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

[Claudiu.Beznea]

On 21.09.2022 08:51, Claudiu Beznea - M18063 wrote:
> 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@xxxxxxxxxxxxx>
> 
> Reviewed-by: Claudiu Beznea <claudiu.beznea@xxxxxxxxxxxxx>

Actually, I'm taking this back at the moment.

> 
> 
>> ---
>>
>>
>> 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))) {

If this condition fails and you previously used GCLK for clock generation
you should remove bits ATMEL_US_USCLKS or ATMEL_UA_BRSRCCK from mode
variable, to avoid configuring MR with GCLK support and clock not being
properly setup.

>> +			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;
>