linux-kernel - Re: [PATCH v3 1/4] serial: 8250: Fix max baud limit in generic 8250 port

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Date:   Fri, 15 May 2020 16:48:27 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Long Cheng <long.cheng@...iatek.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        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>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Stefan Roese <sr@...x.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] serial: 8250: Fix max baud limit in generic 8250
 port

On Thu, May 07, 2020 at 02:31:32AM +0300, Serge Semin wrote:
> Standard 8250 UART ports are designed in a way so they can communicate
> with baud rates up to 1/16 of a reference frequency. It's expected from
> most of the currently supported UART controllers. That's why the former
> version of serial8250_get_baud_rate() method called uart_get_baud_rate()
> with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX)
> and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like
> it was suggested in commit ("serial: 8250_mtk: support big baud rate."),
> caused acceptance of bauds, which was higher than the normal UART
> controllers actually supported. As a result if some user-space program
> requested to set a baud greater than (uartclk / 16) it would have been
> permitted without truncation, but then serial8250_get_divisor(baud)
> (which calls uart_get_divisor() to get the reference clock divisor) would
> have returned a zero divisor. Setting zero divisor will cause an
> unpredictable effect varying from chip to chip. In case of DW APB UART the
> communications just stop.
> 
> Lets fix this problem by getting back the limitation of (uartclk +
> tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek
> 8250 UART ports driver developer shouldn't have touched it in the first
> place  notably seeing he already provided a custom version of set_termios()
> callback in that glue-driver which took into account the extended baud
> rate values and accordingly updated the standard and vendor-specific
> divisor latch registers anyway.

Some of the hardware support PS != 16 (8250_mid), but for now it lies to UART
core for real UART clock because of its (core) hard cored assumption PS == 16
here and there.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> 
> Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
> 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 | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index f77bf820b7a3..4d83c85a7389 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
>  					     struct ktermios *termios,
>  					     struct ktermios *old)
>  {
> +	unsigned int tolerance = port->uartclk / 100;
> +
>  	/*
>  	 * Ask the core to calculate the divisor for us.
>  	 * Allow 1% tolerance at the upper limit so uart clks marginally
> @@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
>  	 */
>  	return uart_get_baud_rate(port, termios, old,
>  				  port->uartclk / 16 / UART_DIV_MAX,
> -				  port->uartclk);
> +				  (port->uartclk + tolerance) / 16);
>  }
>  
>  void
> -- 
> 2.25.1
> 

-- 
With Best Regards,
Andy Shevchenko