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Message-ID: <2025082614-inventive-gharial-c4a3f9@boujee-and-buff>
Date: Tue, 26 Aug 2025 15:08:46 -0400
From: Ben Collins <bcollins@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>,
Antoniu Miclaus <antoniu.miclaus@...log.com>, Lars-Peter Clausen <lars@...afoo.de>,
Michael Hennerich <Michael.Hennerich@...log.com>, linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 4/5] iio: mcp9600: Add support for IIR filter
On Tue, Aug 26, 2025 at 12:20:39PM -0500, David Lechner wrote:
> On 8/25/25 7:10 PM, Ben Collins wrote:
> > MCP9600 supports an IIR filter with 7 levels. Add IIR attribute
> > to allow get/set of this value.
> >
> > Use filter_type[none, ema] for enabling the IIR filter.
> >
> > Signed-off-by: Ben Collins <bcollins@...nel.org>
> > ---
> > drivers/iio/temperature/mcp9600.c | 147 ++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 147 insertions(+)
> >
> > diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
> > index aa42c2b1a369edbd36e0d6d6d1738ed0069fd990..d3309e30628ae5cdc74378403952ba285990f4c0 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 0x05
> > #define MCP9600_SENSOR_TYPE_MASK GENMASK(6, 4)
> > +#define MCP9600_FILTER_MASK GENMASK(2, 0)
> > #define MCP9600_ALERT_CFG1 0x08
> > #define MCP9600_ALERT_CFG(x) (MCP9600_ALERT_CFG1 + (x - 1))
> > #define MCP9600_ALERT_CFG_ENABLE BIT(0)
> > @@ -94,6 +95,27 @@ static const int mcp9600_tc_types[] = {
> > [THERMOCOUPLE_TYPE_R] = 'R',
> > };
> >
> > +enum mcp9600_filter {
> > + MCP9600_FILTER_TYPE_NONE,
> > + MCP9600_FILTER_TYPE_EMA,
> > +};
> > +
> > +static const char * const mcp9600_filter_type[] = {
> > + [MCP9600_FILTER_TYPE_NONE] = "none",
> > + [MCP9600_FILTER_TYPE_EMA] = "ema",
> > +};
> > +
> > +static const int mcp_iir_coefficients_avail[7][2] = {
> > + /* Level 0 is no filter */
> > + { 0, 524549 },
> > + { 0, 243901 },
> > + { 0, 119994 },
> > + { 0, 59761 },
> > + { 0, 29851 },
> > + { 0, 14922 },
> > + { 0, 7461 },
> > +};
> > +
> > static const struct iio_event_spec mcp9600_events[] = {
> > {
> > .type = IIO_EV_TYPE_THRESH,
> > @@ -119,6 +141,8 @@ struct mcp_chip_info {
> > struct mcp9600_data {
> > struct i2c_client *client;
> > u32 thermocouple_type;
> > + int filter_level;
> > + int filter_enabled;
> > };
> >
> > static int mcp9600_config(struct mcp9600_data *data)
> > @@ -129,6 +153,9 @@ static int mcp9600_config(struct mcp9600_data *data)
> >
> > cfg = FIELD_PREP(MCP9600_SENSOR_TYPE_MASK,
> > mcp9600_type_map[data->thermocouple_type]);
> > + /* The chip understands 0 as "none", and 1-7 as ema filter levels. */
> > + if (data->filter_enabled)
> > + FIELD_MODIFY(MCP9600_FILTER_MASK, &cfg, data->filter_level + 1);
> >
> > ret = i2c_smbus_write_byte_data(client, MCP9600_SENSOR_CFG, cfg);
> > if (ret < 0) {
> > @@ -146,7 +173,11 @@ static int mcp9600_config(struct mcp9600_data *data)
> > .address = MCP9600_HOT_JUNCTION, \
> > .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) | \
> > BIT(IIO_CHAN_INFO_THERMOCOUPLE_TYPE) | \
> > + BIT(IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY) | \
>
> Does the filter actually only apply to the hot junction and not the
> cold junction? There is a mismatch between this being info_mask_separate
> and the filter_type being IIO_SHARED_BY_ALL.
