Re: [PATCH v6 2/4] leds: Add driver for Qualcomm LPG

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

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Hello Bjorn,

On Wed, Oct 21, 2020 at 01:12:22PM -0700, Bjorn Andersson wrote:
> +static const unsigned int lpg_clk_table[NUM_PWM_PREDIV][NUM_PWM_CLK] = {
> +	{
> +		1 * (NSEC_PER_SEC / 1024),
> +		1 * (NSEC_PER_SEC / 32768),
> +		1 * (NSEC_PER_SEC / 19200000),
> +	},
> +	{
> +		3 * (NSEC_PER_SEC / 1024),
> +		3 * (NSEC_PER_SEC / 32768),

1000000000 / 32768 is 30517.578125. Because of the parenthesis this is
truncated to 30517. Multiplied by 3 this results in 91551. The exact
result is 91552.734375 however.

> +		3 * (NSEC_PER_SEC / 19200000),
> +	},
> +	{
> +		5 * (NSEC_PER_SEC / 1024),
> +		5 * (NSEC_PER_SEC / 32768),
> +		5 * (NSEC_PER_SEC / 19200000),
> +	},
> +	{
> +		6 * (NSEC_PER_SEC / 1024),
> +		6 * (NSEC_PER_SEC / 32768),
> +		6 * (NSEC_PER_SEC / 19200000),
> +	},
> +};
> +
> +/*
> + * PWM Frequency = Clock Frequency / (N * T)
> + *      or
> + * PWM Period = Clock Period * (N * T)
> + *      where
> + * N = 2^9 or 2^6 for 9-bit or 6-bit PWM size
> + * T = Pre-divide * 2^m, where m = 0..7 (exponent)
> + *
> + * This is the formula to figure out m for the best pre-divide and clock:
> + * (PWM Period / N) = (Pre-divide * Clock Period) * 2^m
> + */
> +static void lpg_calc_freq(struct lpg_channel *chan, unsigned int period_us)
> +{
> +	int             n, m, clk, div;
> +	int             best_m, best_div, best_clk;
> +	unsigned int    last_err, cur_err, min_err;
> +	unsigned int    tmp_p, period_n;
> +
> +	if (period_us == chan->period_us)
> +		return;
> +
> +	/* PWM Period / N */
> +	if (period_us < UINT_MAX / NSEC_PER_USEC)
> +		n = 6;
> +	else
> +		n = 9;
> +
> +	period_n = ((u64)period_us * NSEC_PER_USEC) >> n;
> +
> +	min_err = UINT_MAX;
> +	last_err = UINT_MAX;
> +	best_m = 0;
> +	best_clk = 0;
> +	best_div = 0;
> +	for (clk = 0; clk < NUM_PWM_CLK; clk++) {
> +		for (div = 0; div < NUM_PWM_PREDIV; div++) {
> +			/* period_n = (PWM Period / N) */
> +			/* tmp_p = (Pre-divide * Clock Period) * 2^m */
> +			tmp_p = lpg_clk_table[div][clk];
> +			for (m = 0; m <= NUM_EXP; m++) {
> +				cur_err = abs(period_n - tmp_p);
> +				if (cur_err < min_err) {
> +					min_err = cur_err;
> +					best_m = m;
> +					best_clk = clk;
> +					best_div = div;
> +				}
> +
> +				if (m && cur_err > last_err)
> +					/* Break for bigger cur_err */
> +					break;
> +
> +				last_err = cur_err;
> +				tmp_p <<= 1;

This is inexact. Consider again the case where tmp_p is
3 * (NSEC_PER_SEC / 32768). The values you use and the exact values are:

	m     |       0        |      1       |      2      |      3     | ... |        7 |
	tmp_p |   91551        | 183102       | 366204      | 732408     |     | 11718528 |
        actual|   91552.734375 | 183105.46875 | 366210.9375 | 732421.875 | ... | 11718750 |

So while you save some cycles by precalculating the values in
lpg_clk_table, you trade that for lost precision.

> +			}
> +		}
> +	}

Please don't pick a period that is longer than the requested period (for
the PWM functionality that is).

This can be simplified, you can at least calculate the optimal m
directly.

> +	/* Use higher resolution */
> +	if (best_m >= 3 && n == 6) {
> +		n += 3;
> +		best_m -= 3;
> +	}
> +
> +	chan->clk = best_clk;
> +	chan->pre_div = best_div;
> +	chan->pre_div_exp = best_m;
> +	chan->pwm_size = n;
> +
> +	chan->period_us = period_us;
> +}
> +
> +static void lpg_calc_duty(struct lpg_channel *chan, unsigned int duty_us)
> +{
> +	unsigned int max = (1 << chan->pwm_size) - 1;
> +	unsigned int val = div_u64((u64)duty_us << chan->pwm_size, chan->period_us);

Please use the actually implemented period here instead of the
requested. This improves precision, see commit
8035e6c66a5e98f098edf7441667de74affb4e78 for a similar case.

> +
> +	chan->pwm_value = min(val, max);
> +}
> +
> [...]
> +static const struct pwm_ops lpg_pwm_ops = {
> +	.request = lpg_pwm_request,
> +	.apply = lpg_pwm_apply,

Can you please test your driver with PWM_DEBUG enabled? The first thing
this will critizise is that there is no .get_state callback.

> +	.owner = THIS_MODULE,
> +};
> +
> +static int lpg_add_pwm(struct lpg *lpg)
> +{
> +	int ret;
> +
> +	lpg->pwm.base = -1;

Please drop this assignment.

> +	lpg->pwm.dev = lpg->dev;
> +	lpg->pwm.npwm = lpg->num_channels;
> +	lpg->pwm.ops = &lpg_pwm_ops;
> +
> +	ret = pwmchip_add(&lpg->pwm);
> +	if (ret)
> +		dev_err(lpg->dev, "failed to add PWM chip: ret %d\n", ret);
> +
> +	return ret;
> +}

Best regards
Uwe

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

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