RE: [PATCH V2] pwm: tegra: dynamic clk freq configuration by PWM driver

[Sandipan Patra]

Thanks Jonathan,
Please help reviewing further with my replies inline.


Thanks & Regards,
Sandipan

> -----Original Message-----
> From: Jonathan Hunter <jonathanh@xxxxxxxxxx>
> Sent: Friday, May 22, 2020 3:54 PM
> To: Sandipan Patra <spatra@xxxxxxxxxx>; Thierry Reding
> <treding@xxxxxxxxxx>; robh+dt@xxxxxxxxxx; u.kleine-koenig@xxxxxxxxxxxxxx
> Cc: Bibek Basu <bbasu@xxxxxxxxxx>; Laxman Dewangan
> <ldewangan@xxxxxxxxxx>; linux-pwm@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH V2] pwm: tegra: dynamic clk freq configuration by PWM
> driver
> 
> 
> On 20/04/2020 16:54, Sandipan Patra wrote:
> > Added support for dynamic clock freq configuration in pwm kernel driver.
> > Earlier the pwm driver used to cache boot time clock rate by pwm clock
> > parent during probe. Hence dynamically changing pwm frequency was not
> > possible for all the possible ranges. With this change, dynamic
> > calculation is enabled and it is able to set the requested period from
> > sysfs knob provided the value is supported by clock source.
> >
> > Changes mainly have 2 parts:
> >   - T186 and later chips [1]
> >   - T210 and prior chips [2]
> >
> > For [1] - Changes implemented to set pwm period dynamically and
> >           also checks added to allow only if requested period(ns) is
> >           below or equals to higher range.
> >
> > For [2] - Only checks if the requested period(ns) is below or equals
> >           to higher range defined by max clock limit. The limitation
> >           in T210 or prior chips are due to the reason of having only
> >           one pwm-controller supporting multiple channels. But later
> >           chips have multiple pwm controller instances each having
> > 	  single channel support.
> >
> > Signed-off-by: Sandipan Patra <spatra@xxxxxxxxxx>
> > ---
> > V2:
> > 1. Min period_ns calculation is moved to probe.
> > 2. Added descriptioins for PWM register bits and regarding behaviour
> >    of the controller when new configuration is applied or pwm is disabled.
> > 3. Setting period with possible value when supplied period is below limit.
> > 4. Corrected the earlier code comment:
> >    plus 1 instead of minus 1 during pwm calculation
> >
> >  drivers/pwm/pwm-tegra.c | 110
> > +++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 94 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c index
> > d26ed8f..7a36325 100644
> > --- a/drivers/pwm/pwm-tegra.c
> > +++ b/drivers/pwm/pwm-tegra.c
> > @@ -4,8 +4,39 @@
> >   *
> >   * Tegra pulse-width-modulation controller driver
> >   *
> > - * Copyright (c) 2010, NVIDIA Corporation.
> > - * Based on arch/arm/plat-mxc/pwm.c by Sascha Hauer
> > <s.hauer@xxxxxxxxxxxxxx>
> > + * Copyright (c) 2010-2020, NVIDIA Corporation.
> > + *
> > + * Overview of Tegra Pulse Width Modulator Register:
> > + * 1. 13-bit: Frequency division (SCALE)
> > + * 2. 8-bit : Puls division (DUTY)
> > + * 3. 1-bit : Enable bit
> > + *
> > + * The PWM clock frequency is divided by 256 before subdividing it
> > + based
> > + * on the programmable frequency division value to generate the
> > + required
> > + * frequency for PWM output. The maximum output frequency that can be
> > + * achieved is (max rate of source clock) / 256.
> > + * i.e. if source clock rate is 408 MHz, maximum output frequency cab be:
> > + * 408 MHz/256 = 1.6 MHz.
> > + * This 1.6 MHz frequency can further be divided using SCALE value in PWM.
> > + *
> > + * PWM pulse width: 8 bits are usable [23:16] for varying pulse width.
> > + * To achieve 100% duty cycle, program Bit [24] of this register to
> > + * 1âb1. In which case the other bits [23:16] are set to don't care.
> > + *
> > + * Limitations and known facts:
> > + * -	When PWM is disabled, the output is driven to 0.
> > + * -	It does not allow the current PWM period to complete and
> > + *	stops abruptly.
> > + *
> > + * -	If the register is reconfigured while pwm is running,
> > + *	It does not let the currently running period to complete.
> > + *
> > + * -	Pulse width of the pwm can never be out of bound.
> > + *	It's taken care at HW and SW
> > + * -	If the user input duty is below limit, then driver sets it to
> > + *	minimum possible value.
> > + * -	If anything else goes wrong for setting duty or period,
> > + *	-EINVAL is returned.
> >   */
> >
> >  #include <linux/clk.h>
> > @@ -41,6 +72,7 @@ struct tegra_pwm_chip {
> >  	struct reset_control*rst;
> >
> >  	unsigned long clk_rate;
> > +	unsigned long min_period_ns;
> >
> >  	void __iomem *regs;
> >
> > @@ -67,8 +99,9 @@ static int tegra_pwm_config(struct pwm_chip *chip,
> struct pwm_device *pwm,
> >  			    int duty_ns, int period_ns)
> >  {
> >  	struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
> > -	unsigned long long c = duty_ns, hz;
> > -	unsigned long rate;
> > +	unsigned long long p_width = duty_ns, period_hz;
> > +	unsigned long rate, required_clk_rate;
> > +	unsigned long pfm; /* Frequency divider */
> 
> If it is not necessary to change the variable names, then I would prefer we keep
> them as is as then changes would be less.

