Re: [PATCH 1/3] dt-bindings: iio: adc: add bindings for stm32 mdf filter

[Conor Dooley <conor@kernel.org>]

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On Wed, Oct 07, 2026 at 11:01:28AM +0200, Olivier MOYSAN wrote:
> Hi Conor,
> 
> Thanks for the review
> 
> On 10/1/26 20:32, Conor Dooley wrote:
> > On Thu, Oct 01, 2026 at 04:56:45PM +0200, Olivier Moysan wrote:
> > > Add bindings that describes STM32 MDF settings to support
> > > digital filtering for Pulse Density Modulation (PDM) microphones
> > > and analog sigma delta modulators.
> > > 
> > > Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
> > > ---
> > >   .../bindings/iio/adc/st,stm32-mdf-adc.yaml    | 383 ++++++++++++++++++
> > >   1 file changed, 383 insertions(+)
> > >   create mode 100644 Documentation/devicetree/bindings/iio/adc/st,stm32-mdf-adc.yaml
> > > 
> > > diff --git a/Documentation/devicetree/bindings/iio/adc/st,stm32-mdf-adc.yaml b/Documentation/devicetree/bindings/iio/adc/st,stm32-mdf-adc.yaml
> > > new file mode 100644
> > > index 000000000000..f2fbc3e150e8
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/iio/adc/st,stm32-mdf-adc.yaml
> > > @@ -0,0 +1,383 @@
> > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/iio/adc/st,stm32-mdf-adc.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: STMicroelectronics STM32 Multi-function Digital Filter (MDF) ADC
> > > +
> > > +maintainers:
> > > +  - Olivier Moysan <olivier.moysan@foss.st.com>
> > > +
> > > +description: |
> > > +  STM32 MDF ADC is a sigma delta analog-to-digital converter dedicated to
> > > +  interface external sigma delta modulators to STM32 micro controllers.
> > > +
> > > +properties:
> > > +  compatible:
> > > +    enum:
> > > +      - st,stm32mp25-mdf
> > > +      - st,stm32mp23-mdf
> > > +
> > > +  reg:
> > > +    minItems: 1
> > > +    maxItems: 2
> > 
> > This needs an items list here. The size of the regions seems like crap
> > to begin with...
> > 
> > > +
> > > +  clocks:
> > > +    maxItems: 1
> > > +
> > > +  clock-names:
> > > +    description: Internal clock used for MDF digital processing.
> > > +    items:
> > > +      - const: ker_ck
> > 
> > This is pointless when you only have one.
> > 
> 
> I agree that it could be dropped. However, it is useful for using
> devm_regmap_init_mmio_clk.

Ah, because that function if you pass NULL to it, it doesn't mean no
string, it means no clock.

> 
> > > +
> > > +  "#clock-cells":
> > > +    enum: [0, 1]
> > 
> > Why is this not fixed? Also why are parts of your own device consuming
> > the clocks?

Reading the docs, this has to be 1, you have two clocks?

Not sure why you skipped this and skipped explaining why you are
consuming your own clocks.

> > > +  clock-output-names:
> > > +    description: |
> > > +      CCK0 and CCK1 are optional output clocks, which share the same clock frequency,
> > > +      but can be gated independently to save power.
> > > +    minItems: 1
> > > +    maxItems: 2
> > > +    oneOf:
> > > +      - items:
> > > +          - const: cck0
> > > +      - items:
> > > +          - const: cck1
> > > +      - items:
> > > +          - const: cck0
> > > +          - const: cck1
> > > +
> > > +  clock-frequency:
> > > +    description: |
> > > +      Common clock frequency (Hz) for CCK0 and CCK1 output clocks.
> > > +      The frequency must be a multiple of the "ker_ck" clock frequency.
> > > +    maximum: 25000000
> > 
> > Should not be needed, the consumers request what they need.
> > 
> 
> The CCKx clock frequency depends on the maximum rate supported by the sigma
> delta converters (for instance a digital mic) and the expected decimation
> ratio on the bitstream. Typically this determines the frequency on the SPI
> bus.
> This rate is defined statically and shared by the CCKx clocks. So IMHO, as
> this rate is unique, it can look strange to let the consumer define it.
> Moreover, it seems to me that clock-frequency is already used to configure
> the frequency of a provider in some other bindings.
> For instance: Documentation/devicetree/bindings/clock/silabs,si570.yaml

This is a binding from the age of antiquity, using it to justify your
use is actually harming your argument rather than helping. Your clock
consumers should be providing the information about what the rate should
be, and if you need to do this in dt you can use assigned-clock-rates.

