Date:   Tue, 14 Jul 2020 13:45:01 -0700
From:   Daniel Winkler <danielwinkler@...gle.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Claire Chang <tientzu@...gle.com>,
        Nicolas Boichat <drinkcat@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Aaron Sierra <asierra@...-inc.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Lukas Wunner <lukas@...ner.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        BlueZ <linux-bluetooth@...r.kernel.org>,
        chromeos-bluetooth-upstreaming 
        <chromeos-bluetooth-upstreaming@...omium.org>,
        abhishekpandit@...omium.org, stable@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: 8250_mtk: Fix high-speed baud rates clamping

Thank you Sergey for looking into this. Adding folks working on this
platform to perform validation of the proposed patch.

Best,
Daniel

On Tue, Jul 14, 2020 at 5:41 AM Serge Semin
<Sergey.Semin@...kalelectronics.ru> wrote:
>
> Commit 7b668c064ec3 ("serial: 8250: Fix max baud limit in generic 8250
> port") fixed limits of a baud rate setting for a generic 8250 port.
> In other words since that commit the baud rate has been permitted to be
> within [uartclk / 16 / UART_DIV_MAX; uartclk / 16], which is absolutely
> normal for a standard 8250 UART port. But there are custom 8250 ports,
> which provide extended baud rate limits. In particular the Mediatek 8250
> port can work with baud rates up to "uartclk" speed.
>
> Normally that and any other peculiarity is supposed to be handled in a
> custom set_termios() callback implemented in the vendor-specific
> 8250-port glue-driver. Currently that is how it's done for the most of
> the vendor-specific 8250 ports, but for some reason for Mediatek a
> solution has been spread out to both the glue-driver and to the generic
> 8250-port code. Due to that a bug has been introduced, which permitted the
> extended baud rate limit for all even for standard 8250-ports. The bug
> has been fixed by the commit 7b668c064ec3 ("serial: 8250: Fix max baud
> limit in generic 8250 port") by narrowing the baud rates limit back down to
> the normal bounds. Unfortunately by doing so we also broke the
> Mediatek-specific extended bauds feature.
>
> A fix of the problem described above is twofold. First since we can't get
> back the extended baud rate limits feature to the generic set_termios()
> function and that method supports only a standard baud rates range, the
> requested baud rate must be locally stored before calling it and then
> restored back to the new termios structure after the generic set_termios()
> finished its magic business. By doing so we still use the
> serial8250_do_set_termios() method to set the LCR/MCR/FCR/etc. registers,
> while the extended baud rate setting procedure will be performed later in
> the custom Mediatek-specific set_termios() callback. Second since a true
> baud rate is now fully calculated in the custom set_termios() method we
> need to locally update the port timeout by calling the
> uart_update_timeout() function. After the fixes described above are
> implemented in the 8250_mtk.c driver, the Mediatek 8250-port should
> get back to normally working with extended baud rates.
>
> Link: https://lore.kernel.org/linux-serial/20200701211337.3027448-1-danielwinkler@google.com
>
> Fixes: 7b668c064ec3 ("serial: 8250: Fix max baud limit in generic 8250 port")
> Reported-by: Daniel Winkler <danielwinkler@...gle.com>
> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
>
> ---
>
> Folks, sorry for a delay with the problem fix. A solution is turned out to
> be a bit more complicated than I originally thought in my comment to the
> Daniel revert-patch.
>
> Please also note, that I don't have a Mediatek hardware to test the
> solution suggested in the patch. The code is written as on so called
> the tip of the pen after digging into the 8250_mtk.c and 8250_port.c
> drivers code. So please Daniel or someone with Mediatek 8250-port
> available on a board test this patch first and report about the results in
> reply to this emailing thread. After that, if your conclusion is positive
> and there is no objection against the solution design the patch can be
> merged in.
>
> Cc: Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>
> Cc: Daniel Winkler <danielwinkler@...gle.com>
> Cc: Aaron Sierra <asierra@...-inc.com>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Lukas Wunner <lukas@...ner.de>
> Cc: Vignesh Raghavendra <vigneshr@...com>
> Cc: linux-serial@...r.kernel.org
> Cc: linux-mediatek@...ts.infradead.org
> Cc: BlueZ <linux-bluetooth@...r.kernel.org>
> Cc: chromeos-bluetooth-upstreaming <chromeos-bluetooth-upstreaming@...omium.org>
> Cc: abhishekpandit@...omium.org
> Cc: stable@...r.kernel.org
> ---
>  drivers/tty/serial/8250/8250_mtk.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
> index f839380c2f4c..98b8a3e30733 100644
> --- a/drivers/tty/serial/8250/8250_mtk.c
> +++ b/drivers/tty/serial/8250/8250_mtk.c
> @@ -306,8 +306,21 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios,
>         }
>  #endif
>
> +       /*
> +        * Store the requested baud rate before calling the generic 8250
> +        * set_termios method. Standard 8250 port expects bauds to be
> +        * no higher than (uartclk / 16) so the baud will be clamped if it
> +        * gets out of that bound. Mediatek 8250 port supports speed
> +        * higher than that, therefore we'll get original baud rate back
> +        * after calling the generic set_termios method and recalculate
> +        * the speed later in this method.
> +        */
> +       baud = tty_termios_baud_rate(termios);
> +
>         serial8250_do_set_termios(port, termios, old);
>
> +       tty_termios_encode_baud_rate(termios, baud, baud);
> +
>         /*
>          * Mediatek UARTs use an extra highspeed register (MTK_UART_HIGHS)
>          *
> @@ -339,6 +352,11 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios,
>          */
>         spin_lock_irqsave(&port->lock, flags);
>
> +       /*
> +        * Update the per-port timeout.
> +        */
> +       uart_update_timeout(port, termios->c_cflag, baud);
> +
>         /* set DLAB we have cval saved in up->lcr from the call to the core */
>         serial_port_out(port, UART_LCR, up->lcr | UART_LCR_DLAB);
>         serial_dl_write(up, quot);
> --
> 2.26.2
>

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