linux-kernel - Re: [PATCH V2 1/2] tty: serial: meson: Add a 12MHz internal clock rate to calculate baud rate in order to meet the baud rate requirements of special BT modules

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Message-ID: <CAHp75VeW65dV9jJu8-yUWME+XKnaxZBu5Zv8iEJxP2dizA=HUg@mail.gmail.com>
Date:   Mon, 18 Apr 2022 15:09:48 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Yu Tu <yu.tu@...ogic.com>
Cc:     "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        linux-amlogic <linux-amlogic@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: Re: [PATCH V2 1/2] tty: serial: meson: Add a 12MHz internal clock
 rate to calculate baud rate in order to meet the baud rate requirements of
 special BT modules

On Mon, Apr 18, 2022 at 8:50 AM Yu Tu <yu.tu@...ogic.com> wrote:
>
> A /2 divider over XTAL was introduced since G12A, and is preferred
> to be used over the still present /3 divider since it provides much
> closer frequencies vs the request baudrate.Especially the BT module

'e. E' (mind the space)

> uses 3Mhz baud rate. 8Mhz calculations can lead to baud rate bias,
> causing some problems.

...

> +struct meson_uart_data {
> +       bool has_xtal_div2;

I would prefer to see this as an unsigned int and with a less
particular name, e.g. xtal_div would suffice.

> +};

...

> +               unsigned int xtal_div = 3;

> +               if (private_data && private_data->has_xtal_div2) {
> +                       xtal_div = 2;

Better to define privata data always


> +                       val |= AML_UART_BAUD_XTAL_DIV2;
> +               }
> +               val |= DIV_ROUND_CLOSEST(port->uartclk / xtal_div, baud) - 1;


-- 
With Best Regards,
Andy Shevchenko