[v2,2/2] serial: 8250/ingenic: Add support for the JZ4750/JZ4755

JZ4750/55/60 (but not JZ4760b) have an extra divisor in between extclk
and peripheral clock, called CPCCR.ECS, the driver can't figure out the
real state of the divisor without dirty hack - peek CGU CPCCR register.
However, we can rely on a vendor's bootloader (u-boot 1.1.6) behavior:
if (extclk > 16MHz)
    the divisor is enabled, so the UART driving clock is extclk/2.

This behavior relies on hardware differences: most boards (if not all)
with those SoCs have 12 or 24 MHz oscillators but many peripherals want
12Mhz to operate properly (AIC and USB-PHY at least).

The patch doesn't affect JZ4760's behavior as it is subject for another
patchset with re-classification of all supported ingenic UARTs.

Link: https://github.com/carlos-wong/uboot_jz4755/blob/master/cpu/mips/jz_serial.c#L158
Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
---
 drivers/tty/serial/8250/8250_ingenic.c | 50 ++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 7 deletions(-)

Hi Siarhei,

Le sam. 22 oct. 2022 à 18:12:24 +0300, Siarhei Volkau 
<lis8215@gmail.com> a écrit :
> JZ4750/55/60 (but not JZ4760b) have an extra divisor in between extclk
> and peripheral clock, called CPCCR.ECS, the driver can't figure out 
> the
> real state of the divisor without dirty hack - peek CGU CPCCR 
> register.
> However, we can rely on a vendor's bootloader (u-boot 1.1.6) behavior:
> if (extclk > 16MHz)
>     the divisor is enabled, so the UART driving clock is extclk/2.
> 
> This behavior relies on hardware differences: most boards (if not all)
> with those SoCs have 12 or 24 MHz oscillators but many peripherals 
> want
> 12Mhz to operate properly (AIC and USB-PHY at least).
> 
> The patch doesn't affect JZ4760's behavior as it is subject for 
> another
> patchset with re-classification of all supported ingenic UARTs.
> 
> Link: 
> https://github.com/carlos-wong/uboot_jz4755/blob/master/cpu/mips/jz_serial.c#L158
> Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
> ---
>  drivers/tty/serial/8250/8250_ingenic.c | 50 
> ++++++++++++++++++++++----
>  1 file changed, 43 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_ingenic.c 
> b/drivers/tty/serial/8250/8250_ingenic.c
> index 2b2f5d8d2..3ffa6b722 100644
> --- a/drivers/tty/serial/8250/8250_ingenic.c
> +++ b/drivers/tty/serial/8250/8250_ingenic.c
> @@ -87,24 +87,19 @@ static void __init 
> ingenic_early_console_setup_clock(struct earlycon_device *dev
>  	dev->port.uartclk = be32_to_cpup(prop);
>  }
> 
> -static int __init ingenic_early_console_setup(struct earlycon_device 
> *dev,
> -					      const char *opt)
> +static int __init ingenic_earlycon_setup_tail(struct earlycon_device 
> *dev,
> +					      const char *opt)
>  {
>  	struct uart_port *port = &dev->port;
>  	unsigned int divisor;
>  	int baud = 115200;
> 
> -	if (!dev->port.membase)
> -		return -ENODEV;

You can keep this here - no need to move it (and it'd avoid duplicating 
code).

> -
>  	if (opt) {
>  		unsigned int parity, bits, flow; /* unused for now */
> 
>  		uart_parse_options(opt, &baud, &parity, &bits, &flow);
>  	}
> 
> -	ingenic_early_console_setup_clock(dev);
> -
>  	if (dev->baud)
>  		baud = dev->baud;
>  	divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * baud);
> @@ -129,9 +124,49 @@ static int __init 
> ingenic_early_console_setup(struct earlycon_device *dev,
>  	return 0;
>  }
> 
> +static int __init ingenic_early_console_setup(struct earlycon_device 
> *dev,
> +					      const char *opt)
> +{
> +	if (!dev->port.membase)
> +		return -ENODEV;
> +
> +	ingenic_early_console_setup_clock(dev);
> +
> +	ingenic_earlycon_setup_tail(dev, opt);
> +}
> +
> +static int __init jz4750_early_console_setup(struct earlycon_device 
> *dev,
> +					     const char *opt)
> +{
> +	if (!dev->port.membase)
> +		return -ENODEV;
> +
> +	/*
> +	 * JZ4750/55/60 (not JZ4760b) have an extra divisor
> +	 * between extclk and peripheral clock, the
> +	 * driver can't figure out the real state of the
> +	 * divisor without dirty hacks (peek CGU register).
> +	 * However, we can rely on a vendor's behavior:
> +	 * if (extclk > 16MHz)
> +	 *   the divisor is enabled.
> +	 * This behavior relies on hardware differences:
> +	 * most boards with those SoCs have 12 or 24 MHz
> +	 * oscillators but many peripherals want 12Mhz
> +	 * to operate properly (AIC and USB-phy at least).
> +	 */
> +	ingenic_early_console_setup_clock(dev);
> +	if (dev->port.uartclk > 16000000)
> +		dev->port.uartclk /= 2;

I would assume you could just do:
dev->port.uartclk = 12000000;

Since you'd always get a 12 MHz clock (either with a 12 MHz oscillator 
or a 24 MHz oscillator with a /2 divider).

With that said - I am fine with that code, it would allow to fine-tune 
the oscillator value (although I hightly doubt anybody is going to do 
that).

The 16 MHz value sounds very arbitrary, but I'll give it a pass.

Cheers,
-Paul

> +
> +	ingenic_earlycon_setup_tail(dev, opt);
> +}
> +
>  OF_EARLYCON_DECLARE(jz4740_uart, "ingenic,jz4740-uart",
>  		    ingenic_early_console_setup);
> 
> +OF_EARLYCON_DECLARE(jz4750_uart, "ingenic,jz4750-uart",
> +		    jz4750_early_console_setup);
> +
>  OF_EARLYCON_DECLARE(jz4770_uart, "ingenic,jz4770-uart",
>  		    ingenic_early_console_setup);
> 
> @@ -328,6 +363,7 @@ static const struct ingenic_uart_config 
> x1000_uart_config = {
> 
>  static const struct of_device_id of_match[] = {
>  	{ .compatible = "ingenic,jz4740-uart", .data = &jz4740_uart_config 
> },
> +	{ .compatible = "ingenic,jz4750-uart", .data = &jz4760_uart_config 
> },
>  	{ .compatible = "ingenic,jz4760-uart", .data = &jz4760_uart_config 
> },
>  	{ .compatible = "ingenic,jz4770-uart", .data = &jz4760_uart_config 
> },
>  	{ .compatible = "ingenic,jz4775-uart", .data = &jz4760_uart_config 
> },
> --
> 2.36.1
>