linux-kernel - Re: [PATCH] tty/serial: atmel: add fractional baud rate support

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Message-ID: <0a43b5e9-e449-1a0d-6aef-605e5fb5f5cc@atmel.com>
Date:   Thu, 25 Aug 2016 17:11:30 +0200
From:   Nicolas Ferre <nicolas.ferre@...el.com>
To:     Ludovic Desroches <ludovic.desroches@...el.com>,
        <gregkh@...uxfoundation.org>
CC:     <jslaby@...e.com>, <linux-serial@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] tty/serial: atmel: add fractional baud rate support

Le 25/08/2016 à 15:47, Ludovic Desroches a écrit :
> The USART device provides a fractional baud rate generator to get a more
> accurate baud rate. It can be used only when the USART is configured in
> 'normal mode' and this feature is not available on AT91RM9200 SoC.
> 
> Signed-off-by: Ludovic Desroches <ludovic.desroches@...el.com>

Acked-by: Nicolas Ferre <nicolas.ferre@...el.com>

Thanks, bye,

> ---
>  drivers/tty/serial/atmel_serial.c | 41 +++++++++++++++++++++++++++++++--------
>  include/linux/atmel_serial.h      |  1 +
>  2 files changed, 34 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index 2eaa18d..1759239 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -166,6 +166,7 @@ struct atmel_uart_port {
>  	u32			rts_low;
>  	bool			ms_irq_enabled;
>  	u32			rtor;	/* address of receiver timeout register if it exists */
> +	bool			has_frac_baudrate;
>  	bool			has_hw_timer;
>  	struct timer_list	uart_timer;
>  
> @@ -1745,6 +1746,11 @@ static void atmel_get_ip_name(struct uart_port *port)
>  	dbgu_uart = 0x44424755;	/* DBGU */
>  	new_uart = 0x55415254;	/* UART */
>  
> +	/*
> +	 * Only USART devices from at91sam9260 SOC implement fractional
> +	 * baudrate.
> +	 */
> +	atmel_port->has_frac_baudrate = false;
>  	atmel_port->has_hw_timer = false;
>  
>  	if (name == new_uart) {
> @@ -1753,6 +1759,7 @@ static void atmel_get_ip_name(struct uart_port *port)
>  		atmel_port->rtor = ATMEL_UA_RTOR;
>  	} else if (name == usart) {
>  		dev_dbg(port->dev, "Usart\n");
> +		atmel_port->has_frac_baudrate = true;
>  		atmel_port->has_hw_timer = true;
>  		atmel_port->rtor = ATMEL_US_RTOR;
>  	} else if (name == dbgu_uart) {
> @@ -1764,6 +1771,7 @@ static void atmel_get_ip_name(struct uart_port *port)
>  		case 0x302:
>  		case 0x10213:
>  			dev_dbg(port->dev, "This version is usart\n");
> +			atmel_port->has_frac_baudrate = true;
>  			atmel_port->has_hw_timer = true;
>  			atmel_port->rtor = ATMEL_US_RTOR;
>  			break;
> @@ -2025,8 +2033,9 @@ static void atmel_serial_pm(struct uart_port *port, unsigned int state,
>  static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>  			      struct ktermios *old)
>  {
> +	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
>  	unsigned long flags;
> -	unsigned int old_mode, mode, imr, quot, baud;
> +	unsigned int old_mode, mode, imr, quot, baud, div, cd, fp = 0;
>  
>  	/* save the current mode register */
>  	mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
> @@ -2036,12 +2045,6 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>  		  ATMEL_US_PAR | ATMEL_US_USMODE);
>  
>  	baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);
> -	quot = uart_get_divisor(port, baud);
> -
> -	if (quot > 65535) {	/* BRGR is 16-bit, so switch to slower clock */
> -		quot /= 8;
> -		mode |= ATMEL_US_USCLKS_MCK_DIV8;
> -	}
>  
>  	/* byte size */
>  	switch (termios->c_cflag & CSIZE) {
> @@ -2160,7 +2163,29 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>  		atmel_uart_writel(port, ATMEL_US_CR, rts_state);
>  	}
>  
> -	/* set the baud rate */
> +	/*
> +	 * Set the baud rate:
> +	 * Fractional baudrate allows to setup output frequency more
> +	 * accurately. This feature is enabled only when using normal mode.
> +	 * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
> +	 * Currently, OVER is always set to 0 so we get
> +	 * baudrate = selected clock (16 * (CD + FP / 8))
> +	 */
> +	if (atmel_port->has_frac_baudrate &&
> +	    (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
> +		div = DIV_ROUND_CLOSEST(port->uartclk, baud);
> +		cd = div / 16;
> +		fp = DIV_ROUND_CLOSEST(div % 16, 2);
> +	} else {
> +		cd = uart_get_divisor(port, baud);
> +	}
> +
> +	if (cd > 65535) {	/* BRGR is 16-bit, so switch to slower clock */
> +		cd /= 8;
> +		mode |= ATMEL_US_USCLKS_MCK_DIV8;
> +	}
> +	quot = cd | fp << ATMEL_US_FP_OFFSET;
> +
>  	atmel_uart_writel(port, ATMEL_US_BRGR, quot);
>  	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_RSTSTA | ATMEL_US_RSTRX);
>  	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN | ATMEL_US_RXEN);
> diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
> index 5a4d664..f8e452a 100644
> --- a/include/linux/atmel_serial.h
> +++ b/include/linux/atmel_serial.h
> @@ -118,6 +118,7 @@
>  
>  #define ATMEL_US_BRGR		0x20	/* Baud Rate Generator Register */
>  #define	ATMEL_US_CD		GENMASK(15, 0)	/* Clock Divider */
> +#define ATMEL_US_FP_OFFSET	16	/* Fractional Part */
>  
>  #define ATMEL_US_RTOR		0x24	/* Receiver Time-out Register for USART */
>  #define ATMEL_UA_RTOR		0x28	/* Receiver Time-out Register for UART */
> 


-- 
Nicolas Ferre