Re: [PATCH v2 8/9] media: dt-bindings: Add Intel Displayport RX IP

[Rob Herring <robh@kernel.org>]

On Wed, Feb 28, 2024 at 02:09:33PM +0100, Paweł Anikiel wrote:
> On Wed, Feb 28, 2024 at 1:18 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@linaro.org> wrote:
> >
> > On 28/02/2024 12:05, Paweł Anikiel wrote:
> > > On Tue, Feb 27, 2024 at 3:29 PM Rob Herring <robh@kernel.org> wrote:
> > >>
> > >> On Mon, Feb 26, 2024 at 11:59:42AM +0100, Paweł Anikiel wrote:
> > >>> On Mon, Feb 26, 2024 at 10:13 AM Krzysztof Kozlowski
> > >>> <krzysztof.kozlowski@linaro.org> wrote:
> > >>>>
> > >>>> On 21/02/2024 17:02, Paweł Anikiel wrote:
> > >>>>> The Intel Displayport RX IP is a part of the DisplayPort Intel FPGA IP
> > >>>>> Core. It implements a DisplayPort 1.4 receiver capable of HBR3 video
> > >>>>> capture and Multi-Stream Transport. The user guide can be found here:
> > >>>>>
> > >>>>> https://www.intel.com/programmable/technical-pdfs/683273.pdf
> > >>>>>
> > >>>>> Signed-off-by: Paweł Anikiel <panikiel@google.com>
> > >>>>> ---
> > >>>>>  .../devicetree/bindings/media/intel,dprx.yaml | 160 ++++++++++++++++++
> > >>>>>  1 file changed, 160 insertions(+)
> > >>>>>  create mode 100644 Documentation/devicetree/bindings/media/intel,dprx.yaml
> > >>>>>
> > >>>>> diff --git a/Documentation/devicetree/bindings/media/intel,dprx.yaml b/Documentation/devicetree/bindings/media/intel,dprx.yaml
> > >>>>> new file mode 100644
> > >>>>> index 000000000000..31025f2d5dcd
> > >>>>> --- /dev/null
> > >>>>> +++ b/Documentation/devicetree/bindings/media/intel,dprx.yaml
> > >>>>> @@ -0,0 +1,160 @@
> > >>>>> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > >>>>> +%YAML 1.2
> > >>>>> +---
> > >>>>> +$id: http://devicetree.org/schemas/media/intel,dprx.yaml#
> > >>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > >>>>> +
> > >>>>> +title: Intel DisplayPort RX IP
> > >>>>> +
> > >>>>> +maintainers:
> > >>>>> +  - Paweł Anikiel <panikiel@google.com>
> > >>>>> +
> > >>>>> +description: |
> > >>>>> +  The Intel Displayport RX IP is a part of the DisplayPort Intel FPGA IP
> > >>>>> +  Core. It implements a DisplayPort 1.4 receiver capable of HBR3 video
> > >>>>> +  capture and Multi-Stream Transport.
> > >>>>> +
> > >>>>> +  The IP features a large number of configuration parameters, found at:
> > >>>>> +  https://www.intel.com/content/www/us/en/docs/programmable/683273/23-3-20-0-1/sink-parameters.html
> > >>>>> +
> > >>>>> +  The following parameters have to be enabled:
> > >>>>> +    - Support DisplayPort sink
> > >>>>> +    - Enable GPU control
> > >>>>> +  The following parameters' values have to be set in the devicetree:
> > >>>>> +    - RX maximum link rate
> > >>>>> +    - Maximum lane count
> > >>>>> +    - Support MST
> > >>>>> +    - Max stream count (only if Support MST is enabled)
> > >>>>> +
> > >>>>> +properties:
> > >>>>> +  compatible:
> > >>>>> +    const: intel,dprx-20.0.1
> > >>>>> +
> > >>>>> +  reg:
> > >>>>> +    maxItems: 1
> > >>>>> +
> > >>>>> +  interrupts:
> > >>>>> +    maxItems: 1
> > >>>>> +
> > >>>>> +  intel,max-link-rate:
> > >>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
> > >>>>> +    description: Max link rate configuration parameter
> > >>>>
> > >>>> Please do not duplicate property name in description. It's useless.
> > >>>> Instead explain what is this responsible for.
> > >>>>
> > >>>> Why max-link-rate would differ for the same dprx-20.0.1? And why
> > >>>> standard properties cannot be used?
> > >>>>
> > >>>> Same for all questions below.
> > >>>
> > >>> These four properties are the IP configuration parameters mentioned in
> > >>> the device description. When generating the IP core you can set these
> > >>> parameters, which could make them differ for the same dprx-20.0.1.
> > >>> They are documented in the user guide, for which I also put a link in
> > >>> the description. Is that enough? Or should I also document these
> > >>> parameters here?
> > >>
> > >> Use the standard properties: link-frequencies and data-lanes. Those go
> > >> under the port(s) because they are inheritly per logical link.
> > >
> > > The DP receiver has one input interface (a deserialized DP stream),
> > > and up to four output interfaces (the decoded video streams). The "max
> > > link rate" and "max lane count" parameters only describe the input
> > > interface to the receiver. However, the port(s) I am using here are
> > > for the output streams. They are not affected by those parameters, so
> > > I don't think these properties should go under the output port(s).
> > >
> > > The receiver doesn't have an input port in the DT, because there isn't
> > > any controllable entity on the other side - the deserializer doesn't
> > > have any software interface. Since these standard properties
> > > (link-frequencies and data-lanes) are only defined in
> > > video-interfaces.yaml (which IIUC describes a graph endpoint), I can't
> > > use them directly in the device node.
> >
> > DT describes the hardware, so where does the input come? From something,
> > right? Regardless if you have a driver or not. There is dp-connector
> > binding, if this is physical port.
> 
> Yes, it is a physical port. I agree adding a DT node for the physical
> DP input connector would let us add link-frequencies to the input port
> of the receiver.
> 
> However, dp-connector seems to be a binding for an output port - it's
> under schemas/display/connector, and DP_PWR can be a power supply only
> for an output port (looking at the dp-pwr-supply property). Also, the
> driver for this binding is a DRM bridge driver (display-connector.c)
> which would not be compatible with a v4l2 (sub)device.

So then we should add 'dp-input-connector' because they are different. 
When we haven't defined connectors, properties of the connector have 
been shoved in whatever node is associated with a connector like you 
have done. That works for a while, but then becomes unmanageable. DP on 
USB-C connectors for example.

OTOH, maybe your use here is niche enough to not be worth the trouble. 
Depends if we see the need for video input connectors in general.

Rob