linux-kernel - Re: [PATCH V3 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:   Fri, 22 Apr 2022 14:30:07 +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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: Re: [PATCH V3 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 22/04/2022 13:13, Yu Tu 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
> uses 3Mhz baud rate. 8Mhz calculations can lead to baud rate bias,
> causing some problems.
> 
> Signed-off-by: Yu Tu <yu.tu@...ogic.com>
> ---
>   drivers/tty/serial/meson_uart.c | 17 +++++++++++++++--
>   1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
> index 8e59624935af..58bd2723c004 100644
> --- a/drivers/tty/serial/meson_uart.c
> +++ b/drivers/tty/serial/meson_uart.c
> @@ -68,6 +68,7 @@
>   #define AML_UART_BAUD_MASK		0x7fffff
>   #define AML_UART_BAUD_USE		BIT(23)
>   #define AML_UART_BAUD_XTAL		BIT(24)
> +#define AML_UART_BAUD_XTAL_DIV2		BIT(27)
>   
>   #define AML_UART_PORT_NUM		12
>   #define AML_UART_PORT_OFFSET		6
> @@ -80,6 +81,10 @@ static struct uart_driver meson_uart_driver;
>   
>   static struct uart_port *meson_ports[AML_UART_PORT_NUM];
>   
> +struct meson_uart_data {
> +	bool has_xtal_div2;
> +};
> +
>   static void meson_uart_set_mctrl(struct uart_port *port, unsigned int mctrl)
>   {
>   }
> @@ -293,13 +298,20 @@ static int meson_uart_startup(struct uart_port *port)
>   
>   static void meson_uart_change_speed(struct uart_port *port, unsigned long baud)
>   {
> -	u32 val;
> +	const struct meson_uart_data *private_data = port->private_data;
> +	u32 val = 0;
>   
>   	while (!meson_uart_tx_empty(port))
>   		cpu_relax();
>   
>   	if (port->uartclk == 24000000) {
> -		val = DIV_ROUND_CLOSEST(port->uartclk / 3, baud) - 1;
> +		unsigned int xtal_div = 3;
> +
> +		if (private_data && private_data->has_xtal_div2) {
> +			xtal_div = 2;
> +			val |= AML_UART_BAUD_XTAL_DIV2;
> +		}
> +		val |= DIV_ROUND_CLOSEST(port->uartclk / xtal_div, baud) - 1;
>   		val |= AML_UART_BAUD_XTAL;
>   	} else {
>   		val =  DIV_ROUND_CLOSEST(port->uartclk / 4, baud) - 1;
> @@ -749,6 +761,7 @@ static int meson_uart_probe(struct platform_device *pdev)
>   	port->x_char = 0;
>   	port->ops = &meson_uart_ops;
>   	port->fifosize = fifosize;
> +	port->private_data = (void *)device_get_match_data(&pdev->dev);
>   
>   	meson_ports[pdev->id] = port;
>   	platform_set_drvdata(pdev, port);

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