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Date:   Thu, 31 Oct 2019 19:08:37 +0200
From:   Sakari Ailus <sakari.ailus@iki.fi>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        mchehab@kernel.org, robh+dt@kernel.org,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        c.barrett@framos.com, a.brela@framos.com, peter.griffin@linaro.org
Subject: Re: [PATCH v4 1/2] dt-bindings: media: i2c: Add IMX296 CMOS sensor
 binding
Message-ID: <20191031170837.GN6253@valkosipuli.retiisi.org.uk>
References: <20191030094902.32582-1-manivannan.sadhasivam@linaro.org>
 <20191030094902.32582-2-manivannan.sadhasivam@linaro.org>
 <20191031131538.GA9170@pendragon.ideasonboard.com>
 <20191031134512.GB24273@mani>
 <20191031141141.GD5018@pendragon.ideasonboard.com>
 <20191031142817.GK6253@valkosipuli.retiisi.org.uk>
 <20191031165444.GE5018@pendragon.ideasonboard.com>
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Hi Laurent,

On Thu, Oct 31, 2019 at 06:54:44PM +0200, Laurent Pinchart wrote:
> Hi Sakari,
> 
> On Thu, Oct 31, 2019 at 04:28:17PM +0200, Sakari Ailus wrote:
> > On Thu, Oct 31, 2019 at 04:11:41PM +0200, Laurent Pinchart wrote:
> > > On Thu, Oct 31, 2019 at 07:15:12PM +0530, Manivannan Sadhasivam wrote:
> > >> On Thu, Oct 31, 2019 at 03:15:38PM +0200, Laurent Pinchart wrote:
> > >>> On Wed, Oct 30, 2019 at 03:19:01PM +0530, Manivannan Sadhasivam wrote:
> > >>>> Add YAML devicetree binding for IMX296 CMOS image sensor. Let's also
> > >>>> add MAINTAINERS entry for the binding and driver.
> > >>>> 
> > >>>> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > >>>> ---
> > >>>>  .../devicetree/bindings/media/i2c/imx296.yaml | 94 +++++++++++++++++++
> > >>>>  MAINTAINERS                                   |  8 ++
> > >>>>  2 files changed, 102 insertions(+)
> > >>>>  create mode 100644 Documentation/devicetree/bindings/media/i2c/imx296.yaml
> > >>>> 
> > >>>> diff --git a/Documentation/devicetree/bindings/media/i2c/imx296.yaml b/Documentation/devicetree/bindings/media/i2c/imx296.yaml
> > >>>> new file mode 100644
> > >>>> index 000000000000..c04ec2203268
> > >>>> --- /dev/null
> > >>>> +++ b/Documentation/devicetree/bindings/media/i2c/imx296.yaml
> > >>>> @@ -0,0 +1,94 @@
> > >>>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > >>>> +%YAML 1.2
> > >>>> +---
> > >>>> +$id: http://devicetree.org/schemas/media/i2c/imx296.yaml#
> > >>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > >>>> +
> > >>>> +title: Sony IMX296 1/2.8-Inch CMOS Image Sensor
> > >>>> +
> > >>>> +maintainers:
> > >>>> +  - Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > >>>> +
> > >>>> +description: |-
> > >>>> +  The Sony IMX296 is a 1/2.9-Inch active pixel type CMOS Solid-state image
> > >>>> +  sensor with square pixel array and 1.58 M effective pixels. This chip
> > >>>> +  features a global shutter with variable charge-integration time. It is
> > >>>> +  programmable through I2C and 4-wire interfaces. The sensor output is
> > >>>> +  available via CSI-2 serial data output (1 Lane).
> > >>>> +
> > >>>> +properties:
> > >>>> +  compatible:
> > >>>> +    const: sony,imx296
> > >>>> +
> > >>>> +  reg:
> > >>>> +    maxItems: 1
> > >>>> +
> > >>>> +  clocks:
> > >>>> +    maxItems: 1
> > >>>> +
> > >>>> +  clock-names:
> > >>>> +    description:
> > >>>> +      Input clock for the sensor.
> > >>>> +    items:
> > >>>> +      - const: mclk
> > >>> 
> > >>> The pin is named INCK, let's name the clock accordingly.
> > >> 
> > >> Okay, I thought generic names are preferred here!
> > >>  
> > >>>> +  clock-frequency:
> > >>>> +    description:
> > >>>> +      Frequency of the mclk clock in Hertz.
> > >>> 
> > >>> This shouldn't be needed, you can retrieve the clock frequency at
> > >>> runtime from the clock source.
> > >> 
> > >> Unless the clock source is a fixed one! What if the clock source comes from
> > >> SoC? We need to set the rate, right?
> > > 
> > > In that case, if you want to hardcode the clock in DT, the preferred way
> > > is to use the assigned-clock-rates property. Otherwise, if the driver
> > > requires a specific clock frequency, it's better to hardcode it in the
> > > driver itself. In this specific case, I think assigned-clock-rates is
> > > best as the device can support three different clock frequencies.
> > 
> > Just note that if ACPI support is added to the sensor driver, you'll need
> > the clock-frequency property again, for that's the only way how the driver
> > will get the clock frequency.
> 
> Why is so ? Why can't we implement of assigned-clock-rates for ACPI ?

ACPI doesn't deal with clocks as such. So there's also no ACPI defined way
to access clocks specifically, including the frequency --- instead the
clock is controlled by an AML methods which implement power on and off
sequences for the device.

-- 
Sakari Ailus