linux-kernel - Re: [PATCH v2 2/4] dt-bindings: media: Add bindings for raspberrypi,rp1-cfe

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Message-ID: <Zs8YXVQ4armJJN4X@kekkonen.localdomain>
Date: Wed, 28 Aug 2024 12:30:21 +0000
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Naushir Patuck <naush@...pberrypi.com>,
	Jacopo Mondi <jacopo.mondi@...asonboard.com>,
	Kieran Bingham <kieran.bingham@...asonboard.com>
Subject: Re: [PATCH v2 2/4] dt-bindings: media: Add bindings for
 raspberrypi,rp1-cfe

Heippa,

On Wed, Aug 28, 2024 at 03:14:47PM +0300, Tomi Valkeinen wrote:
> Hi,
> 
> On 28/08/2024 14:12, Laurent Pinchart wrote:
> > On Wed, Aug 28, 2024 at 11:06:23AM +0000, Sakari Ailus wrote:
> > > On Thu, Jun 20, 2024 at 02:07:51PM +0300, Tomi Valkeinen wrote:
> > > > Add DT bindings for raspberrypi,rp1-cfe.
> > > > 
> > > > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> > > > ---
> > > >   .../bindings/media/raspberrypi,rp1-cfe.yaml        | 98 ++++++++++++++++++++++
> > > >   1 file changed, 98 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/media/raspberrypi,rp1-cfe.yaml b/Documentation/devicetree/bindings/media/raspberrypi,rp1-cfe.yaml
> > > > new file mode 100644
> > > > index 000000000000..851533de2305
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/media/raspberrypi,rp1-cfe.yaml
> > > > @@ -0,0 +1,98 @@
> > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > +%YAML 1.2
> > > > +---
> > > > +$id: http://devicetree.org/schemas/media/raspberrypi,rp1-cfe.yaml#
> > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > +
> > > > +title: Raspberry Pi PiSP Camera Front End
> > > > +
> > > > +maintainers:
> > > > +  - Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> > > > +  - Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>
> > > > +
> > > > +description: |
> > > > +  The Raspberry Pi PiSP Camera Front End is a module in Raspberrypi 5's RP1 I/O
> > > > +  controller, that contains:
> > > > +  - MIPI D-PHY
> > > > +  - MIPI CSI-2 receiver
> > > > +  - Simple image processor (called PiSP Front End, or FE)
> > > > +
> > > > +  The FE documentation is available at:
> > > > +  https://datasheets.raspberrypi.com/camera/raspberry-pi-image-signal-processor-specification.pdf
> > > > +
> > > > +  The PHY and CSI-2 receiver part have no public documentation.
> > > > +
> > > > +properties:
> > > > +  compatible:
> > > > +    items:
> > > > +      - const: raspberrypi,rp1-cfe
> > > > +
> > > > +  reg:
> > > > +    items:
> > > > +      - description: CSI-2 registers
> > > > +      - description: D-PHY registers
> > > > +      - description: MIPI CFG (a simple top-level mux) registers
> > > > +      - description: FE registers
> > > > +
> > > > +  interrupts:
> > > > +    maxItems: 1
> > > > +
> > > > +  clocks:
> > > > +    maxItems: 1
> > > > +
> > > > +  port:
> > > > +    $ref: /schemas/graph.yaml#/$defs/port-base
> > > > +    additionalProperties: false
> > > > +    description: CSI-2 RX Port
> > > > +
> > > > +    properties:
> > > > +      endpoint:
> > > > +        $ref: video-interfaces.yaml#
> > > > +        unevaluatedProperties: false
> > > > +
> > > > +        properties:
> > > > +          data-lanes:
> > > > +            minItems: 1
> > > > +            maxItems: 4
> > > > +
> > > > +          clock-lanes:
> > > > +            maxItems: 1
> > > 
> > > minItems needs to be 1 as well.
> 
> Hmm, I see a lot of
> 
> clock-lanes:
>   maxItems: 1
> 
> in the device tree bindings. And
> https://docs.kernel.org/devicetree/bindings/writing-schema.html says "Cases
> that have only a single entry just need to express that with maxItems".

Fair enough.

> 
> > > 
> > > Or... is this actually configurable in hardware?
> > 
> > Looking at the driver, lane reordering is not supported, so we could
> > drop this property. If the hardware is found to support it later, it can
> > easily be added back without any backward compatibility issue.
> 
> Re-ordering is not supported. I guess clock-lanes can be dropped, although I
> feel that if we have the clock lane in the hardware, and the numbering of
> data-lanes must take that into account, then:
> 
> clock-lanes = <0>;
> data-lanes = <1 2>;
> 
> looks better than:
> 
> data-lanes = <1 2>; /* and implicit clk lane 0 */
> 
> But I can't think of any practical benefit it brings...

The clock-lanes property is optional, it only provides any information if
the clock lane is configurable. Please drop it.

-- 
Terveisin,

Sakari Ailus