[PATCH] serial: 8250_mtk: Fix high-speed baud rates clamping

[Claire Chang]

On Wed, Jul 15, 2020 at 4:45 AM Daniel Winkler <danielwinkler at google.com> wrote:
>
> 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 at baikalelectronics.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 at google.com>
> > Signed-off-by: Serge Semin <Sergey.Semin at baikalelectronics.ru>
Tested-by: Claire Chang <tientzu at chromium.org>
> >
> > ---
> >
> > 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.
I tested it with mt8183 + QCA6174.
The UART Bluetooth works fine with this fix.
Thanks!
> >
> > Cc: Alexey Malahov <Alexey.Malahov at baikalelectronics.ru>
> > Cc: Daniel Winkler <danielwinkler at google.com>
> > Cc: Aaron Sierra <asierra at xes-inc.com>
> > Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> > Cc: Lukas Wunner <lukas at wunner.de>
> > Cc: Vignesh Raghavendra <vigneshr at ti.com>
> > Cc: linux-serial at vger.kernel.org
> > Cc: linux-mediatek at lists.infradead.org
> > Cc: BlueZ <linux-bluetooth at vger.kernel.org>
> > Cc: chromeos-bluetooth-upstreaming <chromeos-bluetooth-upstreaming at chromium.org>
> > Cc: abhishekpandit at chromium.org
> > Cc: stable at vger.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
> >