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Date:   Sat, 4 Apr 2020 11:34:46 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Sakari Ailus <sakari.ailus@iki.fi>
Cc:     Robert Foss <robert.foss@linaro.org>,
        Dongchun Zhu <dongchun.zhu@mediatek.com>,
        Fabio Estevam <festevam@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Tomasz Figa <tfiga@chromium.org>,
        linux-media <linux-media@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings
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Hi,

On Sat, Apr 04, 2020 at 02:27:36AM +0300, Sakari Ailus wrote:
> Hi Robert,
>
> On Thu, Apr 02, 2020 at 12:10:00PM +0200, Robert Foss wrote:
> > Hey Maxime,
> >
> > On Wed, 1 Apr 2020 at 10:07, Maxime Ripard <maxime@cerno.tech> wrote:
> > >
> > > Hi,
> > >
> > > On Tue, Mar 31, 2020 at 03:33:44PM +0200, Robert Foss wrote:
> > > > From: Dongchun Zhu <dongchun.zhu@mediatek.com>
> > > >
> > > > This patch adds documentation of device tree in YAML schema for the
> > > > OV8856 CMOS image sensor.
> > > >
> > > > Signed-off-by: Dongchun Zhu <dongchun.zhu@mediatek.com>
> > > > Signed-off-by: Robert Foss <robert.foss@linaro.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@intel.com>
> > > > +  - Dongchun Zhu <dongchun.zhu@mediatek.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.
>
> It's very system specific. Either way, bindings should not assume a
> particular driver implementation.
>
> >
> > >
> > > 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?
>
> The driver shouldn't set the rate here unless it gets it from DT (but that
> was not the intention). So the driver should get the frequency instead.

I'm actually saying the opposite :)

Like you were saying, the binding (or DT, for that matter) shouldn't
assume a particular driver implementation.

So one corollary is that if the driver has some restrictions in Linux,
it shouldn't be part of the binding, right?

This binding uses multiple clock properties but as far as I can see,
the driver retrieves a clock using clocks and makes sure that its rate
match its limitation of 19.2MHz using clock-frequency (which is
redundant on a clk_get_rate on the clocks provided earlier).

I'm suspecting that the parent clock on multiple SoCs can be
configured and is not a fixed rate crystal, so assigned-clocks-rate is
here just to make sure we set the frequency at the one being checked
in the driver's probe so that it all works.

But that 19.2MHz is not a limitation of the device itself, it's a
limitation of our implementation, so we can instead implement
something equivalent in Linux using a clk_set_rate to 19.2MHz (to make
sure that our parent clock is configured at the right rate) and the
clk_get_rate and compare that to 19.2MHz (to make sure that it's not
been rounded too far apart from the frequency we expect).

This is doing exactly the same thing, except that we don't encode our
implementation limitations in the DT, but in the driver instead.

Maxime

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