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

[sashiko-bot@kernel.org]

> __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.
> 
> [ ... ]
> 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.
> 
> Signed-off-by: Nicolas Thibert <nithibert@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260907173000.1254045-1-nithibert@gmail.com?part=1