Re: [PATCH v3 04/10] dt-bindings: iio: dac: ad3552r: add io-backend support

["Nuno Sá" <noname.nuno@gmail.com>]

On Tue, 2024-09-24 at 10:02 +0200, Krzysztof Kozlowski wrote:
> On 23/09/2024 17:50, Angelo Dureghello wrote:
> > Hi Krzysztof,
> > 
> > On 22/09/24 23:02, Krzysztof Kozlowski wrote:
> > > On Thu, Sep 19, 2024 at 11:20:00AM +0200, Angelo Dureghello wrote:
> > > > From: Angelo Dureghello <adureghello@baylibre.com>
> > > > 
> > > > There is a version AXI DAC IP block (for FPGAs) that provides
> > > > a physical bus for AD3552R and similar chips, and acts as
> > > > an SPI controller.
> > > > 
> > > > For this case, the binding is modified to include some
> > > > additional properties.
> > > > 
> > > > Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
> > > > ---
> > > >   .../devicetree/bindings/iio/dac/adi,ad3552r.yaml   | 42
> > > > ++++++++++++++++++++++
> > > >   1 file changed, 42 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/iio/dac/adi,ad3552r.yaml
> > > > b/Documentation/devicetree/bindings/iio/dac/adi,ad3552r.yaml
> > > > index 41fe00034742..aca4a41c2633 100644
> > > > --- a/Documentation/devicetree/bindings/iio/dac/adi,ad3552r.yaml
> > > > +++ b/Documentation/devicetree/bindings/iio/dac/adi,ad3552r.yaml
> > > > @@ -60,6 +60,18 @@ properties:
> > > >       $ref: /schemas/types.yaml#/definitions/uint32
> > > >       enum: [0, 1, 2, 3]
> > > >   
> > > > +  io-backends:
> > > > +    description: The iio backend reference.
> > > > +      An example backend can be found at
> > > > +       
> > > > https://analogdevicesinc.github.io/hdl/library/axi_ad3552r/index.html
> > > > +    maxItems: 1
> > > > +
> > > > +  adi,synchronous-mode:
> > > > +    description: Enable waiting for external synchronization signal.
> > > > +      Some AXI IP configuration can implement a dual-IP layout, with
> > > > internal
> > > > +      wirings for streaming synchronization.
> > > > +    type: boolean
> > > > +
> > > >     '#address-cells':
> > > >       const: 1
> > > >   
> > > > @@ -128,6 +140,7 @@ patternProperties:
> > > >             - custom-output-range-config
> > > >   
> > > >   allOf:
> > > > +  - $ref: /schemas/spi/spi-peripheral-props.yaml#
> > > >     - if:
> > > >         properties:
> > > >           compatible:
> > > > @@ -238,4 +251,33 @@ examples:
> > > >               };
> > > >           };
> > > >       };
> > > > +
> > > > +  - |
> > > > +    axi_dac: spi@44a70000 {
> > > > +        compatible = "adi,axi-ad3552r";
> > > That is either redundant or entire example should go to the parent node,
> > > if this device is fixed child of complex device (IOW, adi,ad3552r cannot
> > > be used outside of adi,axi-ad3552r).
> > 
> > ad3552r can still be used by a generic "classic" spi
> > controller (SCLK/CS/MISO) but at a slower samplerate, fpga
> > controller only (axi-ad3552r) can reach 33MUPS.
> 
> OK, then this is just redundant. Drop the node. Parent example should
> contain the children, though.
> > 
> > > 
> > > > +        reg = <0x44a70000 0x1000>;
> > > > +        dmas = <&dac_tx_dma 0>;
> > > > +        dma-names = "tx";
> > > > +        #io-backend-cells = <0>;
> > > > +        clocks = <&ref_clk>;
> > > > +
> > > > +        #address-cells = <1>;
> > > > +        #size-cells = <0>;
> > > > +
> > > > +        dac@0 {
> > > > +            compatible = "adi,ad3552r";
> > > > +            reg = <0>;
> > > > +            reset-gpios = <&gpio0 92 0>;
> > > Use standard defines for GPIO flags.
> > 
> > fixed, thanks
> > 
> > > > +            io-backends = <&axi_dac>;
> > > Why do you need to point to the parent? How much coupled are these
> > > devices? Child pointing to parent is not usually expected, because
> > > that's obvious.
> > 
> > 
> > "io-backends" is actually the way to refer to the backend module,
> > (used already for i.e. ad9739a),
> > it is needed because the backend is not only acting as spi-controller,
> > but is also providing some APIs for synchronization and bus setup support.
> 
> 
> But if backend is the parent, then this is redundant. You can take it
> from the child-parent relationship. Is this pointing to other devices
> (non-parent) in other ad3552r configurations?
> 

The backend is a provider-consumer type of API. On the consumer side (which is the
driver the child node will probe on), we need to call devm_iio_backend_get() to get
the backend object (which obviously is the parent). For that, 'io-backends' is being
used. We do have another API called __devm_iio_backend_get_from_fwnode_lookup() that
could be used with the parent fwnode and should work. However that was only added to
keep backward compatibility in the first user of the IIO backend framework and it's
not really meant to be used again. We are aware this is awkward at the very least [1]
but hopefully still acceptable.

[1]: https://lore.kernel.org/linux-iio/20240903203935.358a1423@jic23-huawei/
- Nuno Sá