The earlier name was misleading so thought to use a specific name for
which it can be helpful to follow up with the TRM. Since its recommended
to retain the variable names, I will change this in next patch.
 
> 
> >  	u32 val = 0;
> >  	int err;
> >
> > @@ -77,37 +110,77 @@ static int tegra_pwm_config(struct pwm_chip *chip,
> struct pwm_device *pwm,
> >  	 * per (1 << PWM_DUTY_WIDTH) cycles and make sure to round to the
> >  	 * nearest integer during division.
> >  	 */
> > -	c *= (1 << PWM_DUTY_WIDTH);
> > -	c = DIV_ROUND_CLOSEST_ULL(c, period_ns);
> > +	p_width *= (1 << PWM_DUTY_WIDTH);
> > +	p_width = DIV_ROUND_CLOSEST_ULL(p_width, period_ns);
> >
> > -	val = (u32)c << PWM_DUTY_SHIFT;
> > +	val = (u32)p_width << PWM_DUTY_SHIFT;
> > +
> > +	/*
> > +	 *  Period in nano second has to be <= highest allowed period
> > +	 *  based on max clock rate of the pwm controller.
> > +	 *
> > +	 *  higher limit = max clock limit >> PWM_DUTY_WIDTH
> > +	 *  lower limit = min clock limit >> PWM_DUTY_WIDTH >>
> PWM_SCALE_WIDTH
> > +	 */
> > +	if (period_ns < pc->min_period_ns) {
> > +		period_ns = pc->min_period_ns;
> > +		pr_warn("Period is adjusted to allowed value (%d ns)\n",
> > +				period_ns);
> 
> I see that other drivers (pwm-img.c) consider this to be an error and return an
> error. I wonder if adjusting the period makes sense here?
> 
> I wonder if the handling of the min_period, should be a separate change?

I think I misunderstood one of the discussions in initial patch and added this change
to apply the minimum possible value. Understood and will revert this change
with returning error in such case.

> 
> > +	}
> >
> >  	/*
> >  	 * Compute the prescaler value for which (1 << PWM_DUTY_WIDTH)
> >  	 * cycles at the PWM clock rate will take period_ns nanoseconds.
> >  	 */
> > -	rate = pc->clk_rate >> PWM_DUTY_WIDTH;
> > +	if (pc->soc->num_channels == 1) {
> 
> Are you using num_channels to determine if Tegra uses the BPMP? If so then the
> above is not really correct, because num_channels is not really related to what is
> being done here. So maybe you need a new SoC attribute in the soc data.

Here, it tries to find if pwm controller uses multiple channels (like in Tegra210 or older)
or single channel for every pwm instance (i.e. T186, T194). If found multiple channels on
a single controller then it is not correct to configure separate clock rates to each of the
channels. So to distinguish the controller and channel type, num_channels is referred.

> 
> > +		/*
> > +		 * Rate is multiplied with 2^PWM_DUTY_WIDTH so that it
> matches
> > +		 * with the hieghest applicable rate that the controller can
> 
> s/hieghest/highest/

Got it.

