(p, UART_LSR);
 		/*
 		 * To provide required timeing and allow FIFO transfer,
-		 * __stop_tx_rs485 must be called only when both FIFO and
+		 * __stop_tx_rs485() must be called only when both FIFO and
 		 * shift register are empty. It is for device driver to enable
 		 * interrupt on TEMT.
 		 */

@@ -1485,9 +1483,10 @@ static inline void __stop_tx(struct uart
 
 		del_timer(&em485->start_tx_timer);
 		em485->active_timer = NULL;
+
+		__stop_tx_rs485(p);
 	}
 	__do_stop_tx(p);
-	__stop_tx_rs485(p);
 }
 
 static void serial8250_stop_tx(struct uart_port *port)

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/serial-8250_dw-check-the-data-pclk-when-get-apb_pclk.patch queue-4.7/serial-8250_port-fix-runtime-pm-use-in-__do_stop_tx_rs485.patch