> So, it's not clear for me, what is the restriction on clock-frequency
> property.
> > > +
> > > +  ranges: true
> > > +
> > > +  clock-ranges: true
> > > +
> > > +  resets:
> > > +    maxItems: 1
> > > +
> > > +  reset-names:
> > > +    items:
> > > +      - const: mdf

Same applies here as to the clock names fwiw.

> > > +
> > > +  access-controllers:
> > > +    $ref: /schemas/types.yaml#/definitions/phandle-array
> > > +    description: |
> > > +      Phandle to the rifsc device to check access right.
> > > +
> > > +  power-domains:
> > > +    maxItems: 1
> > > +
> > > +  st,interleave:
> > > +    description: |
> > > +      List of phandles of interleaved filters. The indexes of interleaved filters must be
> > > +      consecutives starting from 0 (i.e in range [0..N]). The samples from interleaved filters
> > > +      are muxed in a single channel and retrieved through the device associated to the filter 0.
> > > +      The filters 1..N have to be enabled, but inherit their configuration from filter 0.
> > > +    $ref: /schemas/types.yaml#/definitions/phandle-array
> > 
> > No idea what these even are, but this is probably not the right way to
> > represent the relationship between devices. They're apparently ADCs, but
> > this is also an ADC so I'm not sure what's going on here at all.

Yeah, after more thought this should not be done like this at all,
especially if these filters the phandle point to are part of this device
itself, which I suspect they are. If that's the case, you don't need
phandles to even do this, you just need an int that says what N is.

> > 
> > > +
> > > +required:
> > > +  - compatible
> > > +  - reg
> > > +  - ranges
> > > +  - clocks
> > > +  - clock-names
> > > +  - clock-ranges
> > > +  - "#address-cells"
> > > +  - "#size-cells"
> > > +
> > > +additionalProperties: false
> > > +
> > > +patternProperties:
> > > +  "^sitf@[0-9]+$":
> > > +    type: object
> > > +    description: Serial interface child node
> > 
> > Why is this a child node at all?
> > Probably not worth reviewing more without a link to the docs for this
> > device so I can figure out what on earth is going on!
> > 
> 
> The MDF is a digital filter for sigma-delta bitstreams, rather than the
> analog-to-digital converter itself.
> 
> Here is the link to STM32MP25 reference manuel
> https://www.st.com/resource/en/reference_manual/rm0457-stm32mp2325xx-advanced-armbased-3264bit-mpus-stmicroelectronics.pdf
> Chapter 45: Multi-function digital filter (MDF)
> 
> On STMP32MP25 SoC, the MDF provides 8 serial interfaces (SITF) and 8 digital
> filters (DFLT) which can be connected through a multiplexer.
> 
> Below is a schematic view of typical applications targeted by this
> peripheral
> 
> 
> Audio use case:
> 
> 		mdf
>             +-------------------------------------+
> +------+    | +------+    +------+    +------+    |
> | dmic | -> | | sitf | -> | mux  | -> | dflt | -- | -> audio device
> +------+    | +------+    +------+    +------+    |
>             +-------------------------------------+
> 
> For instance, in an audio use case the serial interface can be configured to
> provide the clock to the digital microphone (cck0, cck1 or both) at a common
> predefined rate, and to retrieve the PDM samples.
> The serial interface provides this samples to a digital filter, which
> delivers PCM samples on its output.
> 
> 
> Analog use case:
> 
> 		mdf
>               +-------------------------------------+
> +--------+    | +------+    +------+    +------+    |
> | sd adc | -> | | sitf | -> | mux  | -> | dflt | -- | -> iio device
> +--------+    | +------+    +------+    +------+    |
>               +-------------------------------------+


So, looking at the docs and a brief chat with Jonathan about this
device, I believe you've got things backwards and this is actually the backend,
and the adc should be the one populating the io-backends property
pointing to this device. You should have io-backend-cells, and I assume
that there should be a cell for the adc to select which dlft it is connected
to.

