Patch "tty: amba-pl011: Fix spurious TX interrupts" has been added to the 4.9-stable tree

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

tty: amba-pl011: Fix spurious TX interrupts

to the 4.9-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:
tty-amba-pl011-fix-spurious-tx-interrupts.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Jayachandran C <[email protected]>
Date: Sat, 1 Apr 2017 19:42:09 +0000
Subject: tty: amba-pl011: Fix spurious TX interrupts

From: Jayachandran C <[email protected]>

[ Upstream commit 7d05587c9e0e4611650bb403812e2d492c178a9f ]

On SMP systems, we see a lot of spurious TX interrupts when a program generates a steady stream of output to the pl011 UART.

The problem can be easily seen when one CPU generates the output while another CPU handles the pl011 interrupts, and the rate of output is low enough not to fill the TX FIFO. The problem seems to be:

-- CPU a -- -- CPU b --
(take port lock)
pl011_start_tx
pl011_start_tx_pio
enable TXIM in REG_IMSC -> causes uart tx intr (pl011_int)
pl011_tx_chars pl011_int
...tx chars, all done... (wait for port lock)
pl011_stop_tx .
disable TXIM .
(release port lock) -> (take port lock)
check for TXIM, not enabled (release port lock) return IRQ_NONE

Enabling the TXIM in pl011_start_tx_pio() causes the interrupt to be generated and delivered to CPU b, even though pl011_tx_chars() is able to complete the TX and then disable the tx interrupt.

Fix this by enabling TXIM only after pl011_tx_chars, if it is needed. pl011_tx_chars will return a boolean indicating whether the TX interrupts have to be enabled.

---
 drivers/tty/serial/amba-pl011.c |   23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

@@ -1302,14 +1302,15 @@ static void pl011_stop_tx(struct uart_po
 	pl011_dma_tx_stop(uap);
 }
 
-static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq);
+static bool pl011_tx_chars(struct uart_amba_port *uap, bool from_irq);
 
 /* Start TX with programmed I/O only (no DMA) */
 static void pl011_start_tx_pio(struct uart_amba_port *uap)
 {
-	uap->im |= UART011_TXIM;
-	pl011_write(uap->im, uap, REG_IMSC);
-	pl011_tx_chars(uap, false);
+	if (pl011_tx_chars(uap, false)) {
+		uap->im |= UART011_TXIM;
+		pl011_write(uap->im, uap, REG_IMSC);
+	}
 }
 
 static void pl011_start_tx(struct uart_port *port)

@@ -1389,25 +1390,26 @@ static bool pl011_tx_char(struct uart_am
 	return true;
 }
 
-static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq)
+/* Returns true if tx interrupts have to be (kept) enabled  */
+static bool pl011_tx_chars(struct uart_amba_port *uap, bool from_irq)
 {
 	struct circ_buf *xmit = &uap->port.state->xmit;
 	int count = uap->fifosize >> 1;
 
 	if (uap->port.x_char) {
 		if (!pl011_tx_char(uap, uap->port.x_char, from_irq))
-			return;
+			return true;
 		uap->port.x_char = 0;
 		--count;
 	}
 	if (uart_circ_empty(xmit) || uart_tx_stopped(&uap->port)) {
 		pl011_stop_tx(&uap->port);
-		return;
+		return false;
 	}
 
 	/* If we are using DMA mode, try to send some characters. */
 	if (pl011_dma_tx_irq(uap))
-		return;
+		return true;
 
 	do {
 		if (likely(from_irq) && count-- == 0)

@@ -1422,8 +1424,11 @@ static void pl011_tx_chars(struct uart_a
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(&uap->port);
 
-	if (uart_circ_empty(xmit))
+	if (uart_circ_empty(xmit)) {
 		pl011_stop_tx(&uap->port);
+		return false;
+	}
+	return true;
 }
 
 static void pl011_modem_status(struct uart_amba_port *uap)

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

queue-4.9/tty-amba-pl011-fix-spurious-tx-interrupts.patch