Re: [PATCH v4 1/2] dt-bindings: media: i2c: Add IMX296 CMOS sensor binding

Laurent Pinchart
2019-11-04T19:02:13Z

Hi Sakari,

On Thu, Oct 31, 2019 at 07:08:37PM +0200, Sakari Ailus wrote:
> On Thu, Oct 31, 2019 at 06:54:44PM +0200, Laurent Pinchart wrote:
> > 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.

It's a shortcoming of ACPI, which should be addressed at the ACPI level.
We shouldn't polute the DT bindings with a clock-frequency property for
this reason.

--
Regards,

Laurent Pinchart