Re: [PATCH V4] pwm: tegra: dynamic clk freq configuration by PWM driver

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello,

On Mon, Jun 01, 2020 at 10:50:36AM +0530, Sandipan Patra wrote:
> diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
> index d26ed8f..1daf591 100644
> --- a/drivers/pwm/pwm-tegra.c
> +++ b/drivers/pwm/pwm-tegra.c
> @@ -4,8 +4,36 @@
>   *
>   * Tegra pulse-width-modulation controller driver
>   *
> - * Copyright (c) 2010, NVIDIA Corporation.
> + * Copyright (c) 2010-2020, NVIDIA Corporation.
>   * Based on arch/arm/plat-mxc/pwm.c by Sascha Hauer <s.hauer@pengutronix.de>
> + *
> + * Overview of Tegra Pulse Width Modulator Register:
> + * 1. 13-bit: Frequency division (SCALE)
> + * 2. 8-bit : Pulse division (DUTY)
> + * 3. 1-bit : Enable bit
> + *
> + * The PWM clock frequency is divided by 256 before subdividing it based
> + * on the programmable frequency division value to generate the required
> + * frequency for PWM output. The maximum output frequency that can be
> + * achieved is (max rate of source clock) / 256.
> + * e.g. if source clock rate is 408 MHz, maximum output frequency can be:
> + * 408 MHz/256 = 1.6 MHz.
> + * This 1.6 MHz frequency can further be divided using SCALE value in PWM.
> + *
> + * PWM pulse width: 8 bits are usable [23:16] for varying pulse width.
> + * To achieve 100% duty cycle, program Bit [24] of this register to
> + * 1’b1. In which case the other bits [23:16] are set to don't care.
> + *
> + * Limitations:
> + * -	When PWM is disabled, the output is driven to inactive.
> + * -	It does not allow the current PWM period to complete and
> + *	stops abruptly.
> + *

I'd prefer to have no empty lines in the in Limitations paragraph to be
able to get all infos using something like:

	sed -rn '/\* Limitations:/,/^ \*\/?$/p' drivers/pwm/pwm-tegra.c

> + * -	If the register is reconfigured while PWM is running,
> + *	it does not complete the currently running period.
> + *
> + * -	If the user input duty is beyond acceptible limits,
> + *	-EINVAL is returned.

s/acceptible/acceptable/ (but in fact this isn't a limitation, so I'd
drop this here, as pointed out in v2).

In v2 I mentioned a few things to add here.

>   */
>  
>  #include <linux/clk.h>
> @@ -41,6 +69,7 @@ struct tegra_pwm_chip {
>  	struct reset_control*rst;
>  
>  	unsigned long clk_rate;
> +	unsigned long min_period_ns;
>  
>  	void __iomem *regs;
>  
> @@ -68,7 +97,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  {
>  	struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
>  	unsigned long long c = duty_ns, hz;
> -	unsigned long rate;
> +	unsigned long rate, required_clk_rate;

In v2 I requested to move this into the if block below. You replied to
want to move it accordingly.

>  	u32 val = 0;
>  	int err;
>  
> @@ -83,9 +112,47 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  	val = (u32)c << PWM_DUTY_SHIFT;
>  
>  	/*
> +	 *  min period = max clock limit >> PWM_DUTY_WIDTH
> +	 */
> +	if (period_ns < pc->min_period_ns)
> +		return -EINVAL;
> +
> +	/*
>  	 * Compute the prescaler value for which (1 << PWM_DUTY_WIDTH)
>  	 * cycles at the PWM clock rate will take period_ns nanoseconds.
> +	 *
> +	 * num_channels: If single instance of PWM controller has multiple
> +	 * channels (e.g. Tegra210 or older) then it is not possible to
> +	 * configure separate clock rates to each of the channels, in such
> +	 * case the value stored during probe will be referred.
> +	 *
> +	 * If every PWM controller instance has one channel respectively, i.e.
> +	 * nums_channels == 1 then only the clock rate can be modified
> +	 * dynamically (e.g. Tegra186 or Tegra194).
>  	 */
> +	if (pc->soc->num_channels == 1) {
> +		/*
> +		 * Rate is multiplied with 2^PWM_DUTY_WIDTH so that it matches
> +		 * with the maximum possible rate that the controller can
> +		 * provide. Any further lower value can be derived by setting
> +		 * PFM bits[0:12].

It looks a bit strange that the algorithm to calculate the clock
settings depends on the number of channels. Looks like a wrong
abstraction.

> +		 *
> +		 * required_clk_rate is a reference rate for source clock and
> +		 * it is derived based on user requested period. By setting the
> +		 * source clock rate as required_clk_rate, PWM controller will
> +		 * be able to configure the requested period.
> +		 */
> +		required_clk_rate =
> +			(NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;
> +
> +		err = clk_set_rate(pc->clk, required_clk_rate);
> +		if (err < 0)
> +			return -EINVAL;
> +
> +		/* Store the new rate for further references */
> +		pc->clk_rate = clk_get_rate(pc->clk);
> +	}
> +
>  	rate = pc->clk_rate >> PWM_DUTY_WIDTH;
>  
>  	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
> @@ -94,7 +161,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  
>  	/*
>  	 * Since the actual PWM divider is the register's frequency divider
> -	 * field minus 1, we need to decrement to get the correct value to
> +	 * field plus 1, we need to decrement to get the correct value to

I would have put this in a separate change.

>  	 * write to the register.
>  	 */
>  	if (rate > 0)
> @@ -205,6 +272,10 @@ static int tegra_pwm_probe(struct platform_device *pdev)
>  	 */
>  	pwm->clk_rate = clk_get_rate(pwm->clk);
>  
> +	/* Set minimum limit of PWM period for the IP */
> +	pwm->min_period_ns =
> +	    (NSEC_PER_SEC / (pwm->soc->max_frequency >> PWM_DUTY_WIDTH)) + 1;

To ensure that required_clk_rate in tegra_pwm_config doesn't get bigger
than pwm->soc->max_frequency this isn't the right formula I think. I'd
use

	pwm->min_period_ns = DIV_ROUNDUP(NSEC_PER_SEC, pwm->soc->max_frequency >> PWM_DUTY_WIDTH);

. Can you confirm?

Best regards
Uwe

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