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

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Message-ID: <b3fe348b-60a3-4480-8a8e-89760c5bb7ae@ideasonboard.com>
Date: Wed, 28 Aug 2024 15:14:47 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: 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

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".

>>
>> 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...

  Tomi