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

Clément Péron
2019-11-21T21:21:44Z

Hi Uwe,

On Thu, 21 Nov 2019 at 22:16, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>
> 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.

This bit is not guaranteed to be reserved for all the SoC variants.

I don't think it's a good idea to set to 0 a bit which is undefined.

Regards,
Clement

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