> 
> > +		 * provide. Any further lower value can be derived by setting
> > +		 * PFM bits[0:12].
> > +		 * Higher mark is taken since BPMP has round-up mechanism
> > +		 * implemented.
> > +		 */
> > +		required_clk_rate =
> > +			(NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;
> > +
> 
> Should be we checking the rate against the max rate supported?

If the request rate is beyond max supported rate, then the clk_set_rate will be failing
and can get caught with error check followed by. Otherwise it will fail through fitting in
the register's frequency divider filed. So I think it is not required to check against max rate.
Please advise if I am not able to follow with what you are suggesting.

> 
> > +		err = clk_set_rate(pc->clk, required_clk_rate);
> > +		if (err < 0)
> > +			return -EINVAL;
> > +
> > +		rate = clk_get_rate(pc->clk) >> PWM_DUTY_WIDTH;
> 
> Do we need to update the pwm->clk_rate here?

This return rate is basically from the factor that requested clk_set_rate and the actual rate set
mostly will have a little deviation based on the clock divider and other factors while setting
a new rate. So capturing the actual rate for further calculation and conversion to Hz.
Whenever it is required to use pwm->clk_rate we are no longer depending upon the cached value
for num_channels == 1. So in my opinion it does not need to be cached. However it is kept
stored for the SoCs having num_channels > 1.
Please suggest if I am missing any case where we need to keep the value stored.

> 
> > +	} else {
> > +		/*
> > +		 * This is the case for SoCs who support multiple channels:
> > +		 *
> > +		 * clk_set_rate() can not be called again in config because
> > +		 * T210 or any prior chip supports one pwm-controller and
> > +		 * multiple channels. Hence in this case cached clock rate
> > +		 * will be considered which was stored during probe.
> > +		 */
> > +		rate = pc->clk_rate >> PWM_DUTY_WIDTH;
> > +	}
> >
> >  	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
> > -	hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC, period_ns);
> > -	rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz);
> > +	period_hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC,
> period_ns);
> > +	pfm = DIV_ROUND_CLOSEST_ULL(100ULL * rate, period_hz);
> >
> >  	/*
> >  	 * Since the actual PWM divider is the register's frequency divider
> > -	 * field minus 1, we need to decrement to get the correct value to
> > +	 * field plus 1, we need to decrement to get the correct value to
> >  	 * write to the register.
> >  	 */
> > -	if (rate > 0)
> > -		rate--;
> > +	if (pfm > 0)
> > +		pfm--;
> >
> >  	/*
> > -	 * Make sure that the rate will fit in the register's frequency
> > +	 * Make sure that pfm will fit in the register's frequency
> >  	 * divider field.
> >  	 */
> > -	if (rate >> PWM_SCALE_WIDTH)
> > +	if (pfm >> PWM_SCALE_WIDTH)
> >  		return -EINVAL;
> >
> > -	val |= rate << PWM_SCALE_SHIFT;
> > +	val |= pfm << PWM_SCALE_SHIFT;
> >
> >  	/*
> >  	 * If the PWM channel is disabled, make sure to turn on the clock @@
> > -205,6 +278,10 @@ static int tegra_pwm_probe(struct platform_device
> *pdev)
> >  	 */
> >  	pwm->clk_rate = clk_get_rate(pwm->clk);
> >
> > +	/* Set minimum limit of PWM period for the IP */
> > +	pwm->min_period_ns =
> > +	    (NSEC_PER_SEC / (pwm->soc->max_frequency >>
> PWM_DUTY_WIDTH)) +
> > +1;
> > +
> >  	pwm->rst = devm_reset_control_get_exclusive(&pdev->dev, "pwm");
> >  	if (IS_ERR(pwm->rst)) {
> >  		ret = PTR_ERR(pwm->rst);
> > @@ -313,4 +390,5 @@ module_platform_driver(tegra_pwm_driver);
> >
> >  MODULE_LICENSE("GPL");
> >  MODULE_AUTHOR("NVIDIA Corporation");
> > +MODULE_AUTHOR("Sandipan Patra <spatra@xxxxxxxxxx>");
> >  MODULE_ALIAS("platform:tegra-pwm");
> >
> 
> --
> nvpublic