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

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Date:   Mon, 25 Oct 2021 17:37:58 -0700
From:   Subbaraman Narayanamurthy <subbaram@...eaurora.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Lee Jones <lee.jones@...aro.org>
Cc:     linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-pwm@...r.kernel.org,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Yassine Oudjana <y.oudjana@...tonmail.com>,
        Luca Weiss <luca@...tu.xyz>
Subject: Re: [PATCH v10 2/2] leds: Add driver for Qualcomm LPG

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