Re: [PATCH v2] serial: Deassert Transmit Enable on probe in driver-specific way

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Sep 21, 2022 at 06:39:33AM +0200, Lukas Wunner wrote:
> When a UART port is newly registered, uart_configure_port() seeks to
> deassert RS485 Transmit Enable by setting the RTS bit in port->mctrl.
> However a number of UART drivers interpret a set RTS bit as *assertion*
> instead of deassertion:  Affected drivers include those using
> serial8250_em485_config() (except 8250_bcm2835aux.c) and some using
> mctrl_gpio (e.g. imx.c).
> 
> Since the interpretation of the RTS bit is driver-specific, it is not
> suitable as a means to centrally deassert Transmit Enable in the serial
> core.  Instead, the serial core must call on drivers to deassert it in
> their driver-specific way.  One way to achieve that is to call
> ->rs485_config().  It implicitly deasserts Transmit Enable.
> 
> So amend uart_configure_port() and uart_resume_port() to invoke
> uart_rs485_config().  That allows removing calls to uart_rs485_config()
> from drivers' ->probe() hooks and declaring the function static.
> 
> Skip any invocation of ->set_mctrl() if RS485 is enabled.  RS485 has no
> hardware flow control, so the modem control lines are irrelevant and
> need not be touched.  When leaving RS485 mode, reset the modem control
> lines to the state stored in port->mctrl.  That way, UARTs which are
> muxed between RS485 and RS232 transceivers drive the lines correctly
> when switched to RS232.  (serial8250_do_startup() historically raises
> the OUT1 modem signal because otherwise interrupts are not signaled on
> ancient PC UARTs, but I believe that no longer applies to modern,
> RS485-capable UARTs and is thus safe to be skipped.)
> 
> imx.c modifies port->mctrl whenever Transmit Enable is asserted and
> deasserted.  Stop it from doing that so port->mctrl reflects the RS232
> line state.
> 
> 8250_omap.c deasserts Transmit Enable on ->runtime_resume() by calling
> ->set_mctrl().  Because that is now a no-op in RS485 mode, amend the
> function to call serial8250_em485_stop_tx().
> 
> fsl_lpuart.c retrieves and applies the RS485 device tree properties
> after registering the UART port.  Because applying now happens on
> registration in uart_configure_port(), move retrieval of the properties
> ahead of uart_add_one_port().
> 
> Fixes: d3b3404df318 ("serial: Fix incorrect rs485 polarity on uart open")
> Reported-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Tested-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Link: https://lore.kernel.org/all/20220329085050.311408-1-matthias.schiffer@ew.tq-group.com/
> Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
> Link: https://lore.kernel.org/all/8f538a8903795f22f9acc94a9a31b03c9c4ccacb.camel@ginzinger.com/
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: stable@vger.kernel.org # v4.14+
> ---
>  v1 -> v2:
>  Deassert RTS in serial8250_em485_init() only if no transmission is
>  currently ongoing (Ilpo)
> 
>  Based on v6.0-rc3 + this dependency:
>  https://lore.kernel.org/linux-serial/72fb646c1b0b11c989850c55f52f9ff343d1b2fa.1662884345.git.lukas@wunner.de/

This message never made it to lore.kernel.org, so I can't seem to apply
it using `b4`.

Can you resend it so that it does make it to the public archives?

thanks,

greg k-h