[PATCH V2] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[PATCH V2] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[Sherry Sun]

According to the LPUART reference manual, TXRTSE and TXRTSPOL of MODIR
register only can be changed when the transmitter is disabled.
So disable the transmitter before changing RS485 related registers and
re-enable it after the change is done.

Fixes: 67b01837861c ("tty: serial: lpuart: Add RS485 support for 32-bit uart flavour")
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
---
Changes in V2:
1. Add TE bit polling read to ensure TE is really become 0 before proceeding.
2. Add Reviewed-by tag.
---
 drivers/tty/serial/fsl_lpuart.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 91d02c55c470..6b2f3a73a367 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1484,6 +1484,19 @@ static int lpuart32_config_rs485(struct uart_port *port, struct ktermios *termio
 
 	unsigned long modem = lpuart32_read(&sport->port, UARTMODIR)
 				& ~(UARTMODIR_TXRTSPOL | UARTMODIR_TXRTSE);
+	u32 ctrl;
+
+	/* TXRTSE and TXRTSPOL only can be changed when transmitter is disabled. */
+	ctrl = lpuart32_read(&sport->port, UARTCTRL);
+	if (ctrl & UARTCTRL_TE) {
+		/* wait transmit engin complete */
+		lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);
+		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE, UARTCTRL);
+
+		while (lpuart32_read(&sport->port, UARTCTRL) & UARTCTRL_TE)
+			cpu_relax();
+	}
+
 	lpuart32_write(&sport->port, modem, UARTMODIR);
 
 	if (rs485->flags & SER_RS485_ENABLED) {
@@ -1503,6 +1516,10 @@ static int lpuart32_config_rs485(struct uart_port *port, struct ktermios *termio
 	}
 
 	lpuart32_write(&sport->port, modem, UARTMODIR);
+
+	if (ctrl & UARTCTRL_TE)
+		lpuart32_write(&sport->port, ctrl, UARTCTRL);
+
 	return 0;
 }
 
-- 
2.34.1

Re: [PATCH V2] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[Jiri Slaby]

On 11. 03. 25, 3:55, Sherry Sun wrote:
> According to the LPUART reference manual, TXRTSE and TXRTSPOL of MODIR
> register only can be changed when the transmitter is disabled.
> So disable the transmitter before changing RS485 related registers and
> re-enable it after the change is done.
> 
> Fixes: 67b01837861c ("tty: serial: lpuart: Add RS485 support for 32-bit uart flavour")
> Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
> Reviewed-by: Frank Li <Frank.Li@nxp.com>
> ---
> Changes in V2:
> 1. Add TE bit polling read to ensure TE is really become 0 before proceeding.
> 2. Add Reviewed-by tag.
> ---
>   drivers/tty/serial/fsl_lpuart.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index 91d02c55c470..6b2f3a73a367 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -1484,6 +1484,19 @@ static int lpuart32_config_rs485(struct uart_port *port, struct ktermios *termio
>   
>   	unsigned long modem = lpuart32_read(&sport->port, UARTMODIR)
>   				& ~(UARTMODIR_TXRTSPOL | UARTMODIR_TXRTSE);

This is unrelated, but why is the above ulong?

> +	u32 ctrl;
> +
> +	/* TXRTSE and TXRTSPOL only can be changed when transmitter is disabled. */
> +	ctrl = lpuart32_read(&sport->port, UARTCTRL);
> +	if (ctrl & UARTCTRL_TE) {
> +		/* wait transmit engin complete */

wait for the transmit engine to complete

> +		lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);

Both this ^^ and:

> +		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE, UARTCTRL);
> +
> +		while (lpuart32_read(&sport->port, UARTCTRL) & UARTCTRL_TE)
> +			cpu_relax();

this ^^ are unbound loops in case the hardware gets mad :(.

Anyway, IIUC, after the TE clear from CTRL by the above write, the TE 
bit is really cleared by the HW from CTRL only after it is really 
disabled, so has to be checked?

