[PATCH RFC] tty: serial: meson: add new compatible for gxl uart

[Neil Armstrong]

Hi,

On 25/04/2022 22:39, Heiner Kallweit wrote:
> My SC2 test system (ah212-based HK1 RBOX X4) uses an uboot console
> speed of 921600 bps. Being able to use the same speed in Linux
> requires using a 12MHz clock source. Ideally we'd set the new
> compatible to the first chip version supporting the
> AML_UART_BAUD_XTAL_DIV2 bit.

Actually G12A is the first chip having AML_UART_BAUD_XTAL_DIV2, but SC2 is
the first one requiring it to achieve 921600 bps correctly.

> Not having access to chip documentation
> I'd appreciate a hint on when this bit was introduced.
> If fine with you in general the final series would include a
> separate patch for the new compatible.

This has been already handled in:
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-next&id=368ab68b18de104719f386a5cfe3595673cc96de
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-next&id=00a7fa836dbc454faf5b7027ad67519af7c6c15b
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-next&id=9b92cc5ee2d10e6c4d327d1e4ceb77aa8b1081ee

> 
> Note: This patch doesn't consider "tty: serial: meson:
> Use DIV_ROUND_CLOSEST to calculate baud rates" yet and therefore
> won't apply cleanly on linux-next.
> I locally used the same DIV_ROUND_CLOSEST() changes before.
> ----------------------------------------------------------------
> Newer SoC versions support using XTAL / 2 (12MHz) as clock source
> for the UART. This clock source allows to support 921600 bps as used
> by vendor uboot on certain systems like ah212 ref board.
> 
> 12MHz / 13 = 923076 bps
> With the currently used 8MHz clock source only 888888bps or
> 1000000 bps are possible.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
> ---
>   drivers/tty/serial/meson_uart.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
> index 2bf1c57e0..0a0a0636f 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
> @@ -299,10 +300,15 @@ 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 |= AML_UART_BAUD_XTAL;
> +		if (of_device_is_compatible(port->dev->of_node, "amlogic,meson-gxl-uart")) {
> +			val = DIV_ROUND_CLOSEST(port->uartclk, 2 * baud) - 1;
> +			val |= AML_UART_BAUD_XTAL | AML_UART_BAUD_XTAL_DIV2;
> +		} else {
> +			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);
> @@ -782,6 +788,7 @@ static const struct of_device_id meson_uart_dt_match[] = {
>   	{ .compatible = "amlogic,meson8-uart" },
>   	{ .compatible = "amlogic,meson8b-uart" },
>   	{ .compatible = "amlogic,meson-gx-uart" },
> +	{ .compatible = "amlogic,meson-gxl-uart" },
>   	{ /* sentinel */ },
>   };
>   MODULE_DEVICE_TABLE(of, meson_uart_dt_match);