Message-ID: <82f53f02-15d3-43fa-9734-8091b360f4e0@baylibre.com>
Date: Sat, 16 Aug 2025 12:16:44 -0500
From: David Lechner <dlechner@...libre.com>
To: Ben Collins <bcollins@...ter.com>, Jonathan Cameron <jic23@...nel.org>
Cc: Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] iio: mcp9600: Add support for IIR filter

On 8/16/25 10:33 AM, Ben Collins wrote:
> 
>> On Aug 16, 2025, at 11:19 AM, Ben Collins <bcollins@...ter.com> wrote:
>>
>>>
>>> On Aug 16, 2025, at 11:08 AM, Jonathan Cameron <jic23@...nel.org> wrote:
>>>
>>> On Sat, 16 Aug 2025 09:12:37 -0400
>>> Ben Collins <bcollins@...ter.com> wrote:
>>>
>>>>> On Aug 16, 2025, at 5:54 AM, Jonathan Cameron <jic23@...nel.org> wrote:
>>>>>
>>>>> On Wed, 13 Aug 2025 17:52:04 -0500
>>>>> David Lechner <dlechner@...libre.com> wrote:
>>>>>
>>>>>> On 8/13/25 10:15 AM, Ben Collins wrote:  
>>>>>>> MCP9600 supports an IIR filter with 7 levels. Add IIR attribute
>>>>>>> to allow get/set of this value.
>>>>>>>
>>>>>>> Signed-off-by: Ben Collins <bcollins@...ter.com>
>>>>>>> ---
>>>>>>> drivers/iio/temperature/mcp9600.c | 43 +++++++++++++++++++++++++++++++
>>>>>>> 1 file changed, 43 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
>>>>>>> index 5ead565f1bd8c..5bed3a35ae65e 100644
>>>>>>> --- a/drivers/iio/temperature/mcp9600.c
>>>>>>> +++ b/drivers/iio/temperature/mcp9600.c
>>>>>>> @@ -31,6 +31,7 @@
>>>>>>> #define MCP9600_STATUS_ALERT(x) BIT(x)
>>>>>>> #define MCP9600_SENSOR_CFG 0x5
>>>>>>> #define MCP9600_SENSOR_TYPE_MASK GENMASK(6, 4)
>>>>>>> +#define MCP9600_FILTER_MASK GENMASK(2, 0)
>>>>>>> #define MCP9600_ALERT_CFG1 0x8
>>>>>>> #define MCP9600_ALERT_CFG(x) (MCP9600_ALERT_CFG1 + (x - 1))
>>>>>>> #define MCP9600_ALERT_CFG_ENABLE BIT(0)
>>>>>>> @@ -111,6 +112,7 @@ static const struct iio_event_spec mcp9600_events[] = {
>>>>>>> .address = MCP9600_HOT_JUNCTION,        \
>>>>>>> .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |        \
>>>>>>>     BIT(IIO_CHAN_INFO_SCALE) |       \
>>>>>>> +       BIT(IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY) |  \
>>>>>>>     BIT(IIO_CHAN_INFO_THERMOCOUPLE_TYPE), \
>>>>>>> .event_spec = &mcp9600_events[hj_ev_spec_off],        \
>>>>>>> .num_event_specs = hj_num_ev,        \
>>>>>>> @@ -149,6 +151,7 @@ static const struct iio_chan_spec mcp9600_channels[][2] = {
>>>>>>> struct mcp9600_data {
>>>>>>> struct i2c_client *client;
>>>>>>> u32 thermocouple_type;
>>>>>>> + u32 filter_level;
>>>>>>> };
>>>>>>>
>>>>>>> static int mcp9600_read(struct mcp9600_data *data,
>>>>>>> @@ -186,6 +189,9 @@ static int mcp9600_read_raw(struct iio_dev *indio_dev,
>>>>>>> case IIO_CHAN_INFO_THERMOCOUPLE_TYPE:
>>>>>>> *val = mcp9600_tc_types[data->thermocouple_type];
>>>>>>> return IIO_VAL_CHAR;
>>>>>>> + case IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY:
>>>>>>> + *val = data->filter_level;    
>>>>>>
>>>>>> We can't just pass the raw value through for this. The ABI is defined
>>>>>> in Documentation/ABI/testing/sysfs-bus-iio and states that the value
>>>>>> is the frequency in Hz.
>>>>>>
>>>>>> So we need to do the math to convert from the register value to
>>>>>> the required value.
>>>>>>
>>>>>> I'm a bit rusty on my discrete time math, so I had chatgpt help me
>>>>>> do the transform of the function from the datasheet to a transfer
>>>>>> function and use that to find the frequency response.
>>>>>>
>>>>>> It seemed to match what my textbook was telling me, so hopefully
>>>>>> it got it right.
>>>>>>
>>>>>> Then it spit out the following program that can be used to make
>>>>>> a table of 3dB points for a given sampling frequency. If I read the
>>>>>> datasheet right, the sampling frequency depends on the number of
>>>>>> bits being read.
>>>>>>
>>>>>> For example, for 3 Hz sample rate (18-bit samples), I got:
>>>>>>
>>>>>> n  f_3dB (Hz)
>>>>>> 1  0.58774
>>>>>> 2  0.24939
>>>>>> 3  0.12063
>>>>>> 4  0.05984
>>>>>> 5  0.02986
>>>>>> 6  0.01492
>>>>>> 7  0.00746
>>>>>>
>>>>>> I had to skip n=0 though since that is undefined. Not sure how we
>>>>>> handle that since it means no filter. Maybe Jonathan can advise?  
>>>>>
>>>>> This is always a fun corner case.  Reality is there is always
>>>>> some filtering going on due to the analog side of things we
>>>>> just have no idea what it is if the nicely defined filter is
>>>>> turned off.  I can't remember what we have done in the past,
>>>>> but one option would be to just have anything bigger than 0.58774
>>>>> defined as being filter off and return a big number. Not elegant
>>>>> though.  Or just don't bother supporting it if we think no one
>>>>> will ever want to run with not filter at all.
>>>>>
>>>>> Hmm. or given this is a digital filter on a sampled signal, can we establish
>>>>> an effective frequency that could be detected without aliasing and
>>>>> use that?  Not sure - I'm way to rusty on filter theory (and was
>>>>> never that good at it!)  
>>>>
>>>> I’ve seen another driver use { U64_MAX, U64_MAX } for this case. It
>>>> didn’t seem very clean. I thought to use { 999999, 999999 } or even
>>>> { 1, 0 }, but anything other than “off” just felt odd.
>>> Ah.  Could we use filter_type? (additional attribute)
>>>
>>> That already has a 'none' option.  Nothing there yet that works for the 'on'
>>> option here.  These are always tricky to name unless they are a very
>>> well known class of filter.   The datasheet calls this one an Exponential
>>> Moving Average filter. Not a term I'd encountered before, but google did
>>> find me some references.  so maybe ema as a filter type?
>>
>> In the docs I have, it says:
>>
>> In addition, this device integrates a first order recursive
>> Infinite Impulse Response (IIR) filter, also known as
>> Exponential Moving Average (EMA).
>>
>> The EMA formula I’ve used for an adc-attached thermistor was the same
>> formula I’ve seen used in IIR, so I think they are generally the same.
> 
> Clarification: An EMA is a 1-pole IIR filter, while IIR filters can be
> many other types besides 1-pole.
> 

We already have a "sinc5+avg" filter, so I would call this one "avg".

I don't think we need to get too specific. The main point of the names
is that for chips with more than one filter, it is obvious which one
is which. For a chip with only "none" and "avg" is will be obvious
that "avg" turns the filter on.

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