Re: [PATCH 3/5] pwm: adp5585: Clamp on/off counters to the 16 bit register range

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] pwm: adp5585: clamp on before computing off in
  pwm_adp5585_apply()
--

commit 636e863f3fd6ed7ec6fcc05f5eed5bd16bc080a9
Author: Shao Mingyin <shao.mingyin@zte.com.cn>

pwm: adp5585: Clamp on/off counters to the 16 bit register range

This commit clamps the on and off counter values in pwm_adp5585_apply() to
0xffff before writing them to the 16-bit hardware registers.

> diff --git a/drivers/pwm/pwm-adp5585.c b/drivers/pwm/pwm-adp5585.c
> index 9a1dbba75bc14..da83fcd35e1d7 100644
> --- a/drivers/pwm/pwm-adp5585.c
> +++ b/drivers/pwm/pwm-adp5585.c
> @@ -96,6 +96,13 @@ static int pwm_adp5585_apply(struct pwm_chip *chip,
>  	on = div_u64(duty_cycle, NSEC_PER_SEC / ADP5585_PWM_OSC_FREQ_HZ);
>  	off = div_u64(period, NSEC_PER_SEC / ADP5585_PWM_OSC_FREQ_HZ) - on;
>  
> +	/*
> +	 * The on/off registers are 16 bits wide, clamp the counter values so
> +	 * that cpu_to_le16() below can't silently truncate them.
> +	 */
> +	on = min_t(u32, on, 0xffff);
> +	off = min_t(u32, off, 0xffff);
> +
>  	val = cpu_to_le16(off);
>  	ret = regmap_bulk_write(regmap, info->pwm_offt_low, &val, 2);
>  	if (ret)

[Severity: Medium]
Should on be clamped to 0xffff before subtracting it to compute off?

The hardware PWM period is the sum of the on and off counters:

drivers/pwm/pwm-adp5585.c:pwm_adp5585_get_state() {
	...
	state->duty_cycle = on * (NSEC_PER_SEC / ADP5585_PWM_OSC_FREQ_HZ);
	state->period = (on + off) * (NSEC_PER_SEC / ADP5585_PWM_OSC_FREQ_HZ);
	...
}

When duty_cycle exceeds 0xffff oscillator ticks (65,535,000 ns), subtracting
the unclamped on value from the period ticks and then clamping on to 0xffff
drops (unclamped_on - 0xffff) ticks from the total programmed period (and
triggers a warning in pwm_apply_debug() under CONFIG_PWM_DEBUG if the same
period was previously configured with a smaller duty_cycle).  Because period
is already clamped to 2 * 0xffff ticks, clamping on before computing off
preserves the requested period and guarantees off <= 0xffff whenever on is
saturated.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107000724697MsR-XmE4V-ADPPMr3wGom@zte.com.cn?part=3