>
> Not related to this patch, but the comment same applies to
> IIO_CHAN_INFO_THERMOCOUPLE_TYPE - I missed that in previous reviews.
The Thermocouple Sensor Configuration register only applies to the
thermocouple (hot-junction), which is what sets the filter and
thermocouple configuration.
More specifically, the filter formula applies to T-delta (thermocouple
hot-junction).
So these are correct.
> > BIT(IIO_CHAN_INFO_SCALE), \
> > + .info_mask_separate_available = \
> > + BIT(IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY), \
> > + .ext_info = mcp9600_ext_filter, \
> > .event_spec = &mcp9600_events[hj_ev_spec_off], \
> > .num_event_specs = hj_num_ev, \
> > }, \
> > @@ -162,6 +193,57 @@ static int mcp9600_config(struct mcp9600_data *data)
> > }, \
> > }
> >
> > +static int mcp9600_get_filter(struct iio_dev *indio_dev,
> > + struct iio_chan_spec const *chan)
> > +{
> > + struct mcp9600_data *data = iio_priv(indio_dev);
> > +
> > + return data->filter_enabled ? MCP9600_FILTER_TYPE_EMA :
> > + MCP9600_FILTER_TYPE_NONE;
> > +}
> > +
> > +static int mcp9600_set_filter(struct iio_dev *indio_dev,
> > + struct iio_chan_spec const *chan,
> > + unsigned int mode)
> > +{
> > + struct mcp9600_data *data = iio_priv(indio_dev);
> > + int new_type;
>
> This variable name is a little confusing. It looks like it should
> rather be:
>
> bool new_filter_enabled;
I can change that.
> > +
> > + switch (mode) {
> > + case MCP9600_FILTER_TYPE_NONE:
> > + new_type = 0;
> > + break;
> > +
> > + case MCP9600_FILTER_TYPE_EMA:
> > + new_type = 1;
> > + break;
> > +
> > + default:
> > + return -EINVAL;
> > + }
> > +
> > + /* Do not reset the filter if we don't need to. */
> > + if (data->filter_enabled == new_type)
> > + return 0;
> > +
> > + data->filter_enabled = new_type;
> > + return mcp9600_config(data);
> > +}
> > +
> > +static const struct iio_enum mcp9600_filter_enum = {
> > + .items = mcp9600_filter_type,
> > + .num_items = ARRAY_SIZE(mcp9600_filter_type),
> > + .get = mcp9600_get_filter,
> > + .set = mcp9600_set_filter,
> > +};
> > +
> > +static const struct iio_chan_spec_ext_info mcp9600_ext_filter[] = {
> > + IIO_ENUM("filter_type", IIO_SHARED_BY_ALL, &mcp9600_filter_enum),
> > + IIO_ENUM_AVAILABLE("filter_type", IIO_SHARED_BY_ALL,
> > + &mcp9600_filter_enum),
> > + { }
> > +};
> > +
I guess I need to make this IIO_SEPARATE/IIO_SHARED_BY_TYPE, but I need
to make sure it's only for the in_temp_raw and not in_temp_ambient_raw.
> > static const struct iio_chan_spec mcp9600_channels[][2] = {
> > MCP9600_CHANNELS(0, 0, 0, 0), /* Alerts: - - - - */
> > MCP9600_CHANNELS(1, 0, 0, 0), /* Alerts: 1 - - - */
> > @@ -216,6 +298,69 @@ 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:
> > + if (!data->filter_enabled)
> > + return IIO_VAL_EMPTY;
>
> This brings us back to the earlier discussion of what should this
> be when the filter is disabled. Mathematically, it would be infinite.
>
> I wonder if it would be reasonable to return "inf" here since many
> floating point parsers will already handle that as a valid value.
I'll defer to the other thread for further discussion on this.
--
Ben Collins
https://libjwt.io
https://github.com/benmcollins
--
3EC9 7598 1672 961A 1139 173A 5D5A 57C7 242B 22CF
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