[PATCH v2] pwm: meson: modify and simplify calculation in meson_pwm_get_state

[Uwe Kleine-König]

On Mon, May 01, 2023 at 04:03:16PM +0200, Heiner Kallweit wrote:
> I don't see a reason why we should treat the case lo < hi differently
> and return 0 as period and duty_cycle. The current logic was added with
> c375bcbaabdb ("pwm: meson: Read the full hardware state in
> meson_pwm_get_state()"), Martin as original author doesn't remember why
> it was implemented this way back then.
> So let's handle it as normal use case and also remove the optimization
> for lo == 0. I think the improved readability is worth it.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
> ---
> v2:
> - improve commit description
> ---
>  drivers/pwm/pwm-meson.c | 14 ++------------
>  1 file changed, 2 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
> index 5732300eb..3865538dd 100644
> --- a/drivers/pwm/pwm-meson.c
> +++ b/drivers/pwm/pwm-meson.c
> @@ -351,18 +351,8 @@ static int meson_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  	channel->lo = FIELD_GET(PWM_LOW_MASK, value);
>  	channel->hi = FIELD_GET(PWM_HIGH_MASK, value);
>  
> -	if (channel->lo == 0) {
> -		state->period = meson_pwm_cnt_to_ns(chip, pwm, channel->hi);
> -		state->duty_cycle = state->period;
> -	} else if (channel->lo >= channel->hi) {
> -		state->period = meson_pwm_cnt_to_ns(chip, pwm,
> -						    channel->lo + channel->hi);
> -		state->duty_cycle = meson_pwm_cnt_to_ns(chip, pwm,
> -							channel->hi);
> -	} else {
> -		state->period = 0;
> -		state->duty_cycle = 0;
> -	}

The last else branch is even wrong, isn't it? .apply() can for a greater
than 50% relative duty cycle well have lo < hi, right? So this is not a
mere optimisation but a fix?!

> +	state->period = meson_pwm_cnt_to_ns(chip, pwm, channel->lo + channel->hi);
> +	state->duty_cycle = meson_pwm_cnt_to_ns(chip, pwm, channel->hi);

Note that meson_pwm_calc() has a similar construct that can be
simplified in a similar way. All three variants have

	channel->pre_div = pre_div;

and the last else branch is universal and can replace the others.

Another issue I just spotted is that

	duty = state->duty_cycle

is wrong for state->duty_cycle > UINT_MAX. (Ditto the assignment to
period.) Making both duty and period u64 shoudl fix that. After that
duty_cnt > 0xffff cannot happen as the core ensures that duty_cycle <=
period.

Having said that, the proposed change here is an improvement, so:

Reviewed-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>

I also suggest to add a Fixes line, i.e.

Fixes: c375bcbaabdb ("pwm: meson: Read the full hardware state in meson_pwm_get_state()")

Best regards
Uwe

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