> +	}
> +


thanks,
-- 
js
suse labs

RE: [PATCH V2] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[Sherry Sun]

> -----Original Message-----
> From: Jiri Slaby <jirislaby@kernel.org>
> Sent: Tuesday, March 11, 2025 11:45 AM
> To: Sherry Sun <sherry.sun@nxp.com>; gregkh@linuxfoundation.org
> Cc: linux-serial@vger.kernel.org; linux-kernel@vger.kernel.org;
> imx@lists.linux.dev; Shenwei Wang <shenwei.wang@nxp.com>; Frank Li
> <frank.li@nxp.com>
> Subject: Re: [PATCH V2] tty: serial: fsl_lpuart: disable transmitter before
> changing RS485 related registers
> 
> On 11. 03. 25, 3:55, Sherry Sun wrote:
> > According to the LPUART reference manual, TXRTSE and TXRTSPOL of
> MODIR
> > register only can be changed when the transmitter is disabled.
> > So disable the transmitter before changing RS485 related registers and
> > re-enable it after the change is done.
> >
> > Fixes: 67b01837861c ("tty: serial: lpuart: Add RS485 support for
> > 32-bit uart flavour")
> > Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
> > Reviewed-by: Frank Li <Frank.Li@nxp.com>
> > ---
> > Changes in V2:
> > 1. Add TE bit polling read to ensure TE is really become 0 before proceeding.
> > 2. Add Reviewed-by tag.
> > ---
> >   drivers/tty/serial/fsl_lpuart.c | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> >
> > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > b/drivers/tty/serial/fsl_lpuart.c index 91d02c55c470..6b2f3a73a367
> > 100644
> > --- a/drivers/tty/serial/fsl_lpuart.c
> > +++ b/drivers/tty/serial/fsl_lpuart.c
> > @@ -1484,6 +1484,19 @@ static int lpuart32_config_rs485(struct
> > uart_port *port, struct ktermios *termio
> >
> >   	unsigned long modem = lpuart32_read(&sport->port, UARTMODIR)
> >   				& ~(UARTMODIR_TXRTSPOL |
> UARTMODIR_TXRTSE);
> 
> This is unrelated, but why is the above ulong?

Hi Jiri, the following lpuart cleanup patch will change all the "unsigned long"
type to the correct u32.

> 
> > +	u32 ctrl;
> > +
> > +	/* TXRTSE and TXRTSPOL only can be changed when transmitter is
> disabled. */
> > +	ctrl = lpuart32_read(&sport->port, UARTCTRL);
> > +	if (ctrl & UARTCTRL_TE) {
> > +		/* wait transmit engin complete */
> 
> wait for the transmit engine to complete

Thanks, will fix.

> 
> > +		lpuart32_wait_bit_set(&sport->port, UARTSTAT,
> UARTSTAT_TC);
> 
> Both this ^^ and:
> 
> > +		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE,
> UARTCTRL);
> > +
> > +		while (lpuart32_read(&sport->port, UARTCTRL) &
> UARTCTRL_TE)
> > +			cpu_relax();
> 
> this ^^ are unbound loops in case the hardware gets mad :(.
> 
> Anyway, IIUC, after the TE clear from CTRL by the above write, the TE bit is
> really cleared by the HW from CTRL only after it is really disabled, so has to be
> checked?
> 
> > +	}
> > +

Description of TE bit in LPUART RM: "After this field becomes 0, the field reads 1 until
the transmitter has completed the current character and the TXD pin is tristated".

I also tried to read back the TE bit right after write 0, actually it didn't become 0
immediately, so I added the polling read here.
You can check the discussion between Shenwei and me for more details. Thanks!
https://lore.kernel.org/imx/20250307021950.1000221-1-sherry.sun@nxp.com/T/#m14098058b083333213bd74414095aa1ef3299780

Best Regards
Sherry