Re: [PATCH v4 1/3] pwm: driver for qualcomm ipq6018 pwm block

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Baruch,

On Wed, Jul 07, 2021 at 07:58:01AM +0300, Baruch Siach wrote:
> On Mon, Jul 05 2021, Uwe Kleine-König wrote:
> > On Sun, Jun 27, 2021 at 08:24:04AM +0300, Baruch Siach wrote:
> >> +/*
> >> + * 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 PWM_ENABLE		0x80000000
> >> +#define PWM_UPDATE		0x40000000
> >> +
> >> +/* The frequency range supported is 1Hz to 100MHz */
> >> +#define MIN_PERIOD_NS	10
> >> +#define MAX_PERIOD_NS	1000000000
> >
> > Please use a driver prefix for these defines.
> 
> I take this to refer also to the defines below, right?

right.

> >> +
> >> +/*
> >> + * The max value specified for each field is based on the number of bits
> >> + * in the pwm control register for that field
> >> + */
> >> +#define MAX_PWM_CFG		0xFFFF
> >> +
> >> +#define PWM_CTRL_HI_SHIFT	16
> >> +
> >> +#define PWM_CFG_REG0 0 /*PWM_DIV PWM_HI*/
> >> +#define PWM_CFG_REG1 1 /*ENABLE UPDATE PWM_PRE_DIV*/
> 
> ...
> 
> >> +static void config_div_and_duty(struct pwm_device *pwm, int pre_div,
> >> +			unsigned long long pwm_div, unsigned long period_ns,
> >> +			unsigned long long duty_ns)
> >
> > Please also use a consistent prefix for function names.
> >
> > I suggest to use u64 for some of the parameters. While this doesn't
> > change anything, it is cleaner as the caller passes variables of this
> > type.
> 
> Actually for pre_div and pwm_div the caller passes int values. I agree
> this is inconsistent.
> 
> ...
> 
> >> +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;
> >> +	int pre_div, close_pre_div, close_pwm_div;
> >> +	int 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;
> >> +
> >> +	if (state->period < MIN_PERIOD_NS)
> >> +		return -ERANGE;
> >
> > MIN_PERIOD_NS depends on clk_get_rate(ipq_chip->clk), doesn't it?
> 
> probe sets this clock to the fixed 100MHz rate (CLK_SRC_FREQ). Would you
> prefer to derive MIN_PERIOD_NS from CLK_SRC_FREQ?

I'd like to either have all of ipq_pwm_apply use this information that
the clock rate is 100MHz or nothing, but having MIN_PERIOD_NS hardcoding
that information and use clk_get_rate(ipq_chip->clk) for calculating
fin_ps is strange.

> >> +	freq = div64_u64(NSEC_PER_SEC, period_ns);
> >> +	fin_ps = div64_u64(NSEC_PER_SEC * 1000ULL, rate);
> >> +	close_pre_div = MAX_PWM_CFG;
> >> +	close_pwm_div = MAX_PWM_CFG;
> >> +
> >> +	for (pre_div = 0; pre_div <= MAX_PWM_CFG; pre_div++) {
> >> +		pwm_div = DIV64_U64_ROUND_CLOSEST(period_ns * 1000,
> >> +						  fin_ps * (pre_div + 1));
> >> +		pwm_div--;
> >> +		if (pwm_div < 0 || pwm_div > MAX_PWM_CFG)
> >> +			continue;
> >> +
> >> +		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 didn't check deeply, but I assume this calculation can be done more
> > efficiently.
> 
> The thing is that we have two dividers to play with. I can't think of a
> cleaner way to find the best match for a given target frequency.

OK, with two equal dividers it might indeed be necessary to do it this
way. After seeing a get_state implemententation I will think about this
some more.

Best regards
Uwe

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

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