[PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios

[PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios

[Tate Whiteberg]

If mid8250_set_termios is called while data is still in transmission,
the corresponding register updates will corrupt the transmission.

Fix this by locking the port and and waiting for the transmitter to
empty before performing updates. It is necessary to wait for both
UART_LSR_THRE and UART_LSR_TEMT to ensure the final character is sent.

Signed-off-by: Tate Whiteberg <whiteberg@arista.com>
---
 drivers/tty/serial/8250/8250.h      |  1 +
 drivers/tty/serial/8250/8250_mid.c  |  5 +++++
 drivers/tty/serial/8250/8250_port.c | 13 +++++++------
 3 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 9337fec9394e..f99620b1cab7 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -180,6 +180,7 @@ void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p);
 void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
 				       struct nbcon_write_context *wctxt,
 				       unsigned int count);
+void serial8250_wait_for_xmitr(struct uart_8250_port *up, int bits);
 
 void serial8250_rpm_get(struct uart_8250_port *p);
 void serial8250_rpm_put(struct uart_8250_port *p);
diff --git a/drivers/tty/serial/8250/8250_mid.c b/drivers/tty/serial/8250/8250_mid.c
index 82656645b8a6..c77e117b7822 100644
--- a/drivers/tty/serial/8250/8250_mid.c
+++ b/drivers/tty/serial/8250/8250_mid.c
@@ -210,6 +210,7 @@ static void mid8250_set_termios(struct uart_port *p, struct ktermios *termios,
 {
 	unsigned int baud = tty_termios_baud_rate(termios);
 	struct mid8250 *mid = p->private_data;
+	struct uart_8250_port *up = up_to_u8250p(p);
 	unsigned short ps = 16;
 	unsigned long fuart = baud * ps;
 	unsigned long w = BIT(24) - 1;
@@ -218,6 +219,7 @@ static void mid8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	/* Gracefully handle the B0 case: fall back to B9600 */
 	fuart = fuart ? fuart : 9600 * 16;
 
+	uart_port_lock_irq(p);
 	if (mid->board->freq < fuart) {
 		/* Find prescaler value that satisfies Fuart < Fref */
 		if (mid->board->freq > baud)
@@ -231,11 +233,14 @@ static void mid8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	}
 
 	rational_best_approximation(fuart, mid->board->freq, w, w, &mul, &div);
+
+	serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 	p->uartclk = fuart * 16 / ps;		/* core uses ps = 16 always */
 
 	writel(ps, p->membase + INTEL_MID_UART_PS);		/* set PS */
 	writel(mul, p->membase + INTEL_MID_UART_MUL);		/* set MUL */
 	writel(div, p->membase + INTEL_MID_UART_DIV);
+	uart_port_unlock_irq(p);
 
 	serial8250_do_set_termios(p, termios, old);
 }
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 38fa45e74a37..4eaf417f1fd7 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2024,7 +2024,7 @@ static bool wait_for_lsr(struct uart_8250_port *up, int bits)
 }
 
 /* Wait for transmitter and holding register to empty with timeout */
-static void wait_for_xmitr(struct uart_8250_port *up, int bits)
+void serial8250_wait_for_xmitr(struct uart_8250_port *up, int bits)
 {
 	unsigned int tmout;
 	bool tx_ready;
@@ -2052,6 +2052,7 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 		}
 	}
 }
+EXPORT_SYMBOL_NS_GPL(serial8250_wait_for_xmitr, "SERIAL_8250");
 
 #ifdef CONFIG_CONSOLE_POLL
 /*
@@ -2098,7 +2099,7 @@ static void serial8250_put_poll_char(struct uart_port *port,
 	ier = serial_port_in(port, UART_IER);
 	__serial8250_clear_IER(up);
 
-	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
+	serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 	/*
 	 *	Send the character out.
 	 */
@@ -2108,7 +2109,7 @@ static void serial8250_put_poll_char(struct uart_port *port,
 	 *	Finally, wait for transmitter to become empty
 	 *	and restore the IER
 	 */
-	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
+	serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 	serial_port_out(port, UART_IER, ier);
 }
 
@@ -2223,7 +2224,7 @@ static void serial8250_THRE_test(struct uart_port *port)
 	 * Synchronize UART_IER access against the console.
 	 */
 	scoped_guard(uart_port_lock_irqsave, port) {
-		wait_for_xmitr(up, UART_LSR_THRE);
+		serial8250_wait_for_xmitr(up, UART_LSR_THRE);
 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
 		udelay(1); /* allow THRE to set */
 		iir_noint1 = serial_port_in(port, UART_IIR) & UART_IIR_NO_INT;
@@ -3293,7 +3294,7 @@ static void serial8250_console_wait_putchar(struct uart_port *port, unsigned cha
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
-	wait_for_xmitr(up, UART_LSR_THRE);
+	serial8250_wait_for_xmitr(up, UART_LSR_THRE);
 	serial8250_console_putchar(port, ch);
 }
 
