linux-kernel - Re: [PATCH 04/12] tty: serial: samsung: Init USI to keep clocks running

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Message-ID: <a1701931-136e-235c-8392-a3f64c050d74@canonical.com>
Date:   Fri, 30 Jul 2021 18:31:56 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Tomasz Figa <tomasz.figa@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>, Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>,
        Ryu Euiyoul <ryu.real@...sung.com>,
        Tom Gall <tom.gall@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Amit Pundir <amit.pundir@...aro.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH 04/12] tty: serial: samsung: Init USI to keep clocks
 running

On 30/07/2021 16:49, Sam Protsenko wrote:
> UART block is a part of USI (Universal Serial Interface) IP-core in
> Samsung SoCs since Exynos9810 (e.g. in Exynos850). USI allows one to
> enable one of three types of serial interface: UART, SPI or I2C. That's
> possible because USI shares almost all internal circuits within each
> protocol. USI also provides some additional registers so it's possible
> to configure it.
> 
> One USI register called USI_OPTION has reset value of 0x0. Because of
> this the clock gating behavior is controlled by hardware (HWACG =
> Hardware Auto Clock Gating), which simply means the serial won't work
> after reset as is. In order to make it work, USI_OPTION[2:1] bits must
> be set to 0b01, so that HWACG is controlled manually (by software).
> Bits meaning:
>   - CLKREQ_ON = 1: clock is continuously provided to IP
>   - CLKSTOP_ON = 0: drive IP_CLKREQ to High (needs to be set along with
>                     CLKREQ_ON = 1)
> 
> USI is not present on older chips, like s3c2410, s3c2412, s3c2440,
> s3c6400, s5pv210, exynos5433, exynos4210. So the new boolean field
> '.has_usi' was added to struct s3c24xx_uart_info. USI registers will be
> only actually accessed when '.has_usi' field is set to "1".
> 
> This feature is needed for further serial enablement on Exynos850, but
> some other new Exynos chips (like Exynos9810) may benefit from this
> feature as well.
> 
> Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
> ---
>  drivers/tty/serial/samsung_tty.c | 33 +++++++++++++++++++++++++++++++-
>  include/linux/serial_s3c.h       |  9 +++++++++
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> index 9fbc61151c2e..0f3cbd0b37e3 100644
> --- a/drivers/tty/serial/samsung_tty.c
> +++ b/drivers/tty/serial/samsung_tty.c
> @@ -65,6 +65,7 @@ enum s3c24xx_port_type {
>  struct s3c24xx_uart_info {
>  	char			*name;
>  	enum s3c24xx_port_type	type;
> +	unsigned int		has_usi;
>  	unsigned int		port_type;
>  	unsigned int		fifosize;
>  	unsigned long		rx_fifomask;
> @@ -1352,6 +1353,29 @@ static int apple_s5l_serial_startup(struct uart_port *port)
>  	return ret;
>  }
>  
> +static void exynos_usi_init(struct uart_port *port)
> +{
> +	struct s3c24xx_uart_port *ourport = to_ourport(port);
> +	struct s3c24xx_uart_info *info = ourport->info;
> +
> +	if (!info->has_usi)
> +		return;
> +
> +	/*
> +	 * USI_RESET is an active High signal. Reset value of USI_RESET is 0x1
> +	 * to drive stable value to PAD. Due to this feature, the USI_RESET must
> +	 * be cleared (set as 0x0) before starting a transaction.

"before starting a transaction" suggests it is related with transaction
or something before starting it. Don't you need it simply after reset or
resume?

> +	 */
> +	wr_regl(port, USI_CON, USI_RESET);

You are clearing entire register, not only USI_RESET bitfield. Is it
really what you want?

> +	udelay(1);
> +
> +	/*
> +	 * Set the HWACG option bit in case of UART Rx mode.
> +	 * CLKREQ_ON = 1, CLKSTOP_ON = 0 (set USI_OPTION[2:1] = 0x1).
> +	 */
> +	wr_regl(port, USI_OPTION, USI_HWACG_CLKREQ_ON);
> +}
> +
>  /* power power management control */
>  
>  static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> @@ -1379,6 +1403,7 @@ static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
>  		if (!IS_ERR(ourport->baudclk))
>  			clk_prepare_enable(ourport->baudclk);
>  
> +		exynos_usi_init(port);
>  		break;
>  	default:
>  		dev_err(port->dev, "s3c24xx_serial: unknown pm %d\n", level);
> @@ -2102,6 +2127,8 @@ static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
>  	if (ret)
>  		pr_warn("uart: failed to enable baudclk\n");
>  
> +	exynos_usi_init(port);
> +
>  	/* Keep all interrupts masked and cleared */
>  	switch (ourport->info->type) {
>  	case TYPE_S3C6400:
> @@ -2750,10 +2777,11 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
>  #endif
>  
>  #if defined(CONFIG_ARCH_EXYNOS)
> -#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
> +#define EXYNOS_COMMON_SERIAL_DRV_DATA_USI(_has_usi)		\
>  	.info = &(struct s3c24xx_uart_info) {			\
>  		.name		= "Samsung Exynos UART",	\
>  		.type		= TYPE_S3C6400,			\
> +		.has_usi	= _has_usi,			\
>  		.port_type	= PORT_S3C6400,			\
>  		.has_divslot	= 1,				\
>  		.rx_fifomask	= S5PV210_UFSTAT_RXMASK,	\
> @@ -2773,6 +2801,9 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
>  		.has_fracval	= 1,				\
>  	}							\
>  
> +#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
> +	EXYNOS_COMMON_SERIAL_DRV_DATA_USI(0)
> +
>  static struct s3c24xx_serial_drv_data exynos4210_serial_drv_data = {
>  	EXYNOS_COMMON_SERIAL_DRV_DATA,
>  	.fifosize = { 256, 64, 16, 16 },
> diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> index f6c3323fc4c5..013c2646863e 100644
> --- a/include/linux/serial_s3c.h
> +++ b/include/linux/serial_s3c.h
> @@ -28,6 +28,15 @@
>  #define S3C2410_UFSTAT	  (0x18)
>  #define S3C2410_UMSTAT	  (0x1C)
>  
> +/* USI Control Register offset */
> +#define USI_CON			(0xC4)
> +/* USI Option Register offset */
> +#define USI_OPTION		(0xC8)
> +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
> +#define USI_RESET		(0<<0)

Just 0x0. I understand you wanted to hint it is a bit field, but the
shift of 0 actually creates more questions.


Best regards,
Krzysztof