Subject: [PATCH 07/16] serial: mvebu-uart: add function to change baudrate

From: [email-masked] (Gregory CLEMENT)
Date: Fri, 06 Oct 2017 14:39:35 +0200

Hi Miquel,
 
 On ven., oct. 06 2017, Miquel Raynal <[email-masked]> wrote:

> From: Allen Yan <[email-masked]>
>
> Until now, the first UART port baudrate was set by the bootloader.
>
> Add a function allowing to change the baudrate. Changes may be done
> from userspace but also at probe time by the kernel. Use the simplest
> method: baudrate divisor.
>
> Works for all UART ports until 230400 baud. To achieve higher baudrates,
> software should implement the fractional divisor feature that allows
> more accuracy for higher rates.
>
> Signed-off-by: Allen Yan <[email-masked]>
> [<[email-masked]>: changed termios handling]
> Signed-off-by: Miquel Raynal <[email-masked]>

Reviewed-by: Gregory CLEMENT <[email-masked]>

Thanks,

Gregory

> ---
>  drivers/tty/serial/mvebu-uart.c | 69 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 65 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
> index da756cfec0bb..81a3d2714fd3 100644
> --- a/drivers/tty/serial/mvebu-uart.c
> +++ b/drivers/tty/serial/mvebu-uart.c
> @@ -72,6 +72,7 @@
>  				 | STAT_PAR_ERR | STAT_OVR_ERR)
>  
>  #define UART_BRDV		0x10
> +#define  BRDV_BAUD_MASK         0x3FF
>  
>  #define MVEBU_NR_UARTS		1
>  
> @@ -344,6 +345,31 @@ static void mvebu_uart_shutdown(struct uart_port *port)
>  	free_irq(port->irq, port);
>  }
>  
> +static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
> +{
> +	struct mvebu_uart *mvuart = to_mvuart(port);
> +	unsigned int baud_rate_div;
> +	u32 brdv;
> +
> +	if (IS_ERR(mvuart->clk))
> +		return -PTR_ERR(mvuart->clk);
> +
> +	/*
> +	 * The UART clock is divided by the value of the divisor to generate
> +	 * UCLK_OUT clock, which is 16 times faster than the baudrate.
> +	 * This prescaler can achieve all standard baudrates until 230400.
> +	 * Higher baudrates could be achieved for the extended UART by using the
> +	 * programmable oversampling stack (also called fractional divisor).
> +	 */
> +	baud_rate_div = DIV_ROUND_UP(port->uartclk, baud * 16);
> +	brdv = readl(port->membase + UART_BRDV);
> +	brdv &= ~BRDV_BAUD_MASK;
> +	brdv |= baud_rate_div;
> +	writel(brdv, port->membase + UART_BRDV);
> +
> +	return 0;
> +}
> +
>  static void mvebu_uart_set_termios(struct uart_port *port,
>  				   struct ktermios *termios,
>  				   struct ktermios *old)
> @@ -367,11 +393,30 @@ static void mvebu_uart_set_termios(struct uart_port *port,
>  	if ((termios->c_cflag & CREAD) == 0)
>  		port->ignore_status_mask |= STAT_RX_RDY(port) | STAT_BRK_ERR;
>  
> -	if (old)
> -		tty_termios_copy_hw(termios, old);
> +	/*
> +	 * Maximum achievable frequency with simple baudrate divisor is 230400.
> +	 * Since the error per bit frame would be of more than 15%, achieving
> +	 * higher frequencies would require to implement the fractional divisor
> +	 * feature.
> +	 */
> +	baud = uart_get_baud_rate(port, termios, old, 0, 230400);
> +	if (mvebu_uart_baud_rate_set(port, baud)) {
> +		/* No clock available, baudrate cannot be changed */
> +		if (old)
> +			baud = uart_get_baud_rate(port, old, NULL, 0, 230400);
> +	} else {
> +		tty_termios_encode_baud_rate(termios, baud, baud);
> +		uart_update_timeout(port, termios->c_cflag, baud);
> +	}
>  
> -	baud = uart_get_baud_rate(port, termios, old, 0, 460800);
> -	uart_update_timeout(port, termios->c_cflag, baud);
> +	/* Only the following flag changes are supported */
> +	if (old) {
> +		termios->c_iflag &= INPCK | IGNPAR;
> +		termios->c_iflag |= old->c_iflag & ~(INPCK | IGNPAR);
> +		termios->c_cflag &= CREAD | CBAUD;
> +		termios->c_cflag |= old->c_cflag & ~(CREAD | CBAUD);
> +		termios->c_lflag = old->c_lflag;
> +	}
>  
>  	spin_unlock_irqrestore(&port->lock, flags);
>  }
> @@ -647,12 +692,28 @@ static int mvebu_uart_probe(struct platform_device *pdev)
>  	if (!mvuart)
>  		return -ENOMEM;
>  
> +	/* Get controller data depending on the compatible string */
>  	mvuart->data = (struct mvebu_uart_driver_data *)match->data;
>  	mvuart->port = port;
>  
>  	port->private_data = mvuart;
>  	platform_set_drvdata(pdev, mvuart);
>  
> +	/* Get fixed clock frequency */
> +	mvuart->clk = devm_clk_get(&pdev->dev, NULL);
> +	if (IS_ERR(mvuart->clk)) {
> +		if (PTR_ERR(mvuart->clk) == -EPROBE_DEFER)
> +			return PTR_ERR(mvuart->clk);
> +
> +		if (IS_EXTENDED(port)) {
> +			dev_err(&pdev->dev, "unable to get UART clock\n");
> +			return PTR_ERR(mvuart->clk);
> +		}
> +	} else {
> +		if (!clk_prepare_enable(mvuart->clk))
> +			port->uartclk = clk_get_rate(mvuart->clk);
> +	}
> +
>  	/* UART Soft Reset*/
>  	writel(CTRL_SOFT_RST, port->membase + UART_CTRL(port));
>  	udelay(1);
> -- 
> 2.11.0
>
>
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-- 
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com