linux-kernel - Re: [PATCH v11 3/4] pwm: add microchip soft ip corePWM driver

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Date:   Tue, 8 Nov 2022 16:50:41 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v11 3/4] pwm: add microchip soft ip corePWM driver

Hello,

On Fri, Oct 07, 2022 at 12:35:12PM +0100, Conor Dooley wrote:
> [...]
> +static u64 mchp_core_pwm_calc_duty(struct pwm_chip *chip, struct pwm_device *pwm,
> +				   const struct pwm_state *state, u8 prescale, u8 period_steps)
> +{
> +	struct mchp_core_pwm_chip *mchp_core_pwm = to_mchp_core_pwm(chip);
> +	u64 duty_steps, tmp;
> +	u16 prescale_val = PREG_TO_VAL(prescale);
> +
> +	/*
> +	 * Calculate the duty cycle in multiples of the prescaled period:
> +	 * duty_steps = duty_in_ns / step_in_ns
> +	 * step_in_ns = (prescale * NSEC_PER_SEC) / clk_rate
> +	 * The code below is rearranged slightly to only divide once.
> +	 */
> +	duty_steps = state->duty_cycle * clk_get_rate(mchp_core_pwm->clk);
> +	tmp = prescale_val * NSEC_PER_SEC;
> +	return div64_u64(duty_steps, tmp);

The assignment to duty_steps can overflow. So you have to use
mul_u64_u64_div_u64 here, too.

> +}
> +
> [...]
> +
> +static int mchp_core_pwm_apply_locked(struct pwm_chip *chip, struct pwm_device *pwm,
> +				      const struct pwm_state *state)
> +{
> +	struct mchp_core_pwm_chip *mchp_core_pwm = to_mchp_core_pwm(chip);
> +	struct pwm_state current_state = pwm->state;
> +	bool period_locked;
> +	u64 duty_steps;
> +	u16 prescale;
> +	u8 period_steps;
> +
> +	if (!state->enabled) {
> +		mchp_core_pwm_enable(chip, pwm, false, current_state.period);
> +		return 0;
> +	}
> +
> +	/*
> +	 * If the only thing that has changed is the duty cycle or the polarity,
> +	 * we can shortcut the calculations and just compute/apply the new duty
> +	 * cycle pos & neg edges
> +	 * As all the channels share the same period, do not allow it to be
> +	 * changed if any other channels are enabled.
> +	 * If the period is locked, it may not be possible to use a period
> +	 * less than that requested. In that case, we just abort.
> +	 */
> +	period_locked = mchp_core_pwm->channel_enabled & ~(1 << pwm->hwpwm);
> +
> +	if (period_locked) {
> +		u16 hw_prescale;
> +		u8 hw_period_steps;
> +
> +		mchp_core_pwm_calc_period(chip, state, &prescale, &period_steps);
> +		hw_prescale = readb_relaxed(mchp_core_pwm->base + MCHPCOREPWM_PRESCALE);
> +		hw_period_steps = readb_relaxed(mchp_core_pwm->base + MCHPCOREPWM_PERIOD);
> +
> +		if ((period_steps + 1) * (prescale + 1) <
> +		    (hw_period_steps + 1) * (hw_prescale + 1))
> +			return -EINVAL;
> +
> +		/*
> +		 * It is possible that something could have set the period_steps
> +		 * register to 0xff, which would prevent us from setting a 100%
> +		 * or 0% relative duty cycle, as explained above in
> +		 * mchp_core_pwm_calc_period().
> +		 * The period is locked and we cannot change this, so we abort.
> +		 */
> +		if (hw_period_steps == MCHPCOREPWM_PERIOD_STEPS_MAX)
> +			return -EINVAL;
> +
> +		prescale = hw_prescale;
> +		period_steps = hw_period_steps;
> +	} else {
> +		int ret;
> +
> +		ret = mchp_core_pwm_calc_period(chip, state, &prescale, &period_steps);
> +		if (ret)
> +			return ret;
> +
> +		mchp_core_pwm_apply_period(mchp_core_pwm, prescale, period_steps);
> +	}
> +
> +	duty_steps = mchp_core_pwm_calc_duty(chip, pwm, state, prescale, period_steps);

Both mchp_core_pwm_calc_period and mchp_core_pwm_calc_duty call
clk_get_rate(), I suggest call this only once and pass the rate to these
two functions.

Both branches of the if above start with calling
mchp_core_pwm_calc_period, this could be simplified, too. (Hmm, in
exactly one of them you check the return code, wouldn't that be sensible
for both callers?)

> +
> +	/*
> +	 * Because the period is per channel, it is possible that the requested
> +	 * duty cycle is longer than the period, in which case cap it to the
> +	 * period, IOW a 100% duty cycle.
> +	 */
> +	if (duty_steps > period_steps)
> +		duty_steps = period_steps + 1;
> +
> +	mchp_core_pwm_apply_duty(chip, pwm, state, duty_steps, period_steps);
> +
> +	mchp_core_pwm_enable(chip, pwm, true, state->period);
> +
> +	return 0;
> +}

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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