linux-kernel - Re: [PATCH v9 1/2] Add OV5647 device tree documentation

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Date:   Wed, 22 Feb 2017 00:37:51 +0200
From:   Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To:     Sakari Ailus <sakari.ailus@....fi>
CC:     Ramiro Oliveira <Ramiro.Oliveira@...opsys.com>,
        <linux-kernel@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <CARLOS.PALMINHA@...opsys.com>,
        Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Mark Rutland <mark.rutland@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Pali Rohár <pali.rohar@...il.com>,
        Pavel Machek <pavel@....cz>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Rob Herring <robh+dt@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Steve Longerbeam <slongerbeam@...il.com>
Subject: Re: [PATCH v9 1/2] Add OV5647 device tree documentation

Hi Sakari,

On 02/21/2017 11:48 PM, Sakari Ailus wrote:
> Hi, Vladimir!
> 
> How do you do? :-)

deferring execution of boring tasks by doing code review :)

> On Tue, Feb 21, 2017 at 10:48:09PM +0200, Vladimir Zapolskiy wrote:
>> Hi Ramiro,
>>
>> On 02/21/2017 10:13 PM, Ramiro Oliveira wrote:
>>> Hi Vladimir,
>>>
>>> Thank you for your feedback
>>>
>>> On 2/21/2017 3:58 PM, Vladimir Zapolskiy wrote:
>>>> Hi Ramiro,
>>>>
>>>> On 02/17/2017 03:14 PM, Ramiro Oliveira wrote:
>>>>> Create device tree bindings documentation.
>>>>>
>>>>> Signed-off-by: Ramiro Oliveira <roliveir@...opsys.com>
>>>>> Acked-by: Rob Herring <robh@...nel.org>
>>>>> ---
>>>>>  .../devicetree/bindings/media/i2c/ov5647.txt       | 35 ++++++++++++++++++++++
>>>>>  1 file changed, 35 insertions(+)
>>>>>  create mode 100644 Documentation/devicetree/bindings/media/i2c/ov5647.txt
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/media/i2c/ov5647.txt b/Documentation/devicetree/bindings/media/i2c/ov5647.txt
>>>>> new file mode 100644
>>>>> index 000000000000..31956426d3b9
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/media/i2c/ov5647.txt
>>>>> @@ -0,0 +1,35 @@
>>>>> +Omnivision OV5647 raw image sensor
>>>>> +---------------------------------
>>>>> +
>>>>> +OV5647 is a raw image sensor with MIPI CSI-2 and CCP2 image data interfaces
>>>>> +and CCI (I2C compatible) control bus.
>>>>> +
>>>>> +Required properties:
>>>>> +
>>>>> +- compatible		: "ovti,ov5647".
>>>>> +- reg			: I2C slave address of the sensor.
>>>>> +- clocks		: Reference to the xclk clock.
>>>>
>>>> Is "xclk" clock a pixel clock or something else?
>>>>
>>>
>>> It's an external oscillator.
>>
>> hmm, I suppose a clock of any type could serve as a clock for the sensor.
>> It can be an external oscillator on a particular board, or it can be
>> something else on another board.
> 
> Any clock source could be used I presume.
> 

That's exactly my point, and it is a reason to rename "xclk" to something
more generic.

>>
>> Can you please describe what for does ov5647 sensor need this clock, what
>> is its function?
> 
> Camera modules (sensors) quite commonly require an external clock as they do
> not have an oscillator on their own. A lot of devices under
> Documentation/devicetree/bindings/media/i2c/ have similar properties.
> 

So, what should be a better replacement of "xclk" in the description above?

E.g.

- clocks		: Phandle to a device supply clock.

>>
>>>
>>>>> +- clock-names		: Should be "xclk".

We got an agreement that "clock-names" property is removed, nevertheless
if it is added back, is should not be "xclk".

>>>>
>>>> You can remove this property, because there is only one source clock.
>>>>
>>>
>>> Ok.
>>>
>>>>> +- clock-frequency	: Frequency of the xclk clock.
>>>>
>>>> And after the last updates in the driver this property can be removed as well.
>>>>
>>>
>>> But I'm still using clk_get_rate in the driver, if I remove the frequency here
>>> the probing will fail.
>>>
>>
>> I doubt it, there should be no connection between a custom "clock-frequency"
>> device tree property in a clock consumer device node and clk_get_rate() function
>> from the CCF, which takes a clock provider as its argument.
> 
> The purpose is to make sure the clock frequency is really usable for the
> device, in this particular case the driver can work with one particular
> frequency.
> 
> That said, the driver does not appear to use the property at the moment. It
> should.
> 
> It'd be good to verify that the rate matches: clk_set_rate() is not
> guaranteed to produce the requested clock rate, and the driver could
> conceivably be updated with support for more frequencies. There are
> typically a few frequencies that a SoC such a sensor is connected can
> support, and 25 MHz is not one of the common frequencies. With this
> property, the frequency would be always there explicitly.
> 

I can provide my arguments given at v8 review time again, since I don't
see a contradiction with my older comments.

Briefly "clock-frequency" as a device tree property on a consumer side
can be considered as redundant, because there is a mechanism to specify
a wanted clock frequency on a clock provider side right in a board DTB.

So, the clock frequency set up is delegated to CCF, and when any other
than 25 MHz frequencies are got supported, that's only the matter of
driver updates, DTBs won't be touched.

--
With best wishes,
Vladimir