[PATCH v4 9/9] tty: serial: atmel: Make the driver aware of the existence of GCLK

[Claudiu.Beznea at microchip.com]

On 19.09.2022 18:08, Sergiu Moga wrote:
> Previously, the atmel serial driver did not take into account the
> possibility of using the more customizable generic clock as its
> baudrate generator. Unless there is a Fractional Part available to
> increase accuracy, there is a high chance that we may be able to
> generate a baudrate closer to the desired one by using the GCLK as the
> clock source. Now, depending on the error rate between
> the desired baudrate and the actual baudrate, the serial driver will
> fallback on the generic clock. The generic clock must be provided
> in the DT node of the serial that may need a more flexible clock source.
> 
> Signed-off-by: Sergiu Moga <sergiu.moga at microchip.com>

Reviewed-by: Claudiu Beznea <claudiu.beznea at microchip.com>


> ---
> 
> 
> v1 -> v2:
> - take into account the different placement of the baudrate clock source
> into the IP's Mode Register (USART vs UART)
> - don't check for atmel_port->gclk != NULL
> - use clk_round_rate instead of clk_set_rate + clk_get_rate
> - remove clk_disable_unprepare from the end of the probe method
> 
> 
> 
> v2 -> v3:
> - add the error rate calculation function as an inline function instead of
> a macro definition
> - add `gclk_fail` goto
> - replace `goto err` with `goto err_clk_disable_unprepare;`
> 
> 
> 
> v3 -> v4:
> - Nothing, this was previously [PATCH 14]
> 
> 
>  drivers/tty/serial/atmel_serial.c | 59 ++++++++++++++++++++++++++++++-
>  1 file changed, 58 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index c983798a4ab2..426f9d4f9a5a 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -15,6 +15,7 @@
>  #include <linux/init.h>
>  #include <linux/serial.h>
>  #include <linux/clk.h>
> +#include <linux/clk-provider.h>
>  #include <linux/console.h>
>  #include <linux/sysrq.h>
>  #include <linux/tty_flip.h>
> @@ -110,6 +111,7 @@ struct atmel_uart_char {
>  struct atmel_uart_port {
>  	struct uart_port	uart;		/* uart */
>  	struct clk		*clk;		/* uart clock */
> +	struct clk		*gclk;		/* uart generic clock */
>  	int			may_wakeup;	/* cached value of device_may_wakeup for times we need to disable it */
>  	u32			backup_imr;	/* IMR saved during suspend */
>  	int			break_active;	/* break being received */
> @@ -229,6 +231,11 @@ static inline int atmel_uart_is_half_duplex(struct uart_port *port)
>  		(port->iso7816.flags & SER_ISO7816_ENABLED);
>  }
>  
> +static inline int atmel_error_rate(int desired_value, int actual_value)
> +{
> +	return 100 - (desired_value * 100) / actual_value;
> +}
> +
>  #ifdef CONFIG_SERIAL_ATMEL_PDC
>  static bool atmel_use_pdc_rx(struct uart_port *port)
>  {
> @@ -2117,6 +2124,8 @@ static void atmel_serial_pm(struct uart_port *port, unsigned int state,
>  		 * This is called on uart_close() or a suspend event.
>  		 */
>  		clk_disable_unprepare(atmel_port->clk);
> +		if (__clk_is_enabled(atmel_port->gclk))
> +			clk_disable_unprepare(atmel_port->gclk);
>  		break;
>  	default:
>  		dev_err(port->dev, "atmel_serial: unknown pm %d\n", state);
> @@ -2132,7 +2141,9 @@ static void atmel_set_termios(struct uart_port *port,
>  {
>  	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
>  	unsigned long flags;
> -	unsigned int old_mode, mode, imr, quot, baud, div, cd, fp = 0;
> +	unsigned int old_mode, mode, imr, quot, div, cd, fp = 0;
> +	unsigned int baud, actual_baud, gclk_rate;
> +	int ret;
>  
>  	/* save the current mode register */
>  	mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
> @@ -2302,6 +2313,46 @@ static void atmel_set_termios(struct uart_port *port,
>  		cd = min_t(unsigned int, cd, ATMEL_US_CD);
>  	}
>  
> +	/*
> +	 * If there is no Fractional Part, there is a high chance that
> +	 * we may be able to generate a baudrate closer to the desired one
> +	 * if we use the GCLK as the clock source driving the baudrate
> +	 * generator.
> +	 */
> +	if (!atmel_port->has_frac_baudrate) {
> +		if (__clk_is_enabled(atmel_port->gclk))
> +			clk_disable_unprepare(atmel_port->gclk);
> +		gclk_rate = clk_round_rate(atmel_port->gclk, 16 * baud);
> +		actual_baud = clk_get_rate(atmel_port->clk) / (16 * cd);
> +		if (gclk_rate && abs(atmel_error_rate(baud, actual_baud)) >
> +		    abs(atmel_error_rate(baud, gclk_rate / 16))) {
> +			clk_set_rate(atmel_port->gclk, 16 * baud);
> +			ret = clk_prepare_enable(atmel_port->gclk);
> +			if (ret)
> +				goto gclk_fail;
> +
> +			if (atmel_port->is_usart) {
> +				mode &= ~ATMEL_US_USCLKS;
> +				mode |= ATMEL_US_USCLKS_GCLK;
> +			} else {
> +				mode &= ~ATMEL_UA_BRSRCCK;
> +				mode |= ATMEL_UA_BRSRCCK_GCLK;
> +			}
> +
> +			/*
> +			 * Set the Clock Divisor for GCLK to 1.
> +			 * Since we were able to generate the smallest
> +			 * multiple of the desired baudrate times 16,
> +			 * then we surely can generate a bigger multiple
> +			 * with the exact error rate for an equally increased
> +			 * CD. Thus no need to take into account
> +			 * a higher value for CD.
> +			 */
> +			cd = 1;
> +		}
> +	}
> +
> +gclk_fail:
>  	quot = cd | fp << ATMEL_US_FP_OFFSET;
>  
>  	if (!(port->iso7816.flags & SER_ISO7816_ENABLED))
> @@ -2897,6 +2948,12 @@ static int atmel_serial_probe(struct platform_device *pdev)
>  	if (ret)
>  		goto err;
>  
> +	atmel_port->gclk = devm_clk_get_optional(&pdev->dev, "gclk");
> +	if (IS_ERR(atmel_port->gclk)) {
> +		ret = PTR_ERR(atmel_port->gclk);
> +		goto err_clk_disable_unprepare;
> +	}
> +
>  	ret = atmel_init_port(atmel_port, pdev);
>  	if (ret)
>  		goto err_clk_disable_unprepare;