linux-kernel - Re: [PATCH 2/2] iio: frequency: add amlogic clock measure support

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Message-ID: <190ccd62-0803-46fa-87ee-0f9cef5a784e@baylibre.com>
Date: Tue, 25 Jun 2024 09:52:30 -0500
From: David Lechner <dlechner@...libre.com>
To: Jerome Brunet <jbrunet@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen
 <lars@...afoo.de>, Neil Armstrong <neil.armstrong@...aro.org>,
 Kevin Hilman <khilman@...libre.com>, linux-kernel@...r.kernel.org,
 linux-amlogic@...ts.infradead.org, linux-iio@...r.kernel.org,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>
Subject: Re: [PATCH 2/2] iio: frequency: add amlogic clock measure support

On 6/25/24 3:31 AM, Jerome Brunet wrote:
> On Mon 24 Jun 2024 at 17:51, David Lechner <dlechner@...libre.com> wrote:
> 
>> On 6/24/24 12:31 PM, Jerome Brunet wrote:
>>> Add support for the HW found in most Amlogic SoC dedicated to measure
>>> system clocks.
>>>
>>
>>
>>
>>> +static int cmsr_read_raw(struct iio_dev *indio_dev,
>>> +			 struct iio_chan_spec const *chan,
>>> +			 int *val, int *val2, long mask)
>>> +{
>>> +	struct amlogic_cmsr *cm = iio_priv(indio_dev);
>>> +
>>> +	guard(mutex)(&cm->lock);
>>> +
>>> +	switch (mask) {
>>> +	case IIO_CHAN_INFO_RAW:
>>> +		*val = cmsr_measure_unlocked(cm, chan->channel);
>>
>> Is this actually returning an alternating voltage magnitutde?
>> Most frequency drivers don't have a raw value, only frequency.
> 
> No it is not the magnitude, it is the clock rate (frequency) indeed.
> Maybe altvoltage was not the right pick for that but nothing obvious
> stands out for Hz measurements

I'm certainly not an expert on the subject, but looking at the other
frequency drivers, using altvoltage looks correct.

But, we in those drivers, nearly all only have a "frequency" attribute
but don't have a "raw" attribute. The ones that do have a "raw" attribute
are frequency generators that use the raw attribute determine the output
voltage.

> 
>>
>>> +		if (*val < 0)
>>> +			return *val;
>>> +		return IIO_VAL_INT;
>>> +
>>> +	case IIO_CHAN_INFO_PROCESSED: /* Result in Hz */
>>
>> Shouldn't this be IIO_CHAN_INFO_FREQUENCY?
> 
> How would I get raw / processed / scale with IIO_CHAN_INFO_FREQUENCY ?
> 
>>
>> Processed is just (raw + offset) * scale which would be a voltage
>> in this case since the channel type is IIO_ALTVOLTAGE.
> 
> This is was Processed does here, along with selecting the most
> appropriate scale to perform the measurement.
> 
>>
>>> +		*val = cmsr_measure_processed_unlocked(cm, chan->channel, val2);
>>> +		if (*val < 0)
>>> +			return *val;
>>> +		return IIO_VAL_INT_64;
>>> +
>>> +	case IIO_CHAN_INFO_SCALE:
>>
>> What is this attribute being used for?
> 
> Hz
> 
>>
>> (clearly not used to convert the raw value to millivolts :-) )
>>
>> Maybe IIO_CHAN_INFO_INT_TIME is the right one for this? Although
>> so far, that has only been used with light sensors.
> 
> I think you are mixing up channel info and type here.
> I do want the info
>  * IIO_CHAN_INFO_RAW
>  * IIO_CHAN_INFO_PROCESSED
>  * IIO_CHAN_INFO_SCALE
> 
> I want those info to represent an alternate voltage frequency in Hz.
> I thought type 'IIO_ALTVOLTAGE' was the right pick for that. Apparently
> it is not. What is the appropriate type then ? Should I add a new one ?

The documentation at Documentation/ABI/testing/sysfs-bus-iio explains
what the combination of a channel type and info means.

For example, out_altvoltageY_raw is defined as it used for the frequency
generator case that I mentioned above. in_altvoltageY_raw is not defined
which means probably no one has needed it yet. But it would still be the
voltage value, not the frequency.

There also isn't a in_altvoltageY_input defined ("_input" is the sysfs
name for IIO_CHAN_INFO_PROCESSED). But in general, "input" (processed) in
IIO just means the value in standard units instead of a raw value. It
doesn't mean that any extra signal processing was done. And "scale" is
just a way to convert "raw" to "input" (processed). So channels will either
have "raw" and "scale" or only have "input" (processed), but not both.

Could you describe at a higher level what the use case for the various
values being exposed here are? I think that would be helpful in figuring
out where they actually fit in to the standard IIO attributes.

> 
>>
>>> +		*val2 = cmsr_get_time_unlocked(cm);
>>> +		*val = 1000000;
>>> +		return IIO_VAL_FRACTIONAL;
>>> +
>>> +	default:
>>> +		return -EINVAL;
>>> +	}
>>> +}
>>> +
>