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Date:   Mon, 24 Oct 2022 13:50:50 +0100
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH v2 2/2] serial: 8250/ingenic: Add support for the
 JZ4750/JZ4755
To:     Siarhei Volkau <lis8215@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        linux-serial@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org
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In-Reply-To: <20221022151224.4000238-3-lis8215@gmail.com>
References: <20221022151224.4000238-1-lis8215@gmail.com>
        <20221022151224.4000238-3-lis8215@gmail.com>
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Hi Siarhei,

Le sam. 22 oct. 2022 =E0 18:12:24 +0300, Siarhei Volkau=20
<lis8215@gmail.com> a =E9crit :
> 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=20
> the
> real state of the divisor without dirty hack - peek CGU CPCCR=20
> 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.
>=20
> This behavior relies on hardware differences: most boards (if not all)
> with those SoCs have 12 or 24 MHz oscillators but many peripherals=20
> want
> 12Mhz to operate properly (AIC and USB-PHY at least).
>=20
> The patch doesn't affect JZ4760's behavior as it is subject for=20
> another
> patchset with re-classification of all supported ingenic UARTs.
>=20
> Link:=20
> https://github.com/carlos-wong/uboot_jz4755/blob/master/cpu/mips/jz_seria=
l.c#L158
> Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
> ---
>  drivers/tty/serial/8250/8250_ingenic.c | 50=20
> ++++++++++++++++++++++----
>  1 file changed, 43 insertions(+), 7 deletions(-)
>=20
> diff --git a/drivers/tty/serial/8250/8250_ingenic.c=20
> 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=20
> ingenic_early_console_setup_clock(struct earlycon_device *dev
>  	dev->port.uartclk =3D be32_to_cpup(prop);
>  }
>=20
> -static int __init ingenic_early_console_setup(struct earlycon_device=20
> *dev,
> -					      const char *opt)
> +static int __init ingenic_earlycon_setup_tail(struct earlycon_device=20
> *dev,
> +					      const char *opt)
>  {
>  	struct uart_port *port =3D &dev->port;
>  	unsigned int divisor;
>  	int baud =3D 115200;
>=20
> -	if (!dev->port.membase)
> -		return -ENODEV;

You can keep this here - no need to move it (and it'd avoid duplicating=20
code).

> -
>  	if (opt) {
>  		unsigned int parity, bits, flow; /* unused for now */
>=20
>  		uart_parse_options(opt, &baud, &parity, &bits, &flow);
>  	}
>=20
> -	ingenic_early_console_setup_clock(dev);
> -
>  	if (dev->baud)
>  		baud =3D dev->baud;
>  	divisor =3D DIV_ROUND_CLOSEST(port->uartclk, 16 * baud);
> @@ -129,9 +124,49 @@ static int __init=20
> ingenic_early_console_setup(struct earlycon_device *dev,
>  	return 0;
>  }
>=20
> +static int __init ingenic_early_console_setup(struct earlycon_device=20
> *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=20
> *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 /=3D 2;

I would assume you could just do:
dev->port.uartclk =3D 12000000;

Since you'd always get a 12 MHz clock (either with a 12 MHz oscillator=20
or a 24 MHz oscillator with a /2 divider).

With that said - I am fine with that code, it would allow to fine-tune=20
the oscillator value (although I hightly doubt anybody is going to do=20
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);
>=20
> +OF_EARLYCON_DECLARE(jz4750_uart, "ingenic,jz4750-uart",
> +		    jz4750_early_console_setup);
> +
>  OF_EARLYCON_DECLARE(jz4770_uart, "ingenic,jz4770-uart",
>  		    ingenic_early_console_setup);
>=20
> @@ -328,6 +363,7 @@ static const struct ingenic_uart_config=20
> x1000_uart_config =3D {
>=20
>  static const struct of_device_id of_match[] =3D {
>  	{ .compatible =3D "ingenic,jz4740-uart", .data =3D &jz4740_uart_config=20
> },
> +	{ .compatible =3D "ingenic,jz4750-uart", .data =3D &jz4760_uart_config=20
> },
>  	{ .compatible =3D "ingenic,jz4760-uart", .data =3D &jz4760_uart_config=20
> },
>  	{ .compatible =3D "ingenic,jz4770-uart", .data =3D &jz4760_uart_config=20
> },
>  	{ .compatible =3D "ingenic,jz4775-uart", .data =3D &jz4760_uart_config=20
> },
> --
> 2.36.1
>=20