I think all of these child nodes should just get deleted - all your
devices here seem completely fake, since you've got stuff that's only a
single word wide. With that, you could also throw away these mickey
mouse nodes with sound-dai-cels of 0, and use 1 instead, with that cell
again determining which dlft is in use.

The only child node I can really thing of any justification for here is
channels - albeit not adc channels since this is not an adc - so that
you can individually configure a dlft using those channel nodes.

These serial inputs, what is the source for the data that is ever
actually sent there? I am assuming it's effectively another IIO device,
just not the onboard adc? Are the outputs exposed on pins on the
package?

I would encourage you to remodel this system, where this device is the
backend and also try to get rid of as much of the consuming your own
clock as possible. Certainly consuming ker_clk should never be required.
It's hard to reason about what should happen with the serial interface
clocks, when I have no idea what the input/data source device actually
is in those cases. Being a clock controller does make sense, because the
device exposes CCK0/1 to the outside world, it's the internal routing to
the SITF blocks that I am not sure how to deal with. A ouroboros device
that is consuming a clock it provides is not really a done thing,
hopefully the IIO guys can tell you how these kinds of things are
typically done here, since I'm sure there's plenty of other devices
that have some sort of ability to change the clock source for a channel.

Probably you need to add two optional input clocks to this device, since
mdf_cck0/1 can be inputs too?

Let me know if any of this feedback is intractable stuff, and maybe hang
on for someone like David or Jonathan to provide some review before
going about a significant rework. It's worth bearing in mind that I am a
binding reviewer and not a domain expert in ADCs etc.

Thanks,
Conor.

