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

[Serge Semin <fancer.lancer@gmail.com>]

Hi Andy,

On Wed, Feb 14, 2024 at 03:45:16PM +0200, Andy Shevchenko wrote:
> 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?

What an old thread you've digged out.)

Well, AFAIR I didn't create a common baud-rate/clock-update method
because the baud-rate change was just a two stages action:
1. calculate divisor+quot couple based on the new clock,
2. update the divisor+quot (+ update the timeout).
The first stage didn't need to have the IRQsafe lock being held and
the runtime-PM being enabled, meanwhile the later one needed those.
So unless the nested locking or try-lock-based pattern is implemented
each stage required dedicated function introduced, which would have
been an overkill for that. But even if I got to implement the
try-lock-based solution with a single function containing both stages
I still couldn't avoid having the serial8250_get_baud_rate() and
serial8250_get_divisor() methods executed in the atomic context, which
isn't required for them and which would needlessly pro-long the CPU
executing with the IRQs disabled. As you well know it's better to
speed up the atomic context execution as much as possible. 

Secondly I didn't know much about the tty/serial subsystem internals
back then. So I was afraid to break some parts I didn't aware of if
the baud-rate/ref-clock change code had some implicit dependencies
from the surrounding code and vice-versa (like the LCR DLAB flag
state).

Finally frankly it didn't seem like that much worth bothering about.
Basically AFAICS there were only four methods which invocation I
would have needed to move to a separate function:

serial8250_get_baud_rate();
serial8250_get_divisor();
// spin-lock
uart_update_timeout(port, termios->c_cflag, baud);
serial8250_set_divisor(port, baud, quot, frac);
// spin-unlock

So I decided to take a simplest and safest path, and created a
dedicated method for the just the ref-clock updates case leaving the
baud-rate change task implemented in the framework of the standard
serial8250_do_set_termios() method.


Regarding doing vise-versa and calling the serial8250_do_set_termios()
method from serial8250_update_uartclk() instead. To be honest I didn't
consider that option. That might work though, but AFAICS the
serial8250_do_set_termios() function will do much more than it's
required in case if the ref-clock has changed.

-Serge(y)

> 
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>