Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value

Hans de Goede
2020-07-07T17:31:36Z

Hi,

On 7/7/20 9:34 AM, Uwe Kleine-König wrote:
> On Mon, Jul 06, 2020 at 10:53:08PM +0200, Hans de Goede wrote:
>> Hi,
>>
>> Thank you for your review and sorry for the slow reply.
>
> No problem for me, I didn't hold my breath :-)
>
>>>> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
>>>> index 43b1fc634af1..80d0f9c64f9d 100644
>>>> --- a/drivers/pwm/pwm-lpss.c
>>>> +++ b/drivers/pwm/pwm-lpss.c
>>>> @@ -97,6 +97,9 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>>>> freq *= base_unit_range;
>>>> base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
>>>
>>> DIV_ROUND_CLOSEST_ULL is most probably wrong, too. But I didn't spend
>>> the time to actually confirm that.
>>
>> Yes I saw your comment elsewhere that the PWM API defines rounding
>> in a certain direction, but fixing that falls outside of this patch.
>
> Yeah, sure.
>
>> [...]
>> I hope this helps to explain what is going on a bit.
>
> I will try to make sense of that and reply to the patch directly when I
> succeeded.
>
>> ###
>>
>> As for the behavior on base_unit==0 in the get_state method,
>> as mentioned above I wrote that when I did not fully understood
>> how the controller works.
>>
>> We really should never encounter this.
>>
>> But if we do then I think closest to the truth would be:
>>
>> state->period = UINT_MAX;
>> state->duty_cycle = 0;
>
> I'd say state->period = 1 & state->duty_cycle = 0 is a better
> representation.

But that would suggest the output is configured for an
infinitely high output frequency, but the frequency is
actually 0, the reason why get_state needs to treat a
base_unit val of 0 special at all is to avoid a division
by 0, and in math dividing by 0 gives infinite, isn't
UINT_MAX a better way to represent infinity ?

Regards,

Hans