> > > +
> > > +    properties:
> > > +      compatible:
> > > +        enum:
> > > +          - st,stm32mp25-sitf-mdf
> > > +
> > > +      reg:
> > > +        description: Specify the SITF serial interface instance
> > > +        maxItems: 1
> > > +
> > > +      clocks:
> > > +        description: |
> > > +          Serial interface clock (optional depending on interface mode)
> > > +        maxItems: 1
> > > +
> > > +      st,sitf-mode:
> > > +        description: |
> > > +          Select serial interface protocol
> > > +          - spi: SPI mode
> > > +          - lf_spi: low frequency SPI mode for low power applications
> > > +        $ref: /schemas/types.yaml#/definitions/string
> > > +        enum:
> > > +          - spi
> > > +          - lf_spi
> > > +
> > > +    required:
> > > +      - reg
> > > +      - st,sitf-mode
> > > +
> > > +    additionalProperties: false
> > > +
> > > +  "^filter@[0-9]+$":
> > > +    type: object
> > > +    description: Digital filter path child node
> > > +
> > > +    properties:
> > > +      compatible:
> > > +        enum:
> > > +          - st,stm32mp25-mdf-dmic
> > > +          - st,stm32mp25-mdf-adc
> > > +
> > > +      reg:
> > > +        description: Specify the MDF filter instance
> > > +        maxItems: 1
> > > +
> > > +      interrupts:
> > > +        maxItems: 1
> > > +
> > > +      clocks:
> > > +        minItems: 1
> > > +        description: Internal clock used for MDF digital processing and control blocks.
> > > +
> > > +      clock-names:
> > > +        items:
> > > +          - const: ker_ck
> > > +
> > > +      dmas:
> > > +        maxItems: 1
> > > +
> > > +      dma-names:
> > > +        items:
> > > +          - const: rx
> > > +
> > > +      "#io-channel-cells":
> > > +        const: 1
> > > +
> > > +      '#address-cells':
> > > +        const: 1
> > > +
> > > +      '#size-cells':
> > > +        const: 0
> > > +
> > > +      st,cic-mode:
> > > +        description: |
> > > +          Cascaded-integrator-comb (CIC) filter configuration
> > > +          - 0: MCIC & ACIC filters in FastSinc mode
> > > +          - [1-3]: MCIC & ACIC filters in Sinc mode order 1 to 3
> > > +          - [4-5]: Single CIC filter in Sinc mode order 4 to 5
> > > +          For audio purpose it is recommended to use CIC Sinc4 or Sinc5
> > > +          This property is mandatory for filter 0 or filters not used in interleave mode.
> > > +        $ref: /schemas/types.yaml#/definitions/uint32
> > > +        minimum: 0
> > > +        maximum: 5
> > > +
> > > +      st,delay:
> > > +        description: Filter delay in samples
> > > +        $ref: /schemas/types.yaml#/definitions/uint32
> > > +        maximum: 127
> > > +
> > > +      st,rs-filter-bypass:
> > > +        description: Bypass RSFLT reshaping filter.
> > > +        $ref: /schemas/types.yaml#/definitions/flag
> > > +
> > > +      st,hpf-filter-cutoff-bp:
> > > +        description: |
> > > +          High Pass Filter (HPF) cut-off frequency expressed as a fraction of the PCM sampling rate.
> > > +          Cut-off frequency = st,hpf-filter-cutoff-bp x Fpcm / 10000.
> > > +          If this property is not defined the HPF is disabled.
> > > +        enum: [625, 1250, 2500, 9500]
> > > +
> > > +      st,sync:
> > > +        description:
> > > +          Synchronize to another filter.
> > > +          Must contain the phandle of the filter providing the synchronization.
> > > +        allOf:
> > > +          - $ref: /schemas/types.yaml#/definitions/phandle-array
> > > +          - maxItems: 1
> > > +
> > > +      st,sitf:
> > > +        $ref: /schemas/types.yaml#/definitions/phandle-array
> > > +        items:
> > > +          - items:
> > > +              - description: Phandle of the serial interface connected to the digital filter
> > > +              - description: |
> > > +                  The phandle's argument selects the bitstream on the falling or rising edge
> > > +                  of the serial interface clock:
> > > +                  - 0: rising edge
> > > +                  - 1: falling edge
> > > +                enum: [0, 1]
> > > +                default: 0
> > > +        description:
> > > +          Should be phandle/bitstream pair.
> > > +
> > > +    required:
> > > +      - compatible
> > > +      - reg
> > > +      - interrupts
> > > +      - dmas
> > > +      - dma-names
> > > +      - "#io-channel-cells"
> > > +      - "#address-cells"
> > > +      - "#size-cells"
> > > +      - st,sitf
> > > +
> > > +    unevaluatedProperties: false
> > > +
> > > +    patternProperties:
> > > +      "^channel@([0-7])$":
> > > +        type: object
> > > +        $ref: adc.yaml
> > > +        description: Represents the external channel which is connected to the MDF.
> > > +
> > > +        properties:
> > > +          reg:
> > > +            maximum: 7
> > > +
> > > +          io-backends:
> > > +            description:
> > > +              Used to pipe external sigma delta modulator or internal ADC backend to MDF
> > > +              channel.
> > > +            maxItems: 1

