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

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

On Thu, Feb 15, 2024 at 09:39:12PM +0200, Andy Shevchenko wrote:
> On Thu, Feb 15, 2024 at 10:32:18PM +0300, Serge Semin wrote:
> > 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.)
> 
> Indeed :-)
> 
> > 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.
> 

> My point here is that the idea behind clock change is most likely to be
> followed up by ->set_termios(). Why to do it differently if it's the case?

Not always. IIUC what you say is just a one path of the code executed
within the chain:

dw8250_set_termios()->dw8250_do_set_termios()->serial8250_do_set_termios()

But another code-path will be taken if the DW UART port
ref-clock is suddenly and asynchronously changed. In that case the
driver is notified by means of the dw8250_clk_notifier_cb() callback,
which doesn't need the entire set_termios() callback execution but
only what is defined in the serial8250_update_uartclk() method:

dw8250_clk_notifier_cb()
+-> worker:: dw8250_clk_work_cb()->serial8250_update_uartclk().

> And note, ->set_termios() can be called as many times as needed, so if nothing
> changes in between it's also fine. But this makes intention much clearer.
> Do you agree?

If what you suggest is to replace the serial8250_update_uartclk() body
with a direct uart_port::set_termios() invocation then I don't find it
being much clearer really. The serial8250_update_uartclk() is
currently specialized on doing one thing: adjusting the divider in
case of the UART-clock change. If instead the entire
serial8250_set_termios() method is called then for a reader it won't
be easy to understand what is really required for a 8250 serial port
to perceive the ref-clock change. But from the maintainability point
of view I guess that it might be safer to just call
serial8250_set_termios() indeed, since among the other things the
later method implies the divider update too. Thus the maintainer won't
need to support the two clock divider update implementations. From
that perspective I agree, directly calling serial8250_set_termios()
might be more suitable despite of it' doing more than required.

-Serge(y)

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