Re: [PATCH v2 1/4] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present

[Andy Shevchenko]

On Mon, Oct 19, 2026 at 05:06:10PM -0300, Vinicius Pedrosa wrote:
> dw8250_probe() installs dw8250_set_divisor(), which brackets the
> DLAB/DLL/DLH sequence with dw8250_idle_enter() and dw8250_idle_exit()
> so a BUSY UART cannot drop the divisor LCR write.
> 
> dw8250_setup_port() runs afterwards and replaces that hook with the
> dwlib divisor hook whenever it detects DLF. The dwlib hook programs the
> divisor through serial8250_do_set_divisor() without entering idle,
> leaving the LCR|DLAB write exposed to BUSY.
> 
> When termios changes while the UART is transmitting, that LCR|DLAB
> write can be ignored and dw8250_check_lcr() retries it between
> dw8250_idle_enter() and dw8250_idle_exit(). dw8250_idle_exit() then
> restores IER while DLAB is still set, so the value lands in DLH. The
> real IER stays at the zero written by dw8250_idle_enter(), and the port
> gets no further TX or RX interrupts while up->ier says they're enabled.
> 
> Do not replace a divisor hook the driver has already installed. Program
> DLF from the BUSY-safe hook, inside the idle section, through a dwlib
> helper that checks dlf_size.
> 
> Observed on an Allwinner A733 (Radxa Cubie A7Z), where
> dw8250_setup_port() mistakes the RS485 control register for DLF (see the
> next patch): the first interactive tcsetattr() during console output
> left hardware IER at 0 and up->ier at 0x07. The shell never printed its
> prompt or read input. With this change alone the same boot reached the
> prompt and echoed input, and hardware IER matched up->ier afterwards.

...

> +/* Program the fractional part of the divisor, if the UART has one */
> +void __dw8250_set_divisor(struct uart_port *p, unsigned int quot_frac)

Let's reduce amount of double underscore functions. Let's call it
dw8250_update_dfl(). Also call the parameter just 'frac'.

void dw8250_update_dfl(struct uart_port *p, unsigned int frac)

...

> +++ b/drivers/tty/serial/8250/8250_dwlib.h

>  void dw8250_do_set_termios(struct uart_port *p, struct ktermios *termios, const struct ktermios *old);
>  void dw8250_setup_port(struct uart_port *p);
> +void __dw8250_set_divisor(struct uart_port *p, unsigned int quot_frac);

Put it rather after _writel_ext() implementation, because it's just a small
wrapper on top of it. Or even make it 'static inline' there.

-- 
With Best Regards,
Andy Shevchenko