Re: [PATCH v3 1/5] dt-bindings: iio: adc: Add ltc2378

[Marcelo Schmitt]

On 06/17, Conor Dooley wrote:
> On Tue, Jun 16, 2026 at 11:03:11PM -0300, Marcelo Schmitt wrote:
> > Document how to describe LTC2378-20 and similar ADCs in device tree.
> > 
> > Signed-off-by: Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>
> > ---
> > Change log v2 -> v3:
> > - Re-added device tree fallback compatibles for LTC2378 chips, now with options
> >   to provide a single compatible string or a pair of single compatible string
> >   plus a fallback string to a slower sample rate spec in case a driver for the
> >   specific part is not found.
> > 
> >  .../bindings/iio/adc/adi,ltc2378.yaml         | 160 ++++++++++++++++++
> >  MAINTAINERS                                   |   7 +
> >  2 files changed, 167 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/iio/adc/adi,ltc2378.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/iio/adc/adi,ltc2378.yaml b/Documentation/devicetree/bindings/iio/adc/adi,ltc2378.yaml
> > new file mode 100644
> > index 000000000000..7d30a2cade8f
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/iio/adc/adi,ltc2378.yaml
> > @@ -0,0 +1,160 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/iio/adc/adi,ltc2378.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Analog Devices LTC2378 and similar Analog to Digital Converters
> > +
> > +maintainers:
> > +  - Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>
> > +
> > +description: |
> > +  Analog Devices LTC2378 series of ADCs.
> > +  Specifications can be found at:
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/233818fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236416fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236418f.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236716fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236718f.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236816f.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236818f.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/236918fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237016fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237616fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237618fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237620fb.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237716fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237718fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237720fb.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237816fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237818fa.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237820fb.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/237918fb.pdf
> > +    https://www.analog.com/media/en/technical-documentation/data-sheets/238016fb.pdf
> > +
> > +$ref: /schemas/spi/spi-peripheral-props.yaml#
> > +
> > +properties:
> > +  compatible:
> > +    oneOf:
> > +      # Single compatible string match.
> > +      - enum:
> > +          - adi,ltc2338-18
> > +          - adi,ltc2364-16
> > +          - adi,ltc2364-18
> > +          - adi,ltc2367-16
> > +          - adi,ltc2367-18
> > +          - adi,ltc2368-16
> > +          - adi,ltc2368-18
> > +          - adi,ltc2369-18
> > +          - adi,ltc2370-16
> > +          - adi,ltc2376-16
> > +          - adi,ltc2376-18
> > +          - adi,ltc2376-20
> > +          - adi,ltc2377-16
> > +          - adi,ltc2377-18
> > +          - adi,ltc2377-20
> > +          - adi,ltc2378-16
> > +          - adi,ltc2378-18
> > +          - adi,ltc2378-20
> > +          - adi,ltc2379-18
> > +          - adi,ltc2380-16
> > +
> > +      # Low sample rate fallback for 16-bit unipolar sensors.
> > +      - items:
> > +          - enum:
> > +              - adi,ltc2370-16 # 2 MSPS
> > +              - adi,ltc2368-16 # 1 MSPS
> > +              - adi,ltc2367-16 # 500 kSPS
> > +          - const: adi,ltc2364-16 # fallback (250 kSPS)
> 
> Your driver still matches on ltc2370-16, which makes me question the
> value of these fallbacks. That said, the chip info struct contains no
> information about sampling rate.
The info about sample rate is added in patch 4.

> What actually is the impact of the sample rate on the programming model?
If the user tries to set a sample rate beyond the maximum supported, software
can throw an error to indicate that.

> Is there actually a benefit to matching on ltc2370-16, or can you just
> match on the fallback?
The benefit is telling software that it is safe to go up to 2 MSPS. It is also
okay to match on the fallback, but software may restrain operation to a slower
sample rate.

With the code being proposed in this patch set, there is no benefit in matching
the ltc2364-16 fallback if there is a faster ltc2370-16 device connected because
device driver supports that. Though, other operating systems and/or platforms
might not support all device variants and so the fallback might be useful.

> +static const struct ltc2378_chip_info ltc2370_16_chip_info = {
> +	.name = "ltc2370-16",
> +	.resolution = 16,
In a later patch ...
+	.max_sample_rate_hz = 2 * HZ_PER_MHZ,
+	.tconv_ns = 322,
> +	.bipolar = false,
> +};
> 
> +static const struct ltc2378_chip_info ltc2368_16_chip_info = {
> +	.name = "ltc2368-16",
> +	.resolution = 16,
+	.max_sample_rate_hz = HZ_PER_MHZ,
+	.tconv_ns = 527
> +	.bipolar = false,
> +};
> 
> +static const struct ltc2378_chip_info ltc2367_16_chip_info = {
> +	.name = "ltc2367-16",
> +	.resolution = 16,
+	.max_sample_rate_hz = 500 * HZ_PER_KHZ,
+	.tconv_ns = 1500,
> +	.bipolar = false,
> +};
> 
> +static const struct ltc2378_chip_info ltc2364_16_chip_info = {
> +	.name = "ltc2364-16",
> +	.resolution = 16,
+	.max_sample_rate_hz = 250 * HZ_PER_KHZ,
+	.tconv_ns = 3000,
> +	.bipolar = false,
> +};
> 
...
> > +      # Low sample rate fallback for 20-bit bipolar sensors.
> > +      - items:
> > +          - enum:
> > +              - adi,ltc2378-20 # 1 MSPS
> > +              - adi,ltc2377-20 # 500 kSPS
> > +          - const: adi,ltc2376-20 # fallback (250 kSPS)
> 
> I didn't check these, but I assume they are the same.

Yes, except for small variations on input (unipolar/bipolar) and precision bits,
these devices are pretty much equal.