linux-kernel - Re: [PATCH v4 4/7] pwm: sun4i: Add support to output source clock directly

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Date:   Wed, 13 Nov 2019 09:58:58 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Clément Péron <peron.clem@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Philipp Zabel <pza@...gutronix.de>, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH v4 4/7] pwm: sun4i: Add support to output source clock
 directly

On Fri, Nov 08, 2019 at 09:45:14AM +0100, Clément Péron wrote:
> From: Jernej Skrabec <jernej.skrabec@...l.net>
> 
> PWM core has an option to bypass whole logic and output unchanged source
> clock as PWM output. This is achieved by enabling bypass bit.
> 
> Note that when bypass is enabled, no other setting has any meaning, not
> even enable bit.
> 
> This mode of operation is needed to achieve high enough frequency to
> serve as clock source for AC200 chip which is integrated into same
> package as H6 SoC.
> 
> Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> Signed-off-by: Clément Péron <peron.clem@...il.com>
> ---
>  drivers/pwm/pwm-sun4i.c | 44 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index a10022d6c0fd..9cc928ab47bc 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -3,6 +3,10 @@
>   * Driver for Allwinner sun4i Pulse Width Modulation Controller
>   *
>   * Copyright (C) 2014 Alexandre Belloni <alexandre.belloni@...e-electrons.com>
> + *
> + * Limitations:
> + * - When outputing the source clock directly, the PWM logic will be bypassed
> + *   and the currently running period is not guaranteed to be completed
>   */
>  
>  #include <linux/bitops.h>
> @@ -73,6 +77,7 @@ static const u32 prescaler_table[] = {
>  
>  struct sun4i_pwm_data {
>  	bool has_prescaler_bypass;
> +	bool has_direct_mod_clk_output;
>  	unsigned int npwm;
>  };
>  
> @@ -118,6 +123,20 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip,
>  
>  	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>  
> +	/*
> +	 * PWM chapter in H6 manual has a diagram which explains that if bypass
> +	 * bit is set, no other setting has any meaning. Even more, experiment
> +	 * proved that also enable bit is ignored in this case.
> +	 */
> +	if ((val & BIT_CH(PWM_BYPASS, pwm->hwpwm)) &&
> +	    sun4i_pwm->data->has_direct_mod_clk_output) {
> +		state->period = DIV_ROUND_UP_ULL(NSEC_PER_SEC, clk_rate);
> +		state->duty_cycle = DIV_ROUND_UP_ULL(state->period, 2);

I first thought you're losing precision here by reusing state->period
here, but with a divisor of 2 everything is fine.

> +		state->polarity = PWM_POLARITY_NORMAL;
> +		state->enabled = true;
> +		return;
> +	}
> +
>  	if ((PWM_REG_PRESCAL(val, pwm->hwpwm) == PWM_PRESCAL_MASK) &&
>  	    sun4i_pwm->data->has_prescaler_bypass)
>  		prescaler = 1;
> @@ -204,6 +223,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	struct sun4i_pwm_chip *sun4i_pwm = to_sun4i_pwm_chip(chip);
>  	struct pwm_state cstate;
>  	u32 ctrl;
> +	bool bypass = false;
>  	int ret;
>  	unsigned int delay_us;
>  	unsigned long now;
> @@ -218,9 +238,24 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		}
>  	}
>  
> +	/*
> +	 * Although it would make much more sense to check for bypass in
> +	 * sun4i_pwm_calculate(), value of bypass bit also depends on "enabled".

I don't understand this reasoning. sun4i_pwm_calculate knows about
.enabled and also sun4i_pwm->data->has_direct_mod_clk_output. Maybe just
add a bool *bypass as parameter and move the logic there?

> +	 */
> +	if (state->enabled) {
> +		u32 clk_rate = clk_get_rate(sun4i_pwm->clk);
> +		bypass = (state->period * clk_rate >= NSEC_PER_SEC) &&
> +			 (state->period * clk_rate < 2 * NSEC_PER_SEC) &&
> +			 (state->duty_cycle * clk_rate * 2 >= NSEC_PER_SEC);
> +	}
> +

This looks right now.

Best regards
Uwe

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