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Message-ID: <20191123200058.kz2farbyvruybtjj@pengutronix.de>
Date: Sat, 23 Nov 2019 21:00:58 +0100
From: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>
To: Clément Péron <peron.clem@...il.com>
Cc: Thierry Reding <thierry.reding@...il.com>,
Maxime Ripard <mripard@...nel.org>,
Chen-Yu Tsai <wens@...e.org>,
Philipp Zabel <pza@...gutronix.de>, linux-pwm@...r.kernel.org,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-sunxi <linux-sunxi@...glegroups.com>,
Jernej Skrabec <jernej.skrabec@...l.net>,
Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH v8 5/6] pwm: sun4i: Add support to output source clock
directly
On Sat, Nov 23, 2019 at 03:05:48PM +0100, Clément Péron wrote:
> Hi Uwe,
>
> On Thu, 21 Nov 2019 at 22:21, Clément Péron <peron.clem@...il.com> wrote:
> >
> > Hi Uwe,
> >
> > On Thu, 21 Nov 2019 at 22:16, Uwe Kleine-König
> > <u.kleine-koenig@...gutronix.de> wrote:
> > >
> > > On Thu, Nov 21, 2019 at 08:59:01PM +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 | 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@...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);
> > > > + 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.
>
> Let me know if you agree or not with this and I send the v9 according
> to your answer.
If my suggestion is not safe according to the documentation, it is
obviously wrong. So only take it into account if a zero can be safely
written.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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