linux-kernel - Re: [PATCH v1 1/6] DT bindings: add bindings for ov965x camera module

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Date:   Wed, 28 Jun 2017 12:50:39 +0200
From:   Sylwester Nawrocki <s.nawrocki@...sung.com>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Hugues FRUCHET <hugues.fruchet@...com>,
        Guennadi Liakhovetski <g.liakhovetski@....de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Yannick FERTRE <yannick.fertre@...com>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>
Subject: Re: [PATCH v1 1/6] DT bindings: add bindings for ov965x camera
 module

On 06/28/2017 11:12 AM, H. Nikolaus Schaller wrote:
>> Am 28.06.2017 um 00:57 schrieb Sylwester Nawrocki <snawrocki@...nel.org>:
>> On 06/27/2017 07:48 AM, H. Nikolaus Schaller wrote:
>>>> Am 26.06.2017 um 22:04 schrieb Sylwester Nawrocki <snawrocki@...nel.org>:
>>>> On 06/26/2017 12:35 PM, Hugues FRUCHET wrote:
>>>>>> What I am missing to support the GTA04 camera is the control of the optional "vana-supply".
>>>>>> So the driver does not power up the camera module when needed and therefore probing fails.
>>>>>>
>>>>>>     - vana-supply: a regulator to power up the camera module.
>>>>>>
>>>>>> Driver code is not complex to add:
>>>>
>>>>> Yes, I saw it in your code, but as I don't have any programmable power
>>>>> supply on my setup, I have not pushed this commit.
>>>>
>>>> Since you are about to add voltage supplies to the DT binding I'd suggest
>>>> to include all three voltage supplies of the sensor chip. Looking at the OV9650
>>>> and the OV9655 datasheet there are following names used for the voltage supply
>>>> pins:
>>>>
>>>> AVDD - Analog power supply,
>>>> DVDD - Power supply for digital core logic,
>>>> DOVDD - Digital power supply for I/O.
>>>
>>> The latter two are usually not independently switchable from the SoC power
>>> the module is connected to.
>>>
>>> And sometimes DVDD and DOVDD are connected together.
>>>
>>> So the driver can't make much use of knowing or requesting them because the
>>> 1.8V supply is always active, even during suspend.
>>>
>>>>
>>>> I doubt the sensor can work without any of these voltage supplies, thus
>>>> regulator_get_optional() should not be used. I would just use the regulator
>>>> bulk API to handle all three power supplies.
>>>
>>> The digital part works with AVDD turned off. So the LDO supplying AVDD should
>>> be switchable to save power (&vaux3 on the GTA04 device).>
>>> But not all designs can switch it off. Hence the idea to define it as an
>>> /optional/ regulator. If it is not defined by DT, the driver simply assumes
>>> it is always powered on.
>>
>> I didn't say we can't define regulator supply properties as optional in the DT
>> binding.  If we define them as such and any of these *-supply properties is
>> missing in DT with regulator_get() the regulator core will use dummy regulator
>> for that particular voltage supply.  While with regulator_get_optional()
>> -ENODEV is returned when the regulator cannot be found.
> 
> Ah, ok. I see.
> 
> I had thought that it is the right thing to do like devm_gpiod_get_optional().
> 
> That one it is described as:
> 
> "* This is equivalent to gpiod_get(), except that when no GPIO was assigned to
>   * the requested function it will return NULL. This is convenient for drivers
>   * that need to handle optional GPIOs."
> 
> Seems to be inconsistent definition of what "optional" means.

Indeed, this commit explains it further:

commit de1dd9fd2156874b45803299b3b27e65d5defdd9
regulator: core: Provide hints to the core about optional supplies

> So we indeed should use devm_regulator_get() in this case. Thanks for > pointing out!

>>> So in summary we only need AVDD switched for the GTA04 - but it does not
>>> matter if the others are optional properties. We would not use them.
>>>
>>> It does matter if they are mandatory because it adds DT complexity (size
>>> and processing) without added function.
>>
>> We should not be defining DT binding only with selected use cases/board
>> designs in mind.  IMO all three voltage supplies should be listed in the
>> binding, presumably all can be made optional, with an assumption that when
>> the property is missing selected pin is hooked up to a fixed regulator.
> 
> Ok, then it should just be defined in the bindings but not used by 
> the driver?

Yes, I think so. So we have a possibly complete binding right from the 
beginning. I someone needs handling other supplies than AVDD they could
update the driver in future.


Regards,
Sylwester