linux-kernel - Re: [PATCH 1/3] tty: serial: meson: Use DIV_ROUND

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Date:   Thu, 7 Apr 2022 15:56:09 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Yu Tu <yu.tu@...ogic.com>, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: Re: [PATCH 1/3] tty: serial: meson: Use DIV_ROUND_CLOSEST to
 calculate baud rates

On 07/04/2022 10:13, Yu Tu wrote:
> Due to chip process differences, chip designers recommend using baud
> rates as close to and larger as possible in order to reduce clock
> errors.
> 
> Signed-off-by: Yu Tu <yu.tu@...ogic.com>
> ---
>   drivers/tty/serial/meson_uart.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
> index 2bf1c57e0981..8e59624935af 100644
> --- a/drivers/tty/serial/meson_uart.c
> +++ b/drivers/tty/serial/meson_uart.c
> @@ -299,10 +299,10 @@ static void meson_uart_change_speed(struct uart_port *port, unsigned long baud)
>   		cpu_relax();
>   
>   	if (port->uartclk == 24000000) {
> -		val = ((port->uartclk / 3) / baud) - 1;
> +		val = DIV_ROUND_CLOSEST(port->uartclk / 3, baud) - 1;
>   		val |= AML_UART_BAUD_XTAL;
>   	} else {
> -		val = ((port->uartclk * 10 / (baud * 4) + 5) / 10) - 1;
> +		val =  DIV_ROUND_CLOSEST(port->uartclk / 4, baud) - 1;
>   	}
>   	val |= AML_UART_BAUD_USE;
>   	writel(val, port->membase + AML_UART_REG5);

I check the calculations, and with DIV_ROUND_CLOSEST(), result is always
closer to the required baudrate.

Reviewed-by: Neil Armstrong <narmstrong@...libre.com>