linux-kernel - Re: [PATCH v3 2/4] serial: 8250: Add 8250 port clock update method

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Date:   Fri, 15 May 2020 17:32:54 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiri Slaby <jslaby@...e.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Long Cheng <long.cheng@...iatek.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        <linux-mips@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        Lukas Wunner <lukas@...ner.de>, Stefan Roese <sr@...x.de>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Allison Randal <allison@...utok.net>,
        Yegor Yefremov <yegorslists@...glemail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        <linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/4] serial: 8250: Add 8250 port clock update method

On Fri, May 15, 2020 at 02:45:25PM +0200, Greg Kroah-Hartman wrote:
> On Thu, May 07, 2020 at 02:31:33AM +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.
> > 
> > Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > Cc: Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>
> > Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> > Cc: Paul Burton <paulburton@...nel.org>
> > Cc: Ralf Baechle <ralf@...ux-mips.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Long Cheng <long.cheng@...iatek.com>
> > Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Cc: Maxime Ripard <mripard@...nel.org>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Russell King <linux@...linux.org.uk>
> > Cc: linux-mips@...r.kernel.org
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-mediatek@...ts.infradead.org
> > ---
> >  drivers/tty/serial/8250/8250_port.c | 38 +++++++++++++++++++++++++++++
> >  include/linux/serial_8250.h         |  2 ++
> >  2 files changed, 40 insertions(+)
> > 
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 4d83c85a7389..484ff9df1432 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -2628,6 +2628,44 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
> >  				  (port->uartclk + tolerance) / 16);
> >  }
> >  
> > +/*
> > + * 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);
> > +
> > +	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);
> > +
> > +out_lock:
> > +	mutex_unlock(&port->state->port.mutex);
> > +}
> > +EXPORT_SYMBOL(serial8250_update_uartclk);
> 
> EXPORT_SYMBOL_GPL() please.

Ok. I guess I've just copied the line from some of the export symbol
statements below. My mistake. Sorry.

-Sergey

> 
> thanks,
> 
> greg k-h