linux-kernel - Re: [PATCH v1 tty-next 1/2] 8250: microchip: pci1xxxx: Add driver for the quad-uart function in the multi-function endpoint of pci1xxxx device.

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Message-ID: <CAHp75VfUq-JkfjD1YOj_+ST-eK=aW=8LoHOUW4zqpeA_QUZbyw@mail.gmail.com>
Date:   Fri, 2 Sep 2022 18:02:34 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Kumaravel Thiagarajan - I21417 
        <Kumaravel.Thiagarajan@...rochip.com>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Johan Hovold <johan@...nel.org>,
        Wander Lairson Costa <wander@...hat.com>,
        Eric Tremblay <etremblay@...tech-controls.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Jeremy Kerr <jk@...abs.org>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Lukas Wunner <lukas@...ner.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>
Subject: Re: [PATCH v1 tty-next 1/2] 8250: microchip: pci1xxxx: Add driver for
 the quad-uart function in the multi-function endpoint of pci1xxxx device.

On Fri, Sep 2, 2022 at 2:57 PM <Kumaravel.Thiagarajan@...rochip.com> wrote:
> > -----Original Message-----
> > From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> > Sent: Thursday, September 1, 2022 7:12 PM
> > On Thu, 1 Sep 2022, Kumaravel.Thiagarajan@...rochip.com wrote:

...

> > > > > +       const unsigned int standard_baud_list[] = {50, 75, 110, 134, 150,
> > 300,
> > > > > +                                               600, 1200, 1800, 2000, 2400, 3600,
> > > > > +                                               4800, 7200, 9600, 19200, 38400, 57600,
> > > > > +                                               115200, 125000, 136400, 150000, 166700,
> > > > > +                                               187500, 214300, 250000, 300000, 375000,
> > > > > +                                               500000, 750000,
> > > > > + 1000000, 1500000};
> > > >
> > > > Why?!
> > >
> > > The standard baud rates are handled within serial8250_do_set_termios
> > > which is invoked from within mchp_pci1xxxx_set_termios in first place.
> > > Hence if it matches with any of the standard baudrates, it can return
> > > immediately.
> >
> > Care to explain why the baudrates in your table don't match those in
> > tty_baudrate.c? ...It makes no sense to me that you call these "standard
> > baud rates".
> The baudrates in my table are from our legacy UART IP and these baudrates can be
> generated by the hardware by updating UART_DLL & UART_DLM alone as done by the
> serial8250_do_set_termios.
> I noticed that some of the baud rates in tty_baudrate.c arenot listed in this table
> but will still be handled by the mchp_pci1xxxx_set_termios.
> I can rename standard_baud_list to simply baud_list. Please let me know.

No, the point is avoid repeating what standard APIs already do. Just
make sure you call it properly and provide _get/_set_divisor()
callbacks. Note, your driver can cope with BOTHER and there all
non-standard baud rates go.

-- 
With Best Regards,
Andy Shevchenko