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

[Heiner Kallweit <hkallweit1@gmail.com>]

On 01.05.2023 20:07, Uwe Kleine-König wrote:
> 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@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?!
> 

I *think* too that it's wrong. However I have no test hw and I'm not aware
of any problem caused by the current code. Therefore I was reluctant to make
the patch 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;
> 
The pre_div member will be gone anyway with a patch series that is in
discussion currently ("make full use of CCF").

> 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.
> 
I saw that one too. It's something for a follow-up patch.

> Having said that, the proposed change here is an improvement, so:
> 
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@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()")
> 
OK

> Best regards
> Uwe
> 
Heiner


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