Re: [PATCH 1/3] dt-bindings: media: i2c: Add OV8865 bindings documentation

[Sakari Ailus]

Hi Paul,

On Wed, Nov 04, 2020 at 11:26:43AM +0100, Paul Kocialkowski wrote:
> Hi Sakari and thanks for the review!
> 
> On Tue 03 Nov 20, 01:24, Sakari Ailus wrote:
> > On Fri, Oct 23, 2020 at 07:54:04PM +0200, Paul Kocialkowski wrote:
> > > This introduces YAML bindings documentation for the OV8865
> > > image sensor.
> > > 
> > > Co-developed-by: Kévin L'hôpital <kevin.lhopital@xxxxxxxxxxx>
> > > Signed-off-by: Kévin L'hôpital <kevin.lhopital@xxxxxxxxxxx>
> > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx>
> > > ---
> > >  .../bindings/media/i2c/ovti,ov8865.yaml       | 124 ++++++++++++++++++
> > >  1 file changed, 124 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/media/i2c/ovti,ov8865.yaml
> > > 
> > > diff --git a/Documentation/devicetree/bindings/media/i2c/ovti,ov8865.yaml b/Documentation/devicetree/bindings/media/i2c/ovti,ov8865.yaml
> > > new file mode 100644
> > > index 000000000000..807f1a94afae
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/media/i2c/ovti,ov8865.yaml
> > > @@ -0,0 +1,124 @@
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/media/i2c/ovti,ov8865.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: OmniVision OV8865 Image Sensor Device Tree Bindings
> > > +
> > > +maintainers:
> > > +  - Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx>
> > > +
> > > +properties:
> > > +  compatible:
> > > +    const: ovti,ov8865
> > > +
> > > +  reg:
> > > +    maxItems: 1
> > > +
> > > +  clocks:
> > > +    items:
> > > +      - description: EXTCLK Clock
> > > +
> > > +  clock-names:
> > > +    items:
> > > +      - const: extclk
> > 
> > Is this needed with a single clock?
> 
> Yes I think so: we grab the clock with devm_clk_get which takes a name string
> that matches the clock-names property.

That argument may be NULL.

> 
> > And... shouldn't this also come with assigned-clock-rates etc., to set the
> > clock frequency?
> 
> I'm a bit confused why we would need to do that in the device-tree rather than
> setting the clock rate with clk_set_rate in the driver, like any other driver
> does. I think this was discussed before (on the initial ov8865 series) and the
> conclusion was that there is no particular reason for media i2c drivers to
> behave differently. So I believe this is the correct approach.

I'm not exactly sure about that conclusion.

You can use clk_set_rate() if you get the frequency from DT, but we
recently did conclude that camera sensor drivers can expect to get the
frequency indicated by assigned-clock-rate property.

In other words, the driver may not be specific to a particular board and
SoC you have.

Please also read Documentation/driver-api/media/camera-sensor.rst .

> 
> > > +
> > > +  dvdd-supply:
> > > +    description: Digital Domain Power Supply
> > > +
> > > +  avdd-supply:
> > > +    description: Analog Domain Power Supply (internal AVDD is used if missing)
> > > +
> > > +  dovdd-supply:
> > > +    description: I/O Domain Power Supply
> > > +
> > > +  powerdown-gpios:
> > > +    maxItems: 1
> > > +    description: Power Down Pin GPIO Control (active low)
> > > +
> > > +  reset-gpios:
> > > +    maxItems: 1
> > > +    description: Reset Pin GPIO Control (active low)
> > > +
> > > +  port:
> > > +    type: object
> > > +    description: Input port, connect to a MIPI CSI-2 receiver
> > > +
> > > +    properties:
> > > +      endpoint:
> > > +        type: object
> > > +
> > > +        properties:
> > > +          remote-endpoint: true
> > > +
> > > +          bus-type:
> > > +            const: 4
> > > +
> > > +          clock-lanes:
> > > +            maxItems: 1
> > 
> > I believe you can drop clock-lanes and bus-type; these are both constants.
> 
> I don't understand why bus-type should be dropped because it is constant:
> if bus-type is set to something else, the driver will definitely not probe
> since we're requesting V4L2_MBUS_CSI2_DPHY for v4l2_fwnode_endpoint_parse.
> So I think it's quite important for the bindings to reflect this.

This driver is for a particular device that has MIPI CSI-2 on D-PHY as the
data bus. You can assume that in the driver.

> 
> > I presume the device does not support lane remapping?
> 
> That's correct so this is indeed not something we can configure.
> But shouldn't we instead specift clock-lanes = <0> as a const rather than
> getting rid of it?

Why would you put redundant information to DT?

> 
> > Could you also add link-frequencies, to list which frequencies are known to
> > be good?
> 
> Ah right, I had missed it. I'm a bit unsure about what I should do with the
> information from the driver though: should I refuse to use link frequencies that
> are not in the list?

Yes, please.

-- 
Regards,

Sakari Ailus