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Date:   Tue, 8 Feb 2022 03:36:34 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Alexander Stein <alexander.stein@ew.tq-group.com>
Cc:     jeanmichel.hautbois@ideasonboard.com,
        linux-arm-kernel@lists.infradead.org,
        dave.stevenson@raspberrypi.com, devicetree@vger.kernel.org,
        kernel-list@raspberrypi.com, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linux-rpi-kernel@lists.infradead.org,
        lukasz@jany.st, mchehab@kernel.org, naush@raspberrypi.com,
        robh@kernel.org, tomi.valkeinen@ideasonboard.com,
        nsaenz@kernel.org, bcm-kernel-feedback-list@broadcom.com
Subject: Re: (EXT) Re: (EXT) [RFC PATCH v4 03/12] dt-bindings: media: Add
 bindings for bcm2835-unicam
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Hi Alexander,

On Mon, Feb 07, 2022 at 07:30:25AM +0100, Alexander Stein wrote:
> Am Samstag, 5. Februar 2022, 03:22:51 CET schrieb Laurent Pinchart:
> > On Fri, Feb 04, 2022 at 09:50:06AM +0100, Alexander Stein wrote:
> > > Am Donnerstag, 3. Februar 2022, 18:50:00 CET schrieb Jean-Michel Hautbois:
> > > > Introduce the dt-bindings documentation for bcm2835 CCP2/CSI2 Unicam
> > > > camera interface. Also add a MAINTAINERS entry for it.
> > > > 
> > > > Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
> > > > Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
> > > > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
> > > > 
> > > > ---
> > > > v4:
> > > > - make MAINTAINERS its own patch
> > > > - describe the reg and clocks correctly
> > > > - use a vendor entry for the number of data lanes
> > > > ---
> > > > 
> > > >  .../bindings/media/brcm,bcm2835-unicam.yaml   | 110 ++++++++++++++++++
> > > >  1 file changed, 110 insertions(+)
> > > >  create mode 100644
> > > > 
> > > > Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> > > > 
> > > > diff --git
> > > > a/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> > > > b/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> > > > new file mode 100644
> > > > index 000000000000..0725a0267c60
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> > > > @@ -0,0 +1,110 @@
> > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > +%YAML 1.2
> > > > +---
> > > > +$id: http://devicetree.org/schemas/media/brcm,bcm2835-unicam.yaml#
> > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > +
> > > > +title: Broadcom BCM283x Camera Interface (Unicam)
> > > > +
> > > > +maintainers:
> > > > +  - Raspberry Pi Kernel Maintenance <kernel-list@raspberrypi.com>
> > > > +
> > > > +description: |-
> > > > +  The Unicam block on BCM283x SoCs is the receiver for either
> > > > +  CSI-2 or CCP2 data from image sensors or similar devices.
> > > > +
> > > > +  The main platform using this SoC is the Raspberry Pi family of boards.
> > > > +  On the Pi the VideoCore firmware can also control this hardware block,
> > > > +  and driving it from two different processors will cause issues.
> > > > +  To avoid this, the firmware checks the device tree configuration
> > > > +  during boot. If it finds device tree nodes starting by csi then
> > > > +  it will stop the firmware accessing the block, and it can then
> > > > +  safely be used via the device tree binding.
> > > > +
> > > > +properties:
> > > > +  compatible:
> > > > +    const: brcm,bcm2835-unicam
> > > > +
> > > > +  reg:
> > > > +    items:
> > > > +      - description: Unicam block.
> > > > +      - description: Clock Manager Image (CMI) block.
> > > > +
> > > > +  interrupts:
> > > > +    maxItems: 1
> > > > +
> > > > +  clocks:
> > > > +    items:
> > > > +      - description: Clock to drive the LP state machine of Unicam.
> > > > +      - description: Clock for the vpu (core clock).
> > > > +
> > > > +  clock-names:
> > > > +    items:
> > > > +      - const: lp
> > > > +      - const: vpu
> > > > +
> > > > +  power-domains:
> > > > +    items:
> > > > +      - description: Unicam power domain
> > > > +
> > > > +  brcm,num-data-lanes:
> > > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > > +    enum: [ 2, 4 ]
> > > > +    description: Number of data lanes on the csi bus
> > > 
> > > There is already data-lanes in
> > > Documentation/devicetree/bindings/media/video- interfaces.yaml. AFAICS
> > > these two are identical. Can't the video-
> > > interface.yaml be used for this? I'm no expert TBH.
> > 
> > This is the number of data lanes that the Unicam instance supports.
> > There are two Unicam instances, and they can have 2 or 4 data lanes
> > depending on the SoC. The data-lanes endpoint property indicates the
> > number of lanes used on a particular board.
> 
> Thanks for the explanation. Isn't this something which could be derived from 
> the compatible, e.g. having 2 different ones for 2 and 4 lanes respectively?
> See [1] for a similar situation in the SPI subsystem.
> I don't have a strong opinion, just want to share my feedback.

Yes, it could also be done through compatible strings, but in this case
I think a vendor-specific property is better. The number of lanes routed
from the Unicam IP core to the external of the SoC depends on the exact
SoC model, so we would need to create different compatible strings for
essentially the same IP core.

> [1] https://patchwork.kernel.org/project/spi-devel-general/patch/20211207104114.2720764-1-alexander.stein@ew.tq-group.com/#24641405

-- 
Regards,

Laurent Pinchart