Re: [PATCH v4 2/2] dt-bindings: iio: filter: Add lpf/hpf freq margins

[Krzysztof Kozlowski <krzk@kernel.org>]

On 26/02/2025 23:16, Sam Winchenbach wrote:
>>>>> +  adi,hpf-margin-hz:
>>>>> +    description:
>>>>> +      Sets maximum high-pass corner frequency to the frequency of rf_in minus
>>>>> +      this value when in auto mode.
>>>>
>>>> IIUC, these are two bounds - lower and upper - in relation to something
>>>> else (like rf_in frequency)? If so, make it an array (naming to be
>>>> discuss, I assume you know better what's that):
>>>
>>> It is true that these are both related to rf_in but both the low and high pass
>>> filters can operate independently. Logically, IMO, it makes more sense to have
>>
>>
>> You mean you can set only low or high pass and keep other as default?
>> But what is the default then - something from reset value or "0" means
>> disabled?
> 
> This value isn't setting the corner frequency of the filter, but the minimum
> distance the corner must be from the fundamental frequency. So, for example,
> if rf_in is 3.35 GHz and you set lpf-margin-hz to 0 then the corner frequency
> will be set to 3.35 GHz because that is an exact value supported by the device.
> 
> If lpf-margin-hz is set to 30 MHz (for example), then corner frequency would be
> at least 3.35 GHz + 30 MHz = 3.38 GHz.  3.49 GHz is the closest corner
> frequency without going below 3.38 GHz that is supported by the device, so that
> is what will be selected.
> 
> This prevents the situation where your fundamental frequency falls on, or close
> to, a corner frequency which could result in 3dB (half power) loss in your
> signal.
> 
> This is all completely indepent of the high-pass filter.

Description is confusing a bit, because it suggests the value sets the
corner frequency. It explicitly says this - "sets ... corner frequency"
and such meaning for properties we usually associate with the property
doing this. Here however corner frequency will be always set to rf_in
and you just adjust the value.

> 
>>
>>> them as separate controls but I am happy to put them into an array if that is
>>> the idiomatic approach to situations like this. That said, I am having a
>>> difficult time getting dt_binding_check to pass when I have an array of uint64.
>>>
>>> When listing two items, as in your example below, I get the following:
>>> adi,admv8818.example.dtb: admv8818@0: adi,filter-margins-hz: [[0, 30000000], [0, 30000000]] is too long
>>
>> Tricky to say without seeing your code. Magic crystal ball had
>> malfunction today.
> 
> This is the property:
> 
>   adi,filter-margins-hz:
>     items:
>       - description: |
>           The minimum distance, in Hz, between rf_in and the low-pass corner
>           frequency when the device is used in "auto" mode. If the sum of
>           rf_in and this value is greater than 18.85 GHz then the low-pass
>           filter will be put into bypass mode, otherwise the closest corner
>           frequency that is greater than or equal to the sum of rf_in plus this
>           value will be used.
>         minimum: 0
>         maximum: 0xFFFFFFFFFFFFFFFF
>         default: 0
>       - description: |
>           The minimum distance, in Hz, between rf_in and the high-pass corner
>           frequency when the device is used in "auto" mode. If the difference
>           between rf_in and this value is less than 1.75 GHz then the high-pass
>           filter will be put into bypass mode, otherwise the closest corner
>           frequency that is less than or equal to the difference of rf_in and
>           this value will be used.
>         minimum: 0
>         maximum: 0xFFFFFFFFFFFFFFFF
>         default: 0
> 
> And this is the example:
> 
> examples:
>   - |
>     spi {
>       #address-cells = <1>;
>       #size-cells = <0>;
>       admv8818@0 {
>         compatible = "adi,admv8818";
>         reg = <0>;
>         spi-max-frequency = <10000000>;
>         clocks = <&admv8818_rfin>;
>         clock-names = "rf_in";
>         adi,filter-margins-hz = /bits/ 64 <30000000 30000000>;


foo-hz is in 32-bit, so basically you have here 4 32-bit numbers which
indeed reported by dtschema - property is too long. Drop 64-bit here.

Device allows multiple LPF/HPF values to be stored in LUT tables and it
actually has four independent filters. Shouldn't these be included here?
Maybe not LUT tables, but the configuration for all filters?

Best regards,
Krzysztof