linux-kernel - Re: [PATCH 2/3] tty: serial: meson: Added 12Mhz as the clock source for calculating baud rate

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Message-ID: <bedff186-8ce4-3cfd-aa67-f0375daf29b4@baylibre.com>
Date:   Thu, 7 Apr 2022 16:03:01 +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 2/3] tty: serial: meson: Added 12Mhz as the clock source
 for calculating baud rate

On 07/04/2022 10:13, Yu Tu wrote:
> Starting with the g12A chip, add a 12Mhz clock to calculate the baud
> rate, since the BT module uses 3Mhz baud rate. 8Mhz calculations can
> lead to baud rate bias, causing some problems.

The commit message isn't clear enough, 12MHz is a new intermediate clock
rate, not a new clock source of the UART module.

Please explain 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.

> 
> Signed-off-by: Yu Tu <yu.tu@...ogic.com>
> ---
>   drivers/tty/serial/meson_uart.c | 25 +++++++++++++++++++++++--
>   1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
> index 8e59624935af..7e77693a1318 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;
> +	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->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;
> @@ -691,6 +703,7 @@ static int meson_uart_probe_clocks(struct platform_device *pdev,
>   
>   static int meson_uart_probe(struct platform_device *pdev)
>   {
> +	struct meson_uart_data *private_data;
>   	struct resource *res_mem;
>   	struct uart_port *port;
>   	u32 fifosize = 64; /* Default is 64, 128 for EE UART_0 */
> @@ -733,6 +746,13 @@ static int meson_uart_probe(struct platform_device *pdev)
>   	if (!port)
>   		return -ENOMEM;
>   
> +	private_data = devm_kzalloc(&pdev->dev, sizeof(struct meson_uart_data), GFP_KERNEL);
> +	if (!private_data)
> +		return -ENOMEM;
> +
> +	if ((bool)device_get_match_data(&pdev->dev))
> +		private_data->has_xtal_div2 = true;

It should be much cleaner to pass meson_uart_data to meson_uart_dt_match .data,
and then you can retrieve device_get_match_data() and put the result into
port->private_data.
This will avoid a devm_kzalloc().

Then in meson_uart_change_speed() change the test to check for NULL private_data:
if (private_data && private_data->has_xtal_div2)

> +
>   	ret = meson_uart_probe_clocks(pdev, port);
>   	if (ret)
>   		return ret;
> @@ -749,6 +769,7 @@ static int meson_uart_probe(struct platform_device *pdev)
>   	port->x_char = 0;
>   	port->ops = &meson_uart_ops;
>   	port->fifosize = fifosize;
> +	port->private_data = private_data;
>   
>   	meson_ports[pdev->id] = port;
>   	platform_set_drvdata(pdev, port);

Neil