@@ -3504,7 +3505,7 @@ void serial8250_console_write(struct uart_8250_port *up,
 	 *	Finally, wait for transmitter to become empty
 	 *	and restore the IER
 	 */
-	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
+	serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 
 	if (em485) {
 		mdelay(port->rs485.delay_rts_after_send);
-- 
2.43.0

Re: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios

[Andy Shevchenko]

On Fri, Aug 28, 2026 at 02:45:17PM +0000, Tate Whiteberg wrote:
> If mid8250_set_termios is called while data is still in transmission,

mid8250_set_termios()

> the corresponding register updates will corrupt the transmission.
> 
> Fix this by locking the port and and waiting for the transmitter to
> empty before performing updates. It is necessary to wait for both
> UART_LSR_THRE and UART_LSR_TEMT to ensure the final character is sent.

Fixes tag?

...

> static void mid8250_set_termios(struct uart_port *p, struct ktermios *termios,

>  {
>  	unsigned int baud = tty_termios_baud_rate(termios);
>  	struct mid8250 *mid = p->private_data;
> +	struct uart_8250_port *up = up_to_u8250p(p);

Move it one line up.

>  	unsigned short ps = 16;
>  	unsigned long fuart = baud * ps;
>  	unsigned long w = BIT(24) - 1;

>  	/* Gracefully handle the B0 case: fall back to B9600 */
>  	fuart = fuart ? fuart : 9600 * 16;
>  
> +	uart_port_lock_irq(p);

+ blank line.

Do we need to protect the preliminary calculations?

>  	if (mid->board->freq < fuart) {
>  		/* Find prescaler value that satisfies Fuart < Fref */
>  		if (mid->board->freq > baud)
> @@ -231,11 +233,14 @@ static void mid8250_set_termios(struct uart_port *p, struct ktermios *termios,
>  	}
>  
>  	rational_best_approximation(fuart, mid->board->freq, w, w, &mul, &div);

> +	serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);

Can't it be done one line after? Somewhere...

>  	p->uartclk = fuart * 16 / ps;		/* core uses ps = 16 always */

...here?

>  	writel(ps, p->membase + INTEL_MID_UART_PS);		/* set PS */
>  	writel(mul, p->membase + INTEL_MID_UART_MUL);		/* set MUL */
>  	writel(div, p->membase + INTEL_MID_UART_DIV);

+ blank line.

> +	uart_port_unlock_irq(p);
>  
>  	serial8250_do_set_termios(p, termios, old);

...

> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 38fa45e74a37..4eaf417f1fd7 100644

All below better to split to another prerequisite patch.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios

[kernel test robot]

Hi Tate,

kernel test robot noticed the following build errors:

[auto build test ERROR on tty/tty-testing]
[also build test ERROR on tty/tty-next tty/tty-linus usb/usb-testing usb/usb-next usb/usb-linus linus/master next-20260828]
[cannot apply to v7.2]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Tate-Whiteberg/serial-8250_mid-wait-for-LSR-tx-empty-before-setting-termios/20260828-144517
base:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-testing
patch link:    https://lore.kernel.org/r/20260828144617.1210599-2-whiteberg%40arista.com
patch subject: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios
config: powerpc64-randconfig-001-20260829 (https://download.01.org/0day-ci/archive/20260831/202608310138.0uIPCjfP-lkp@intel.com/config)
compiler: powerpc64-linux-gcc (GCC) 13.4.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260831/202608310138.0uIPCjfP-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202608310138.0uIPCjfP-lkp@intel.com/

All errors (new ones prefixed by >>, old ones prefixed by <<):

>> ERROR: modpost: drivers/tty/serial/8250/8250_mid.ko: module uses symbol 'serial8250_wait_for_xmitr' from namespace 'SERIAL_8250', but does not import it.

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios

[kernel test robot]

Hi Tate,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tty/tty-testing]
[also build test WARNING on tty/tty-next tty/tty-linus usb/usb-testing usb/usb-next usb/usb-linus linus/master next-20260828]
[cannot apply to v7.2]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Tate-Whiteberg/serial-8250_mid-wait-for-LSR-tx-empty-before-setting-termios/20260828-144517
base:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-testing
patch link:    https://lore.kernel.org/r/20260828144617.1210599-2-whiteberg%40arista.com
patch subject: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios
config: alpha-randconfig-r073-20260829 (https://download.01.org/0day-ci/archive/20260831/202608311414.nybi87OD-lkp@intel.com/config)
compiler: alpha-linux-gcc (GCC) 8.5.0
smatch: v0.5.0-9187-g5189e3fb
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260831/202608311414.nybi87OD-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202608311414.nybi87OD-lkp@intel.com/

All warnings (new ones prefixed by >>, old ones prefixed by <<):

WARNING: modpost: vmlinux: 'saved_config' is COMMON symbol
>> WARNING: modpost: drivers/tty/serial/8250/8250_mid.ko: module uses symbol 'serial8250_wait_for_xmitr' from namespace 'SERIAL_8250', but does not import it.

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki