[v5,2/4] pwm: driver for qualcomm ipq6018 pwm block

Driver for the PWM block in Qualcomm IPQ6018 line of SoCs. Based on
driver from downstream Codeaurora kernel tree. Removed support for older
(V1) variants because I have no access to that hardware.

Tested on IPQ6010 based hardware.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
v5:

Use &tcsr_q6 syscon to access registers (Bjorn Andersson)

Address Uwe Kleine-König review comments:

  Implement .get_state()

  Add IPQ_PWM_ prefix to local macros

  Use GENMASK/BIT/FIELD_PREP for register fields access

  Make type of config_div_and_duty() parameters consistent

  Derive IPQ_PWM_MIN_PERIOD_NS from IPQ_PWM_CLK_SRC_FREQ

  Integrate enable/disable into config_div_and_duty() to save register read,
  and reduce frequency glitch on update

  Use min() instead of min_t()

  Fix comment format

  Use dev_err_probe() to indicate probe step failure

  Add missing clk_disable_unprepare() in .remove

  Don't set .owner

v4:

  Use div64_u64() to fix link for 32-bit targets ((kernel test robot
  <lkp@intel.com>, Uwe Kleine-König)

v3:

  s/qcom,pwm-ipq6018/qcom,ipq6018-pwm/ (Rob Herring)

  Fix integer overflow on 32-bit targets (kernel test robot <lkp@intel.com>)

v2:

Address Uwe Kleine-König review comments:

  Fix period calculation when out of range

  Don't set period larger than requested

  Remove PWM disable on configuration change

  Implement .apply instead of non-atomic .config/.enable/.disable

  Don't modify PWM on .request/.free

  Check pwm_div underflow

  Fix various code and comment formatting issues

Other changes:

  Use u64 divisor safe division

  Remove now empty .request/.free
---
 drivers/pwm/Kconfig   |  12 ++
 drivers/pwm/Makefile  |   1 +
 drivers/pwm/pwm-ipq.c | 278 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 291 insertions(+)
 create mode 100644 drivers/pwm/pwm-ipq.c

Hi Uwe,

Thanks again for your detailed review.

I have a few comments and questions below.

On Wed, Jul 14 2021, Uwe Kleine-König wrote:
> On Tue, Jul 13, 2021 at 02:35:43PM +0300, Baruch Siach wrote:
>> --- /dev/null
>> +++ b/drivers/pwm/pwm-ipq.c
>> @@ -0,0 +1,278 @@
>> +// SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0
>> +/*
>> + * Copyright (c) 2016-2017, 2020 The Linux Foundation. All rights reserved.
>> + */
>> +
>> +#include <linux/module.h>
>> +#include <linux/platform_device.h>
>> +#include <linux/pwm.h>
>> +#include <linux/clk.h>
>> +#include <linux/io.h>
>> +#include <linux/math64.h>
>> +#include <linux/of_device.h>
>> +#include <linux/mfd/syscon.h>
>> +#include <linux/regmap.h>
>> +
>> +#define IPQ_PWM_MAX_DEVICES	4
>
> This is only used once. Just doing
>
> 	pwm->chip.npwm = 4;
>
> is better in my book. Does "MAX" suggest that there are variants with
> less PWMs?

I have no idea. I guess not. I'll drop this macro in v6.

>
>> +/* The frequency range supported is 1Hz to 100MHz */
>
> A space between number and unit is usual and makes this better readable.

Quick 'git grep' indicates that '[[:digit:]]\+MHz' is a little more
popular than '[[:digit:]]\+ MHz' in kernel code. But OK, not a big deal.

>> +#define IPQ_PWM_CLK_SRC_FREQ	(100*1000*1000)
>> +#define IPQ_PWM_MIN_PERIOD_NS	(NSEC_PER_SEC / IPQ_PWM_CLK_SRC_FREQ)
>
> You're assuming here that the parent clock runs at exactly the set rate.
> Is this a sensible assumption? If this division didn't have an integer
> result there would be rounding issues.

The code only uses this for period validity check. It saves us some code
for run-time division.

>> +#define IPQ_PWM_MAX_PERIOD_NS	((u64)NSEC_PER_SEC)
>> +
>> +/*
>> + * The max value specified for each field is based on the number of bits
>> + * in the pwm control register for that field
>> + */
>> +#define IPQ_PWM_MAX_DIV		0xFFFF
>> +
>> +#define IPQ_PWM_CFG_REG0 0 /*PWM_DIV PWM_HI*/
>> +#define IPQ_PWM_REG0_PWM_DIV		GENMASK(15, 0)
>> +#define IPQ_PWM_REG0_HI_DURATION	GENMASK(31, 16)
>> +
>> +#define IPQ_PWM_CFG_REG1 1 /*ENABLE UPDATE PWM_PRE_DIV*/
>> +#define IPQ_PWM_REG1_PRE_DIV		GENMASK(15, 0)
>> +/*
>> + * Enable bit is set to enable output toggling in pwm device.
>> + * Update bit is set to reflect the changed divider and high duration
>> + * values in register.
>> + */
>> +#define IPQ_PWM_REG1_UPDATE		BIT(30)
>> +#define IPQ_PWM_REG1_ENABLE		BIT(31)
>> +
>> +
>> +struct ipq_pwm_chip {
>> +	struct pwm_chip chip;
>> +	struct clk *clk;
>> +	struct regmap *regmap;
>> +	u32 regmap_off;
>> +};
>> +
>> +static struct ipq_pwm_chip *to_ipq_pwm_chip(struct pwm_chip *chip)
>> +{
>> +	return container_of(chip, struct ipq_pwm_chip, chip);
>> +}
>> +
>> +static unsigned ipq_pwm_reg_offset(struct pwm_device *pwm, unsigned reg)
>> +{
>> +	return ((pwm->hwpwm * 2) + reg) * 4;
>> +}
>> +
>> +static unsigned int ipq_pwm_reg_read(struct pwm_device *pwm, unsigned reg)
>> +{
>> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(pwm->chip);
>> +	unsigned int off = ipq_chip->regmap_off + ipq_pwm_reg_offset(pwm, reg);
>
> I already stumbled about this in v4 but thought I'd let you do it. As
> I stumbled again I'll say something now:
>
> I would do the register stuff as follows:
>
> 	/* Each PWM has two registers, the offset for PWM #i is at 8 * #i */
> 	#define IPQ_PWM_CFG_REG0	0
> 	#define IPQ_PWM_CFG_REG1	4
>
> and then do:
>
> 	static unsigned int ipq_pwm_reg_read(struct pwm_device *pwm, unsigned reg)
> 	{
> 		struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(pwm->chip);
> 		unsigned int off = ipq_chip->regmap_off + 8 * pwm->hwpwm + reg;
>
> 		...
>
> this is a bit easier to understand IMHO, but might be subjective. I let
> you decide if you want to change that or stay with your approach.
>
>> +	unsigned int val;
>> +
>> +	regmap_read(ipq_chip->regmap, off, &val);
>> +
>> +	return val;
>> +}
>> +
>> +static void ipq_pwm_reg_write(struct pwm_device *pwm, unsigned reg,
>> +		unsigned val)
>> +{
>> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(pwm->chip);
>> +	unsigned int off = ipq_chip->regmap_off + ipq_pwm_reg_offset(pwm, reg);
>> +
>> +	regmap_write(ipq_chip->regmap, off, val);
>> +}
>> +
>> +static void config_div_and_duty(struct pwm_device *pwm, unsigned int pre_div,
>> +			unsigned int pwm_div, u64 period_ns, u64 duty_ns,
>> +			bool enable)
>> +{
>> +	unsigned long hi_dur;
>> +	unsigned long long quotient;
>> +	unsigned long val = 0;
>> +
>> +	/*
>> +	 * high duration = pwm duty * (pwm div + 1)
>> +	 * pwm duty = duty_ns / period_ns
>> +	 */
>> +	quotient = (pwm_div + 1) * duty_ns;
>> +	hi_dur = div64_u64(quotient, period_ns);
>
> this division should use the actual period, not the target period.
> Otherwise the result might be to small.
>
>> +	val = FIELD_PREP(IPQ_PWM_REG0_HI_DURATION, hi_dur) |
>> +		FIELD_PREP(IPQ_PWM_REG0_PWM_DIV, pwm_div);
>> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG0, val);
>> +
>> +	val = FIELD_PREP(IPQ_PWM_REG1_PRE_DIV, pre_div);
>> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
>> +
>> +	/* Enable needs a separate write to REG1 */
>> +	val |= IPQ_PWM_REG1_UPDATE;
>
> Setting this bit results in the two writes above being configured
> atomically so that no mixed settings happen to the output, right?

I guess so. I have no access to hardware documentation, mind you. I
first tried to do only one write to REG1, but it had no effect. The
existence of the UPDATE bit also indicates that hardware works as you
suggest.

> Does the hardware complete the currently running cycle on
> reconfiguration?

No idea.

>> +	if (enable)
>> +		val |= IPQ_PWM_REG1_ENABLE;
>> +	else
>> +		val &= ~IPQ_PWM_REG1_ENABLE;
>
> The else branch has no effect as val is initialized as zero above, so
> please drop it.
>
>> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
>
> How does the hardware behave with the ENABLE bit unset? Does it drive
> the pin to zero?

Yes. That's what experimentation here shows. The pin is pulled up, but
the PWM keeps it low.

>> +}
>> +
>> +static int ipq_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>> +			 const struct pwm_state *state)
>> +{
>> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(chip);
>> +	unsigned long freq;
>> +	unsigned int pre_div, pwm_div, close_pre_div, close_pwm_div;
>> +	long long diff;
>> +	unsigned long rate = clk_get_rate(ipq_chip->clk);
>> +	unsigned long min_diff = rate;
>> +	uint64_t fin_ps;
>> +	u64 period_ns, duty_ns;
>
> You have to refuse the request if state->polarity !=
> PWM_POLARITY_NORMAL.
>
>> +
>> +	if (state->period < IPQ_PWM_MIN_PERIOD_NS)
>
> It's strange that you assume here the hardcoded 100 MHz but below you
> use clk_get_rate(ipq_chip->clk).

As I said above, this is meant to save code for the less critical
case. Should I use clk_get_rate() here as well? If we go with
assigned-clock-rates, as you suggest below, we'll have to do that
anyway.

>
>> +		return -ERANGE;
>> +
>> +	period_ns = min(state->period, IPQ_PWM_MAX_PERIOD_NS);
>> +	duty_ns = min(state->duty_cycle, period_ns);
>> +
>> +	/* freq in Hz for period in nano second */
>> +	freq = div64_u64(NSEC_PER_SEC, period_ns);
>> +	fin_ps = div64_u64(NSEC_PER_SEC * 1000ULL, rate);
>
> I don't understand that factor 1000. This just cancels with the 1000 in
> the calculation of pwm_div below?! Maybe this is to soften the precision
> loss?

That is my understanding of the code intent.

>> +	close_pre_div = IPQ_PWM_MAX_DIV;
>> +	close_pwm_div = IPQ_PWM_MAX_DIV;
>> +
>> +	for (pre_div = 0; pre_div <= IPQ_PWM_MAX_DIV; pre_div++) {
>> +		pwm_div = DIV64_U64_ROUND_CLOSEST(period_ns * 1000,
>> +						  fin_ps * (pre_div + 1));
>
> Having fin_ps in the divisor results in loss of precision. When ever the
> closest rounding division rounds down diff becomes negative below. So
> you should round up here.
>
> Also if you do:
>
> 	pwm_div = round_up((period_ns * rate) / (NSEC_PER_SEC * (pre_div + 1)))
>
> there is no relevant loss of precision. (You might have to care for
> period_ns * rate overflowing though or argue why it doesn't overflow.)

Looks better.

>
>> +		pwm_div--;
>> +		if (pwm_div > IPQ_PWM_MAX_DIV)
>> +			continue;
>
> This check can be dropped if the loop (depending on the other parameters)
> does not start with pre_div = 0 but some bigger number.

That is, calculate the minimum pre_div value for which the division
above always produces pwm_div in range, right?

>
>> +		diff = ((uint64_t)freq * (pre_div + 1) * (pwm_div + 1))
>> +			- (uint64_t)rate;
>> +
>> +		if (diff < 0) /* period larger than requested */
>> +			continue;
>> +		if (diff == 0) { /* bingo */
>> +			close_pre_div = pre_div;
>> +			close_pwm_div = pwm_div;
>> +			break;
>> +		}
>> +		if (diff < min_diff) {
>> +			min_diff = diff;
>> +			close_pre_div = pre_div;
>> +			close_pwm_div = pwm_div;
>
> I would call these best_..._div, not close_..._div which makes the
> purpose clearer.
>
> A big pre_div results in a coarse resolution for duty_cycle. This makes
> other similar drivers chose to hardcode pwm_div to its max value. At
> least you should ensure that pre_div <= pwm_div.
>
>> +		}
>> +	}
>> +
>> +	/* config divider values for the closest possible frequency */
>> +	config_div_and_duty(pwm, close_pre_div, close_pwm_div,
>> +			    period_ns, duty_ns, state->enabled);
>> +
>> +	return 0;
>> +}
>> +
>> +static void ipq_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>> +			      struct pwm_state *state)
>> +{
>> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(chip);
>> +	unsigned long rate = clk_get_rate(ipq_chip->clk);
>> +	unsigned int pre_div, pwm_div, hi_dur;
>> +	u64 effective_div, hi_div;
>> +	u32 reg0, reg1;
>> +
>> +	reg0 = ipq_pwm_reg_read(pwm, IPQ_PWM_CFG_REG0);
>> +	reg1 = ipq_pwm_reg_read(pwm, IPQ_PWM_CFG_REG1);
>> +
>> +	state->polarity = PWM_POLARITY_NORMAL;
>> +	state->enabled = reg1 & IPQ_PWM_REG1_ENABLE;
>> +
>> +	pwm_div = FIELD_GET(IPQ_PWM_REG0_PWM_DIV, reg0);
>> +	hi_dur = FIELD_GET(IPQ_PWM_REG0_HI_DURATION, reg0);
>> +	pre_div = FIELD_GET(IPQ_PWM_REG1_PRE_DIV, reg1);
>> +	effective_div = (pre_div + 1) * (pwm_div + 1);
>
> Please add a comment here that with pre_div and pwm_div <= 0xffff the
> multiplication below doesn't overflow
>
>> +	state->period = div64_u64(effective_div * NSEC_PER_SEC, rate);
>> +
>> +	hi_div = hi_dur * (pre_div + 1);
>
> This suggests that the hardware cannot do 100% relative duty cycle if
> pwm_div == 0xffff? I suggest to clamp pwm_div to 0xfffe then.

What is "100% relative duty"? How does pwm_div clamping helps?

>> +	state->duty_cycle = div64_u64(hi_div * NSEC_PER_SEC, rate);
>> +}
>> +
>> +static struct pwm_ops ipq_pwm_ops = {
>
> const please
>
>> +	.apply = ipq_pwm_apply,
>> +	.get_state = ipq_pwm_get_state,
>> +	.owner = THIS_MODULE,
>> +};
>> +
>> +static int ipq_pwm_probe(struct platform_device *pdev)
>> +{
>> +	struct ipq_pwm_chip *pwm;
>> +	struct device *dev = &pdev->dev;
>> +	struct of_phandle_args args;
>> +	int ret;
>> +
>> +	pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
>> +	if (!pwm)
>> +		return -ENOMEM;
>> +
>> +	platform_set_drvdata(pdev, pwm);
>> +
>> +	ret = of_parse_phandle_with_fixed_args(dev->of_node, "qcom,pwm-regs",
>> +			1, 0, &args);
>> +	if (ret)
>> +		return dev_err_probe(dev, ret, "regs parse failed");
>> +
>> +	pwm->regmap = syscon_node_to_regmap(args.np);
>> +	of_node_put(args.np);
>> +	if (IS_ERR(pwm->regmap))
>> +		return dev_err_probe(dev, PTR_ERR(pwm->regmap),
>> +				"regs map failed");
>> +	pwm->regmap_off = args.args[0];
>
> Does this have to be so complicated? Why doesn't the normal approach
> with the pwm being a child of the syscon device and reg = <...> work
> here?

I'll do that in v6. That's what Bjorn originally suggested in response
to v2.

>
>> +	pwm->clk = devm_clk_get(dev, "core");
>> +	if (IS_ERR(pwm->clk))
>> +		return dev_err_probe(dev, PTR_ERR(pwm->clk),
>> +				"failed to get core clock");
>> +
>> +	ret = clk_set_rate(pwm->clk, IPQ_PWM_CLK_SRC_FREQ);
>> +	if (ret)
>> +		return dev_err_probe(dev, ret, "clock rate set failed");
>
> Would it make more sense to set this in the device tree using
> assigned-clock-rate?

That's 'assigned-clock-rates' I believe. I'll try that.

>
>> +	ret = clk_prepare_enable(pwm->clk);
>> +	if (ret)
>> +		return dev_err_probe(dev, ret, "clock enable failed");
>> +
>> +	pwm->chip.dev = dev;
>> +	pwm->chip.ops = &ipq_pwm_ops;
>> +	pwm->chip.npwm = IPQ_PWM_MAX_DEVICES;
>> +
>> +	ret = pwmchip_add(&pwm->chip);
>> +	if (ret < 0) {
>> +		dev_err_probe(dev, ret, "pwmchip_add() failed\n");
>> +		clk_disable_unprepare(pwm->clk);
>> +		return ret;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +static int ipq_pwm_remove(struct platform_device *pdev)
>> +{
>> +	struct ipq_pwm_chip *pwm = platform_get_drvdata(pdev);
>> +
>> +	clk_disable_unprepare(pwm->clk);
>> +	pwmchip_remove(&pwm->chip);
>
> This is the wrong order. Until pwmchip_remove() returns the PWM must stay
> functional, so disable the clock only after pwmchip_remove().
>
>> +
>> +	return 0;
>> +}
>> +
>> +static const struct of_device_id pwm_ipq_dt_match[] = {
>> +	{ .compatible = "qcom,ipq6018-pwm", },
>> +	{}
>> +};
>> +MODULE_DEVICE_TABLE(of, pwm_ipq_dt_match);
>> +
>> +static struct platform_driver ipq_pwm_driver = {
>> +	.driver = {
>> +		.name = "ipq-pwm",
>> +		.of_match_table = pwm_ipq_dt_match,
>> +	},
>> +	.probe = ipq_pwm_probe,
>> +	.remove = ipq_pwm_remove,
>> +};
>> +
>> +module_platform_driver(ipq_pwm_driver);
>> +
>> +MODULE_LICENSE("Dual BSD/GPL");
>
> Best regards
> Uwe

Hello Baruch,

On Fri, Jul 16, 2021 at 08:51:20AM +0300, Baruch Siach wrote:
> On Wed, Jul 14 2021, Uwe Kleine-König wrote:
> > On Tue, Jul 13, 2021 at 02:35:43PM +0300, Baruch Siach wrote:
> >> +/* The frequency range supported is 1Hz to 100MHz */
> >
> > A space between number and unit is usual and makes this better readable.
> 
> Quick 'git grep' indicates that '[[:digit:]]\+MHz' is a little more
> popular than '[[:digit:]]\+ MHz' in kernel code. But OK, not a big deal.

"usual" was not meant in the sense "How it is used in the kernel" but
what the typesetting rules say. (Not 100% sure about English, but in
German you're supposed to add a space.)

> >> +#define IPQ_PWM_CLK_SRC_FREQ	(100*1000*1000)
> >> +#define IPQ_PWM_MIN_PERIOD_NS	(NSEC_PER_SEC / IPQ_PWM_CLK_SRC_FREQ)
> >
> > You're assuming here that the parent clock runs at exactly the set rate.
> > Is this a sensible assumption? If this division didn't have an integer
> > result there would be rounding issues.
> 
> The code only uses this for period validity check. It saves us some code
> for run-time division.

This check is only completely right if the clock really runs at 100 MHz,
and I'd prefer correct over saving a division. (If you know the clock
will run at 100 MHz for sure, you can better hard code it everywhere
giving the compiler the opportunity to optimize.) So the TL;DR here is:
use one or the other and use that one consistently.

> >> +	unsigned int val;
> >> +
> >> +	regmap_read(ipq_chip->regmap, off, &val);
> >> +
> >> +	return val;
> >> +}
> >> +
> >> +static void ipq_pwm_reg_write(struct pwm_device *pwm, unsigned reg,
> >> +		unsigned val)
> >> +{
> >> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(pwm->chip);
> >> +	unsigned int off = ipq_chip->regmap_off + ipq_pwm_reg_offset(pwm, reg);
> >> +
> >> +	regmap_write(ipq_chip->regmap, off, val);
> >> +}
> >> +
> >> +static void config_div_and_duty(struct pwm_device *pwm, unsigned int pre_div,
> >> +			unsigned int pwm_div, u64 period_ns, u64 duty_ns,
> >> +			bool enable)
> >> +{
> >> +	unsigned long hi_dur;
> >> +	unsigned long long quotient;
> >> +	unsigned long val = 0;
> >> +
> >> +	/*
> >> +	 * high duration = pwm duty * (pwm div + 1)
> >> +	 * pwm duty = duty_ns / period_ns
> >> +	 */
> >> +	quotient = (pwm_div + 1) * duty_ns;
> >> +	hi_dur = div64_u64(quotient, period_ns);
> >
> > this division should use the actual period, not the target period.
> > Otherwise the result might be to small.

I just noticed: Using the period here is also bad for precision as the
actual period is the result of a division.

> >> +	val = FIELD_PREP(IPQ_PWM_REG0_HI_DURATION, hi_dur) |
> >> +		FIELD_PREP(IPQ_PWM_REG0_PWM_DIV, pwm_div);
> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG0, val);
> >> +
> >> +	val = FIELD_PREP(IPQ_PWM_REG1_PRE_DIV, pre_div);
> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
> >> +
> >> +	/* Enable needs a separate write to REG1 */
> >> +	val |= IPQ_PWM_REG1_UPDATE;
> >
> > Setting this bit results in the two writes above being configured
> > atomically so that no mixed settings happen to the output, right?
> 
> I guess so. I have no access to hardware documentation, mind you. I
> first tried to do only one write to REG1, but it had no effect. The
> existence of the UPDATE bit also indicates that hardware works as you
> suggest.

I wouldn't trust HW documentation here. If you have some means to
inspect the waveform this is easy to test. Depending on how long you can
make the periods an LED is enough. If you start with a slower parent
clk, a big pre_div and hi_dur = 0 the LED is supposed to be off. Then
set hi_dur = pwm_div/2 which either make the LED blink slowly or keeps
off. Then setting pre_div = 2 either increased the blink frequency or it
doesn't. ...

> > Does the hardware complete the currently running cycle on
> > reconfiguration?
> 
> No idea.

This is easy to test, too. If you set a big period and duty_cycle and
immediately after that set a small period and duty.

> >> +	if (enable)
> >> +		val |= IPQ_PWM_REG1_ENABLE;
> >> +	else
> >> +		val &= ~IPQ_PWM_REG1_ENABLE;
> >
> > The else branch has no effect as val is initialized as zero above, so
> > please drop it.
> >
> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
> >
> > How does the hardware behave with the ENABLE bit unset? Does it drive
> > the pin to zero?
> 
> Yes. That's what experimentation here shows. The pin is pulled up, but
> the PWM keeps it low.

And with polarity set to inverted the PWM pulls the line up? As the
different hardwares behave differently and some consumers have
expectations here, having this documented would be great.

> >> +static int ipq_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >> +			 const struct pwm_state *state)
> >> +{
> >> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(chip);
> >> +	unsigned long freq;
> >> +	unsigned int pre_div, pwm_div, close_pre_div, close_pwm_div;
> >> +	long long diff;
> >> +	unsigned long rate = clk_get_rate(ipq_chip->clk);
> >> +	unsigned long min_diff = rate;
> >> +	uint64_t fin_ps;
> >> +	u64 period_ns, duty_ns;
> >
> > You have to refuse the request if state->polarity !=
> > PWM_POLARITY_NORMAL.
> >
> >> +
> >> +	if (state->period < IPQ_PWM_MIN_PERIOD_NS)
> >
> > It's strange that you assume here the hardcoded 100 MHz but below you
> > use clk_get_rate(ipq_chip->clk).
> 
> As I said above, this is meant to save code for the less critical
> case. Should I use clk_get_rate() here as well? If we go with
> assigned-clock-rates, as you suggest below, we'll have to do that
> anyway.

Sounds right. (That is: use assigned-clock-rates + use clk_get_rate
consistently)
 
> >> +		return -ERANGE;
> >> +
> >> +	period_ns = min(state->period, IPQ_PWM_MAX_PERIOD_NS);
> >> +	duty_ns = min(state->duty_cycle, period_ns);
> >> +
> >> +	/* freq in Hz for period in nano second */
> >> +	freq = div64_u64(NSEC_PER_SEC, period_ns);
> >> +	fin_ps = div64_u64(NSEC_PER_SEC * 1000ULL, rate);
> >
> > I don't understand that factor 1000. This just cancels with the 1000 in
> > the calculation of pwm_div below?! Maybe this is to soften the precision
> > loss?
> 
> That is my understanding of the code intent.
> 
> >> +	close_pre_div = IPQ_PWM_MAX_DIV;
> >> +	close_pwm_div = IPQ_PWM_MAX_DIV;
> >> +
> >> +	for (pre_div = 0; pre_div <= IPQ_PWM_MAX_DIV; pre_div++) {
> >> +		pwm_div = DIV64_U64_ROUND_CLOSEST(period_ns * 1000,
> >> +						  fin_ps * (pre_div + 1));
> >
> > Having fin_ps in the divisor results in loss of precision. When ever the
> > closest rounding division rounds down diff becomes negative below. So
> > you should round up here.
> >
> > Also if you do:
> >
> > 	pwm_div = round_up((period_ns * rate) / (NSEC_PER_SEC * (pre_div + 1)))
> >
> > there is no relevant loss of precision. (You might have to care for
> > period_ns * rate overflowing though or argue why it doesn't overflow.)
> 
> Looks better.

And doesn't need the factor 1000 to improve precision \o/

> >> +		pwm_div--;
> >> +		if (pwm_div > IPQ_PWM_MAX_DIV)
> >> +			continue;
> >
> > This check can be dropped if the loop (depending on the other parameters)
> > does not start with pre_div = 0 but some bigger number.
> 
> That is, calculate the minimum pre_div value for which the division
> above always produces pwm_div in range, right?

Yes, that was my idea. I didn't do the math but expect this not to be so
difficult.

> [...]
> >> +static void ipq_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> >> +			      struct pwm_state *state)
> >> +{
> >> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(chip);
> >> +	unsigned long rate = clk_get_rate(ipq_chip->clk);
> >> +	unsigned int pre_div, pwm_div, hi_dur;
> >> +	u64 effective_div, hi_div;
> >> +	u32 reg0, reg1;
> >> +
> >> +	reg0 = ipq_pwm_reg_read(pwm, IPQ_PWM_CFG_REG0);
> >> +	reg1 = ipq_pwm_reg_read(pwm, IPQ_PWM_CFG_REG1);
> >> +
> >> +	state->polarity = PWM_POLARITY_NORMAL;
> >> +	state->enabled = reg1 & IPQ_PWM_REG1_ENABLE;
> >> +
> >> +	pwm_div = FIELD_GET(IPQ_PWM_REG0_PWM_DIV, reg0);
> >> +	hi_dur = FIELD_GET(IPQ_PWM_REG0_HI_DURATION, reg0);
> >> +	pre_div = FIELD_GET(IPQ_PWM_REG1_PRE_DIV, reg1);
> >> +	effective_div = (pre_div + 1) * (pwm_div + 1);
> >
> > Please add a comment here that with pre_div and pwm_div <= 0xffff the
> > multiplication below doesn't overflow
> >
> >> +	state->period = div64_u64(effective_div * NSEC_PER_SEC, rate);
> >> +
> >> +	hi_div = hi_dur * (pre_div + 1);
> >
> > This suggests that the hardware cannot do 100% relative duty cycle if
> > pwm_div == 0xffff? I suggest to clamp pwm_div to 0xfffe then.
> 
> What is "100% relative duty"? How does pwm_div clamping helps?

relative duty = duty_cycle / period. So 100% relative duty means period ==
duty_cycle. With pwm_div == 0xffff period is
0x10000 * (pre_div + 1) / rate but duty_cycle cannot achieve that as the
maximum is 0xffff * (pre_div + 1) / rate.

> >> +	pwm->clk = devm_clk_get(dev, "core");
> >> +	if (IS_ERR(pwm->clk))
> >> +		return dev_err_probe(dev, PTR_ERR(pwm->clk),
> >> +				"failed to get core clock");
> >> +
> >> +	ret = clk_set_rate(pwm->clk, IPQ_PWM_CLK_SRC_FREQ);
> >> +	if (ret)
> >> +		return dev_err_probe(dev, ret, "clock rate set failed");
> >
> > Would it make more sense to set this in the device tree using
> > assigned-clock-rate?
> 
> That's 'assigned-clock-rates' I believe. I'll try that.

Ah right, I missed the s.

Best regards
Uwe

Hi Uwe,

On Fri, Jul 16 2021, Uwe Kleine-König wrote:
> On Fri, Jul 16, 2021 at 08:51:20AM +0300, Baruch Siach wrote:
>> On Wed, Jul 14 2021, Uwe Kleine-König wrote:
>> > On Tue, Jul 13, 2021 at 02:35:43PM +0300, Baruch Siach wrote:
>> >> +	val = FIELD_PREP(IPQ_PWM_REG0_HI_DURATION, hi_dur) |
>> >> +		FIELD_PREP(IPQ_PWM_REG0_PWM_DIV, pwm_div);
>> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG0, val);
>> >> +
>> >> +	val = FIELD_PREP(IPQ_PWM_REG1_PRE_DIV, pre_div);
>> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
>> >> +
>> >> +	/* Enable needs a separate write to REG1 */
>> >> +	val |= IPQ_PWM_REG1_UPDATE;
>> >
>> > Setting this bit results in the two writes above being configured
>> > atomically so that no mixed settings happen to the output, right?
>> 
>> I guess so. I have no access to hardware documentation, mind you. I
>> first tried to do only one write to REG1, but it had no effect. The
>> existence of the UPDATE bit also indicates that hardware works as you
>> suggest.
>
> I wouldn't trust HW documentation here. If you have some means to
> inspect the waveform this is easy to test. Depending on how long you can
> make the periods an LED is enough. If you start with a slower parent
> clk, a big pre_div and hi_dur = 0 the LED is supposed to be off. Then
> set hi_dur = pwm_div/2 which either make the LED blink slowly or keeps
> off. Then setting pre_div = 2 either increased the blink frequency or it
> doesn't. ...

I currently have only access to DVM to measure the PWM effect. I'll try
to do more measures when I have access to better equipment.

baruch