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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Date:   Thu, 21 Apr 2022 10:46:57 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        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>,
        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

Hi Andy,

On 18/04/2022 14:09, Andy Shevchenko wrote:
> 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

While I'm in favor of defining private data, here 3 and 2 are actually the values
2 and 3 used to divide.

The code is easy to read and we quickly understand this value is the clock divider.

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