Re: [PATCH v4 04/16] pwm: lpss: Add range limit check for the base_unit register value

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 7/9/20 4:21 PM, Andy Shevchenko wrote:
> On Thu, Jul 09, 2020 at 03:23:13PM +0200, Hans de Goede wrote:
>> On 7/9/20 2:53 PM, Andy Shevchenko wrote:
>>> On Wed, Jul 08, 2020 at 11:14:20PM +0200, Hans de Goede wrote:
>>>> When the user requests a high enough period ns value, then the
>>>> calculations in pwm_lpss_prepare() might result in a base_unit value of 0.
>>>>
>>>> But according to the data-sheet the way the PWM controller works is that
>>>> each input clock-cycle the base_unit gets added to a N bit counter and
>>>> that counter overflowing determines the PWM output frequency. Adding 0
>>>> to the counter is a no-op. The data-sheet even explicitly states that
>>>> writing 0 to the base_unit bits will result in the PWM outputting a
>>>> continuous 0 signal.
>>>
>>> And I don't see how you can get duty 100% / 0% (I don't remember which one is
>>> equivalent to 0 in base unit) after this change. IIRC the problem here that
>>> base unit when non-zero is always being added to the counter and it will
>>> trigger the change of output at some point which is not what we want for 100% /
>>> 0% cases.
>>
>> The base_unit controls the output frequency, not the duty-cycle. So clamping
>> the base_unit, as calculated from the period here, which also only configures
>> output-frequency does not impact the duty-cycle at all.
>>
>> note that AFAICT currently no (in kernel) users actually try to set a period value
>> which would hit the clamp, so for existing users the clamp is a no-op. I just
>> added it to this patch-set for correctness sake and because userspace
>> (sysfs interface) users could in theory set out of range values.
>>
>> As for the duty-cycle thing, first of all let me say that that is a
>> question / issue which is completely orthogonal to this patch, this
>> patch only impacts the period/output frequency NOT the duty-cycle,
> 
> Unfortunately the base unit settings affects duty cycle.
> 
> Documentation says about integer part and fractional, where integer is
> 8 bit and this what's being compared to on time divisor. Thus, if on time
> divisor is 255 and base unit is 1 (in integer part) or 0.25, we can't get 0%.
> (It looks like if 'on time divisor MOD base unit == 0' we won't get 0%)
> 
>> With that said, the documentation is not really helpful here,
>> we need to set the on_time_div to 255 to get a duty-cycle close to 0
>> (and to 0 to get a duty cycle of 100%) but if setting this to 255 gives
>> us a duty-cycle of really really 0%, or just close to 0% is uncleaer.
> 
> It depends on base unit value.
> 
>> We could do a separate patch add ing a hack where if the user asks for
>> 0% duty-cycle we program the base_unit to 0, but that seems like a bad
>> idea for 2 reasons:
> 
>> 1. If the user really wants the output to be constantly 0 the user should
>> just disable the pwm
> 
> I can't take this as an argument. Disabling PWM is orthogonal to what duty cycle is.
> 
>> 2. New base_unit values are latched and not applied until the counter
>> overflows, with a base_unit of 0 the counter never overflows. I have
>> not tested this but I would not be surprised if after programming a
>> base_unit value of 0, we are unable to ever change the value again
>> through any other means then power-cycling the PWM controller.
>> Even if I could test this on some revisions, we already know that
>> not all revisions work the same wrt the latching. So it is best to
>> just never set base_unit to 0, that is just a recipe asking for trouble.
> 
> This what doc says about zeros:
> • Programming either the PWM_base_unit value or the PWM_on_time_divisor to ‘0’
> will generate an always zero output.
> 
> So, what I'm talking seems about correlation between base unit and on time
> divisor rather than zeros.
> 
> I agree with this patch.
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Thank you.

>>>> When the user requestes a low enough period ns value, then the
>>>> calculations in pwm_lpss_prepare() might result in a base_unit value
>>>> which is bigger then base_unit_range - 1. Currently the codes for this
>>>> deals with this by applying a mask:
>>>>
>>>> 	base_unit &= (base_unit_range - 1);
>>>>
>>>> But this means that we let the value overflow the range, we throw away the
>>>> higher bits and store whatever value is left in the lower bits into the
>>>> register leading to a random output frequency, rather then clamping the
>>>> output frequency to the highest frequency which the hardware can do.
>>>
>>> It would be nice to have an example of calculus here.
>>>
>>>> This commit fixes both issues by clamping the base_unit value to be
>>>> between 1 and (base_unit_range - 1).
>>>
>>> Eventually I sat and wrote all this on paper. I see now that the problem
>>> is in out of range of the period. And strongly we should clamp rather period
>>> to the supported range, but your solution is an equivalent.
>>
>> Right, the advantage of doing the clamping on the register value is that we
>> avoid some tricky math with possible rounding errors and which is different
>> per controller revision because the number of bits in the base unit being
>> different per controller revision.
> 
> ...
> 
>>>> +	base_unit = clamp_t(unsigned long long, base_unit, 1,
>>>> +			    base_unit_range - 1);
>>>
>>> A nit: one line.
>>
>> Doesn't fit in 80 chars, I guess we could make this one line now with the new 100 chars
>> limit, but that does make it harder to read for people using standard terminal widths
>> and a terminal based editors. So I would prefer to keep this as is.
> 
> You can use clamp_val().

I did not know about that, that will work nicely I will switch to clamp_val
for the next version. I assume it is ok to keep your Reviewed-by with this
very minor change?

Regards,

Hans

>