Re: [PATCH v8 5/6] pwm: sun4i: Add support to output source clock directly

Uwe Kleine-König
2019-11-21T21:17:14Z

On Thu, Nov 21, 2019 at 08:59:01PM +0100, Clément Péron wrote:
> From: Jernej Skrabec <jernej.skrabec@siol.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@siol.net>
> Signed-off-by: Clément Péron <peron.clem@gmail.com>
> ---
> drivers/pwm/pwm-sun4i.c | 48 +++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 46 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index 1fa2057419fb..0fe9c680d6d0 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@free-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);
> + 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;
> @@ -149,13 +168,24 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip,
>
> static int sun4i_pwm_calculate(struct sun4i_pwm_chip *sun4i_pwm,
> const struct pwm_state *state,
> - u32 *dty, u32 *prd, unsigned int *prsclr)
> + u32 *dty, u32 *prd, unsigned int *prsclr,
> + bool *bypass)
> {
> u64 clk_rate, div = 0;
> unsigned int pval, prescaler = 0;
>
> clk_rate = clk_get_rate(sun4i_pwm->clk);
>
> + *bypass = sun4i_pwm->data->has_direct_mod_clk_output &&
> + state->enabled &&
> + (state->period * clk_rate >= NSEC_PER_SEC) &&
> + (state->period * clk_rate < 2 * NSEC_PER_SEC) &&
> + (state->duty_cycle * clk_rate * 2 >= NSEC_PER_SEC);
> +
> + /* Skip calculation of other parameters if we bypass them */
> + if (*bypass)
> + return 0;
> +
> if (sun4i_pwm->data->has_prescaler_bypass) {
> /* First, test without any prescaler when available */
> prescaler = PWM_PRESCAL_MASK;
> @@ -206,6 +236,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> int ret;
> unsigned int delay_us, prescaler;
> unsigned long now;
> + bool bypass;
>
> pwm_get_state(pwm, &cstate);
>
> @@ -220,7 +251,8 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> spin_lock(&sun4i_pwm->ctrl_lock);
> ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>
> - ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler);
> + ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler,
> + &bypass);
> if (ret) {
> dev_err(chip->dev, "period exceeds the maximum value\n");
> spin_unlock(&sun4i_pwm->ctrl_lock);
> @@ -229,6 +261,18 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> return ret;
> }
>
> + if (sun4i_pwm->data->has_direct_mod_clk_output) {
> + if (bypass) {
> + ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);
> + /* We can skip other parameter */
> + sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
> + spin_unlock(&sun4i_pwm->ctrl_lock);
> + return 0;
> + } else {
> + ctrl &= ~BIT_CH(PWM_BYPASS, pwm->hwpwm);
> + }
> + }

This could be simplified to:

if (bypass) {
ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);
/*
* Other parameters are not relevant in this mode and so
* writing them can be skipped
*/
sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
spin_unlock(&sun4i_pwm->ctrl_lock);
return 0;
} else {
ctrl &= ~BIT_CH(PWM_BYPASS, pwm->hwpwm);
}

which has the advantage(?) that the bypass bit is also (more obviously)
cleared for SoCs that don't support it and it reduces the indention
level.

Best regards
Uwe

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