linux-kernel - Re: [PATCH v2 4/7] pwm: sun4i: Add support to output source clock directly

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Date:   Mon, 4 Nov 2019 09:38:35 +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>, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH v2 4/7] pwm: sun4i: Add support to output source clock
 directly

On Sun, Nov 03, 2019 at 09:33:31PM +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.

I think the , should be dropped.

> Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> Signed-off-by: Clément Péron <peron.clem@...il.com>
> ---
>  drivers/pwm/pwm-sun4i.c | 39 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index b5e7ac364f59..2441574674d9 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 guaranted to be completed

Typo: guaranted  -> guaranteed

>   */
>  
>  #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)) &&
> +	    data->has_direct_mod_clk_output) {
> +		state->period = DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC, clk_rate);
> +		state->duty_cycle = state->period / 2;
> +		state->polarity = PWM_POLARITY_NORMAL;
> +		state->enabled = true;
> +		return;
> +	}

Not sure how the rest of sun4i_pwm_get_state behaves, but I would prefer
to let .get_state() round up which together with .apply_state() rounding
down yields sound behaviour.

> +
>  	if ((PWM_REG_PRESCAL(val, pwm->hwpwm) == PWM_PRESCAL_MASK) &&
>  	    sun4i_pwm->data->has_prescaler_bypass)
>  		prescaler = 1;
> @@ -203,7 +222,8 @@ 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;
> +	u32 ctrl, clk_rate;
> +	bool bypass;
>  	int ret;
>  	unsigned int delay_us;
>  	unsigned long now;
> @@ -218,6 +238,16 @@ 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".
> +	 * Period is allowed to be rounded up or down.
> +	 */
> +	clk_rate = clk_get_rate(sun4i_pwm->clk);
> +	bypass = ((state->period * clk_rate >= NSEC_PER_SEC &&
> +		   state->period * clk_rate < NSEC_PER_SEC + clk_rate) &&
> +		  state->enabled);

I guess the compiler is smart enough here, but checking for
state->enabled is cheaper than the other checks, so putting this at the
start of the expression seems sensible.

The comment doesn't match the code. You don't round up state->period.
(This is good, please fix the comment.) I think dropping the check

	state->period * clk_rate < NSEC_PER_SEC + clk_rate

would be fine, too.

I'd like to have a check for

	state->duty_cycle * clk_rate >= NSEC_PER_SEC / 2 &&
	state->duty_cycle * clk_rate < NSEC_PER_SEC

here. If this isn't true rather disable the PWM or output a 100% duty
cycle with a larger period.

> +
>  	spin_lock(&sun4i_pwm->ctrl_lock);
>  	ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>  

Best regards
Uwe

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