Re: [PATCH v9 1/4] serial: 8250: Add 8250 port clock update method

[Andy Shevchenko <andy@black.fi.intel.com>]

On Thu, Jul 23, 2020 at 03:33:54AM +0300, Serge Semin wrote:
> Some platforms can be designed in a way so the UART port reference clock
> might be asynchronously changed at some point. In Baikal-T1 SoC this may
> happen due to the reference clock being shared between two UART ports, on
> the Allwinner SoC the reference clock is derived from the CPU clock, so
> any CPU frequency change should get to be known/reflected by/in the UART
> controller as well. But it's not enough to just update the
> uart_port->uartclk field of the corresponding UART port, the 8250
> controller reference clock divisor should be altered so to preserve
> current baud rate setting. All of these things is done in a coherent
> way by calling the serial8250_update_uartclk() method provided in this
> patch. Though note that it isn't supposed to be called from within the
> UART port callbacks because the locks using to the protect the UART port
> data are already taken in there.

...

> +/*
> + * Note in order to avoid the tty port mutex deadlock don't use the next method
> + * within the uart port callbacks. Primarily it's supposed to be utilized to
> + * handle a sudden reference clock rate change.
> + */
> +void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)
> +{
> +	struct uart_8250_port *up = up_to_u8250p(port);
> +	unsigned int baud, quot, frac = 0;
> +	struct ktermios *termios;
> +	unsigned long flags;
> +
> +	mutex_lock(&port->state->port.mutex);
> +
> +	if (port->uartclk == uartclk)
> +		goto out_lock;
> +
> +	port->uartclk = uartclk;
> +	termios = &port->state->port.tty->termios;
> +
> +	baud = serial8250_get_baud_rate(port, termios, NULL);
> +	quot = serial8250_get_divisor(port, baud, &frac);
> +
> +	serial8250_rpm_get(up);
> +	spin_lock_irqsave(&port->lock, flags);
> +
> +	uart_update_timeout(port, termios->c_cflag, baud);
> +
> +	serial8250_set_divisor(port, baud, quot, frac);
> +	serial_port_out(port, UART_LCR, up->lcr);
> +	serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS);
> +
> +	spin_unlock_irqrestore(&port->lock, flags);
> +	serial8250_rpm_put(up);
> +
> +out_lock:
> +	mutex_unlock(&port->state->port.mutex);

While looking for something else I have stumbled over this function.
My Q is, since it has some duplications with
serial8250_do_set_termios(), can we actually call the latter (or
derevative that can be called in both) in the above code instead of
duplicating some lines?

	if (port UART clock has to be updated)
	  call (unlocked version of) serial8250_do_set_termios()

Serge, what do you think?

> +}

-- 
With Best Regards,
Andy Shevchenko