> > > +
> > > +        required:
> > > +          - reg
> > > +
> > > +        unevaluatedProperties: false
> > > +
> > > +    allOf:
> > > +      - if:
> > > +          properties:
> > > +            compatible:
> > > +              contains:
> > > +                const: st,stm32mp25-mdf-adc
> > > +
> > > +        then:
> > > +          patternProperties:
> > > +            "^channel@[0-7]$":
> > > +              required:
> > > +                - io-backends
> > > +
> > > +      - if:
> > > +          properties:
> > > +            compatible:
> > > +              contains:
> > > +                const: st,stm32mp25-mdf-dmic
> > > +
> > > +        then:
> > > +          patternProperties:
> > > +            "^mdf-dai+$":
> > > +              type: object
> > > +              description: child node
> > > +
> > > +              properties:
> > > +                compatible:
> > > +                  enum:
> > > +                    - st,stm32mp25-mdf-dai
> > > +
> > > +                "#sound-dai-cells":
> > > +                  const: 0
> > > +
> > > +                io-channels:
> > > +                  description:
> > > +                    From common IIO binding. Used to pipe external sigma delta
> > > +                    modulator or internal ADC output to MDF channel.
> > > +
> > > +                power-domains:
> > > +                  maxItems: 1
> > > +
> > > +                port:
> > > +                  $ref: /schemas/sound/audio-graph-port.yaml#
> > > +                  unevaluatedProperties: false
> > > +
> > > +              required:
> > > +                - compatible
> > > +                - "#sound-dai-cells"
> > > +                - io-channels
> > > +
> > > +              additionalProperties: false
> > > +
> > > +examples:
> > > +  - |
> > > +    #include <dt-bindings/clock/st,stm32mp25-rcc.h>
> > > +    #include <dt-bindings/interrupt-controller/arm-gic.h>
> > > +    mdf1: mdf@504d0000 {
> > > +      compatible = "st,stm32mp25-mdf";
> > > +      ranges = <0 0x504d0000 0x1000>;
> > > +      reg = <0x504d0000 0x8>, <0x504d0ff0 0x10>;
> > > +      #address-cells = <1>;
> > > +      #size-cells = <1>;
> > > +      clocks = <&rcc CK_KER_MDF1>;
> > > +      clock-names = "ker_ck";
> > > +      clock-ranges;
> > > +      #clock-cells = <1>;
> > > +      clock-output-names = "cck0", "cck1";
> > > +      clock-frequency = <2048000>;
> > > +
> > > +      sitf5: sitf@300 {
> > > +        compatible = "st,stm32mp25-sitf-mdf";
> > > +        reg = <0x300 0x4>;
> > > +        st,sitf-mode = "spi";
> > > +        clocks = <&mdf1 0>;
> > > +      };
> > > +
> > > +      filter0: filter@84 {
> > > +        compatible = "st,stm32mp25-mdf-dmic";
> > > +        reg = <0x84 0x70>;
> > > +        #io-channel-cells = <1>;
> > > +        interrupts = <GIC_SPI 184 IRQ_TYPE_LEVEL_HIGH>;
> > > +        dmas = <&hpdma 63 0x63 0x12 0>;
> > > +        dma-names = "rx";
> > > +        st,cic-mode = <5>;
> > > +        st,sitf = <&sitf5 0>;
> > > +        #address-cells = <1>;
> > > +        #size-cells = <0>;
> > > +
> > > +        channel@0 {
> > > +          reg = <0>;
> > > +        };
> > > +
> > > +        asoc_pdm0: mdf-dai {
> > > +          compatible = "st,stm32mp25-mdf-dai";
> > > +          #sound-dai-cells = <0>;
> > > +          io-channels = <&filter0 0>;
> > > +        };
> > > +      };
> > > +
> > > +      filter1: filter@104  {
> > > +        compatible = "st,stm32mp25-mdf-adc";
> > > +        reg = <0x104 0x70>;
> > > +        #io-channel-cells = <1>;
> > > +        interrupts = <GIC_SPI 185 IRQ_TYPE_LEVEL_HIGH>;
> > > +        dmas = <&hpdma 64 0x63 0x12 0>;
> > > +        dma-names = "rx";
> > > +        st,cic-mode = <2>;
> > > +        st,sitf = <&sitf5 1>;
> > > +        #address-cells = <1>;
> > > +        #size-cells = <0>;
> > > +
> > > +        channel@1 {
> > > +          reg = <1>;
> > > +          settling-time-us = <1000>;
> > > +          io-backends = <&sd_adc1>;
> > > +        };
> > > +      };
> > > +    };
> > > +
> > > +...
> > > -- 
> > > 2.43.0
> > > 
> 

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