Re: [PATCH v10 2/2] leds: Add driver for Qualcomm LPG

Subbaraman Narayanamurthy
2021-10-26T00:38:07Z

Hi Bjorn,

> +#define LPG_RESOLUTION 512

Just a thought. Having this fixed to 9-bit resolution would require a lot of code churn if this driver ends up supporting higher resolution PWM later. Would it be possible to have this as a parameter in "struct lpg_channel" ?

> +static const unsigned int lpg_clk_rates[] = {1024, 32768, 19200000};
> +static const unsigned int lpg_pre_divs[] = {1, 3, 5, 6};
> +
> +static int lpg_calc_freq(struct lpg_channel *chan, uint64_t period)
> +{
> + unsigned int clk, best_clk = 0;
> + unsigned int div, best_div = 0;
> + unsigned int m, best_m = 0;
> + unsigned int error;
> + unsigned int best_err = UINT_MAX;
> + u64 best_period = 0;
> +
> + /*
> + * The PWM period is determined by:
> + *
> + * resolution * pre_div * 2^M
> + * period = --------------------------
> + * refclk
> + *
> + * With resolution fixed at 2^9 bits, pre_div = {1, 3, 5, 6} and
> + * M = [0..7].
> + *
> + * This allows for periods between 27uS and 384s, as the PWM framework
> + * wants a period of equal or lower length than requested, reject
> + * anything below 27uS.
> + */
> + if (period <= (u64)NSEC_PER_SEC * LPG_RESOLUTION / 19200000)
> + return -EINVAL;
> +
> + /* Limit period to largest possible value, to avoid overflows */
> + if (period > (u64)NSEC_PER_SEC * LPG_RESOLUTION * 6 * (1 << LPG_MAX_M) / 1024)
> + period = (u64)NSEC_PER_SEC * LPG_RESOLUTION * 6 * (1 << LPG_MAX_M) / 2014;

s/2014/1024 ?

Thanks,
Subbaraman