[v2] serial: imx: fix tx statemachine deadlock

Commit Message

Paul Geurts Nov. 24, 2023, 12:53 p.m. UTC

When using the serial port as RS485 port, the tx statemachine is used to
control the RTS pin to drive the RS485 transceiver TX_EN pin. When the
TTY port is closed in the middle of a transmission (for instance during
userland application crash), imx_uart_shutdown disables the interface
and disables the Transmission Complete interrupt. afer that,
imx_uart_stop_tx bails on an incomplete transmission, to be retriggered
by the TC interrupt. This interrupt is disabled and therefore the tx
statemachine never transitions out of SEND. The statemachine is in
deadlock now, and the TX_EN remains low, making the interface useless.

imx_uart_stop_tx now checks for incomplete transmission AND whether TC
interrupts are enabled before bailing to be retriggered. This makes sure
the state machine handling is reached, and is properly set to
WAIT_AFTER_SEND.

Fixes: cb1a60923609 serial: imx: implement rts delaying for rs485

Signed-off-by: Paul Geurts <[e-mail]>
---
V1 -> V2: Added fixes line to the commit message

 drivers/tty/serial/imx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Fabio Estevam Nov. 24, 2023, 12:58 p.m. UTC | #1

Hi Paul,

On Fri, Nov 24, 2023 at 9:55 AM Paul Geurts <[e-mail]> wrote:
>
> When using the serial port as RS485 port, the tx statemachine is used to
> control the RTS pin to drive the RS485 transceiver TX_EN pin. When the
> TTY port is closed in the middle of a transmission (for instance during
> userland application crash), imx_uart_shutdown disables the interface
> and disables the Transmission Complete interrupt. afer that,
> imx_uart_stop_tx bails on an incomplete transmission, to be retriggered
> by the TC interrupt. This interrupt is disabled and therefore the tx
> statemachine never transitions out of SEND. The statemachine is in
> deadlock now, and the TX_EN remains low, making the interface useless.
>
> imx_uart_stop_tx now checks for incomplete transmission AND whether TC
> interrupts are enabled before bailing to be retriggered. This makes sure
> the state machine handling is reached, and is properly set to
> WAIT_AFTER_SEND.
>
> Fixes: cb1a60923609 serial: imx: implement rts delaying for rs485

One nit: the correct format for the Fixes tag is:

Fixes: cb1a60923609 ("serial: imx: implement rts delaying for rs485")

And no blank line is needed between the Fixes and Signed-off-by line.

> Signed-off-by: Paul Geurts <[e-mail]>

Greg Kroah-Hartman Nov. 24, 2023, 1:03 p.m. UTC | #2

On Fri, Nov 24, 2023 at 09:58:19AM -0300, Fabio Estevam wrote:
> Hi Paul,
> 
> On Fri, Nov 24, 2023 at 9:55 AM Paul Geurts <[e-mail]> wrote:
> >
> > When using the serial port as RS485 port, the tx statemachine is used to
> > control the RTS pin to drive the RS485 transceiver TX_EN pin. When the
> > TTY port is closed in the middle of a transmission (for instance during
> > userland application crash), imx_uart_shutdown disables the interface
> > and disables the Transmission Complete interrupt. afer that,
> > imx_uart_stop_tx bails on an incomplete transmission, to be retriggered
> > by the TC interrupt. This interrupt is disabled and therefore the tx
> > statemachine never transitions out of SEND. The statemachine is in
> > deadlock now, and the TX_EN remains low, making the interface useless.
> >
> > imx_uart_stop_tx now checks for incomplete transmission AND whether TC
> > interrupts are enabled before bailing to be retriggered. This makes sure
> > the state machine handling is reached, and is properly set to
> > WAIT_AFTER_SEND.
> >
> > Fixes: cb1a60923609 serial: imx: implement rts delaying for rs485
> 
> One nit: the correct format for the Fixes tag is:
> 
> Fixes: cb1a60923609 ("serial: imx: implement rts delaying for rs485")
> 
> And no blank line is needed between the Fixes and Signed-off-by line.

It's not really a "nit", our tools will complain if this is in the wrong
format as so many things depend on this being correct.

Paul, can you make a v3 with this change?

And really, this should have been v3 anyway, as the difference being you
added a changelog from v2 :)

thanks,

greg k-h

Patch

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 708b9852a575..ad36c49c7898 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -415,13 +415,13 @@  static void imx_uart_stop_tx(struct uart_port *port)
 	ucr1 = imx_uart_readl(sport, UCR1);
 	imx_uart_writel(sport, ucr1 & ~UCR1_TRDYEN, UCR1);
 
+	ucr4 = imx_uart_readl(sport, UCR4);
 	usr2 = imx_uart_readl(sport, USR2);
-	if (!(usr2 & USR2_TXDC)) {
+	if ((!(usr2 & USR2_TXDC)) && (ucr4 & UCR4_TCEN)) {
 		/* The shifter is still busy, so retry once TC triggers */
 		return;
 	}
 
-	ucr4 = imx_uart_readl(sport, UCR4);
 	ucr4 &= ~UCR4_TCEN;
 	imx_uart_writel(sport, ucr4, UCR4);