linux-kernel - Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings

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Date:   Thu, 2 Apr 2020 12:10:00 +0200
From:   Robert Foss <robert.foss@...aro.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Dongchun Zhu <dongchun.zhu@...iatek.com>,
        Fabio Estevam <festevam@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-media <linux-media@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings

Hey Maxime,

On Wed, 1 Apr 2020 at 10:07, Maxime Ripard <maxime@...no.tech> wrote:
>
> Hi,
>
> On Tue, Mar 31, 2020 at 03:33:44PM +0200, Robert Foss wrote:
> > From: Dongchun Zhu <dongchun.zhu@...iatek.com>
> >
> > This patch adds documentation of device tree in YAML schema for the
> > OV8856 CMOS image sensor.
> >
> > Signed-off-by: Dongchun Zhu <dongchun.zhu@...iatek.com>
> > Signed-off-by: Robert Foss <robert.foss@...aro.org>
> > ---
> >
> > - Changes since v5:
> >   * Add assigned-clocks and assigned-clock-rates
> >   * robher: dt-schema errors
> >
> > - Changes since v4:
> >   * Fabio: Change reset-gpio to GPIO_ACTIVE_LOW, explain in description
> >   * Add clock-lanes property to example
> >   * robher: Fix syntax error in devicetree example
> >
> > - Changes since v3:
> >   * robher: Fix syntax error
> >   * robher: Removed maxItems
> >   * Fixes yaml 'make dt-binding-check' errors
> >
> > - Changes since v2:
> >   Fixes comments from from Andy, Tomasz, Sakari, Rob.
> >   * Convert text documentation to YAML schema.
> >
> > - Changes since v1:
> >   Fixes comments from Sakari, Tomasz
> >   * Add clock-frequency and link-frequencies in DT
> >
> >  .../devicetree/bindings/media/i2c/ov8856.yaml | 150 ++++++++++++++++++
> >  MAINTAINERS                                   |   1 +
> >  2 files changed, 151 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/media/i2c/ov8856.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/media/i2c/ov8856.yaml b/Documentation/devicetree/bindings/media/i2c/ov8856.yaml
> > new file mode 100644
> > index 000000000000..beeddfbb8709
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/media/i2c/ov8856.yaml
> > @@ -0,0 +1,150 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +# Copyright (c) 2019 MediaTek Inc.
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/media/i2c/ov8856.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Omnivision OV8856 CMOS Sensor Device Tree Bindings
> > +
> > +maintainers:
> > +  - Ben Kao <ben.kao@...el.com>
> > +  - Dongchun Zhu <dongchun.zhu@...iatek.com>
> > +
> > +description: |-
> > +  The Omnivision OV8856 is a high performance, 1/4-inch, 8 megapixel, CMOS
> > +  image sensor that delivers 3264x2448 at 30fps. It provides full-frame,
> > +  sub-sampled, and windowed 10-bit MIPI images in various formats via the
> > +  Serial Camera Control Bus (SCCB) interface. This chip is programmable
> > +  through I2C and two-wire SCCB. The sensor output is available via CSI-2
> > +  serial data output (up to 4-lane).
> > +
> > +properties:
> > +  compatible:
> > +    const: ovti,ov8856
> > +
> > +  reg:
> > +    maxItems: 1
> > +
> > +  clocks:
> > +    maxItems: 1
> > +
> > +  clock-names:
> > +    description:
> > +      Input clock for the sensor.
> > +    items:
> > +      - const: xvclk
> > +
> > +  clock-frequency:
> > +    description:
> > +      Frequency of the xvclk clock in Hertz.
>
> We also had that discussion recently for another omnivision sensor
> (ov5645 iirc), but what is clock-frequency useful for?
>
> It seems that the sensor is passed in clocks, so if you need to
> retrieve the clock rate you should use the clock API instead.
>
> Looking at the driver, it looks like it first retrieves the clock, set
> it to clock-frequency, and then checks that this is OV8856_XVCLK_19_2
> (19.2 MHz).

As far as I understand it, 19.2MHz is requirement for the sensor mode
that currently defaults to. Some modes require higher clock speeds
than this however.

>
> The datasheet says that the sensor can have any frequency in the 6 -
> 27 MHz range, so this is a driver limitation and should be set in the
> driver using the clock API, and you can always bail out if it doesn't
> provide a rate that is not acceptable for the drivers assumption.
>
> In any case, you don't need clock-frequency here...

So your suggestion is that we remove all clocks-rate properties, and
replace the clk_get_rate() calls in the driver with clk_set_rate()
calls for the desired frequencies?

>
> > +  assigned-clocks:
> > +    description:
> > +      Input clock for the sensor.
> > +
> > +  assigned-clock-rates:
> > +    description:
> > +      Frequency of the xvclk clock in Hertz.
>
> And you don't need assigned-clock-rates either.

Ack.

>
> Maxime