Re: [PATCH] serial: 8250_of: set UART_CAP_NOTEMT for rts-gpios RS485 direction

[Nicolas Thibert]

Hello,

Comment was just to understand the modification done. I think the
double submission is because I first try in standard mail but then
with git-email
I tested it on my QCA4531 (QCA953x base) board with an RS485 converter
(CA-IS2092A). The converter use a standard GPIO to drive the bus
between input and output. When declaring such device in dts, the GPIO
was not used remaining in a steady state and preventing from receiving
RS485 answers
Mod neede for such a dts :
&uart {
    pinctrl-names = "default";
    rts-gpios = <&gpio 3 GPIO_ACTIVE_HIGH>;
    linux,rs485-enabled-at-boot-time;
    status = "okay";
};

Regards

Nicolas

Le mer. 23 sept. 2026 à 14:20, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> a écrit :
>
> On Mon, Sep 07, 2026 at 07:30:00PM +0200, Nicolas Thibert wrote:
> > __stop_tx() (8250_port.c) only calls the RS485 rs485_stop_tx() hook
> > (which de-asserts the direction GPIO/RTS line) once it has observed
> > both UART_LSR_THRE and UART_LSR_TEMT for the last byte. If TEMT is
> > never seen and the driver hasn't set UART_CAP_NOTEMT, the function
> > returns without scheduling any retry -- the direction line is left
> > asserted (driver enabled) forever, with nothing to un-stick it short
> > of another kernel-visible LSR event.
> >
> > of_platform_serial_setup()/of_platform_serial_probe() unconditionally
> > wire up the generic em485 GPIO-RTS RS485 support
> > (rs485_config/rs485_start_tx/rs485_stop_tx) for every port they
> > register, but never set UART_CAP_NOTEMT, so any board using this
> > driver whose 16550-compatible core doesn't reliably surface TEMT for
> > its shift register hits the stuck-direction-GPIO case above.
> >
> > Confirmed live on an ath79 QCA9531 board (SoC-internal ns16550a-
> > compatible UART, RS485 transceiver DE/RE tied together on a GPIO via
> > rts-gpios, linux,rs485-enabled-at-boot-time): the direction GPIO
> > correctly asserts for the duration of a transmit, but never
> > de-asserts afterwards -- confirmed by sampling the GPIO's debugfs
> > state through and after a multi-hundred-byte write, on both the first
> > transmit and repeated back-to-back transmits. Setting
> > UART_CAP_NOTEMT, which makes __stop_tx() fall back to a frame-time-
> > based timer instead of waiting indefinitely on TEMT, makes the
> > direction GPIO reliably return low right after each transmit
> > completes.
> >
> > Scope the fix to ports that declare a GPIO-controlled direction line
> > (rts-gpios), rather than setting it unconditionally for every port
> > this driver registers: this is the class of hardware actually
> > affected (RTS state has to be explicitly un-stuck by software, unlike
> > a UART's native RTS pin), and it avoids adding the extra frame-time
> > margin to ports relying on the native RTS pin, which this has not
> > been observed to need.
> >
> > Set it in of_platform_serial_probe(), after the existing
> > "if (port8250.port.fifosize) port8250.capabilities = UART_CAP_FIFO;"
> > assignment, rather than in of_platform_serial_setup(): that later
> > plain assignment (not "|=") unconditionally overwrites
> > port8250.capabilities on any port whose "fifo-size" DT property is
> > set, silently discarding a capability bit set earlier in setup(). Not
> > observed on the reporter's own board (it has no "fifo-size" property,
> > so port.fifosize stays 0 and that assignment is skipped), but a real
> > regression waiting to happen on any other of_platform_serial user
> > that does set "fifo-size" -- which is a common, documented property
> > for this binding.
>
> Please rewrite this to be in english and says things properly.
>
> >
> > Signed-off-by: Nicolas Thibert <nithibert@xxxxxxxxx>
> > Assisted-by: LLM (Claude Sonnet 5, Anthropic)
>
> signed-off-by goes last.
>
> > ---
> >  drivers/tty/serial/8250/8250_of.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > --- a/drivers/tty/serial/8250/8250_of.c
> > +++ b/drivers/tty/serial/8250/8250_of.c
> > @@ -233,6 +233,24 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
> >                       &port8250.overrun_backoff_time_ms) != 0)
> >               port8250.overrun_backoff_time_ms = 0;
> >
> > +     /*
> > +      * This generic driver never enables a dedicated line-status
> > +      * interrupt on TEMT, so for ports whose RS485 direction is
> > +      * controlled via a GPIO (rts-gpios) rather than the native RTS
> > +      * pin, __stop_tx() (8250_port.c) can see THRE without TEMT on the
> > +      * last byte and bail out without ever retrying -- leaving the
> > +      * direction GPIO stuck asserted after the last byte sent, on
> > +      * hardware whose shift register doesn't reliably surface TEMT.
> > +      * UART_CAP_NOTEMT makes it fall back to a frame-time-based timer
> > +      * instead of waiting on that interrupt. Scoped to rts-gpios users
> > +      * only, to avoid changing timing for ports relying on the native
> > +      * RTS pin, which this has not been observed to affect. Set here,
> > +      * after the fifosize-based capabilities assignment above, so it
> > +      * isn't clobbered by it.
> > +      */
>
> Is this comment really needed?
>
>
> > +     if (of_property_present(ofdev->dev.of_node, "rts-gpios"))
>
> What will change from this now on existing systems?
>
> How was this tested?
>
> And why was this sent twice?
>
> thanks,
>
> greg k-h