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

["H. Nikolaus Schaller" <hns@goldelico.com>]

> Am 28.06.2017 um 00:57 schrieb Sylwester Nawrocki <snawrocki@kernel.org>:
> 
> On 06/27/2017 07:48 AM, H. Nikolaus Schaller wrote:
>>> Am 26.06.2017 um 22:04 schrieb Sylwester Nawrocki <snawrocki@kernel.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.

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?

BR and thanks,
Nikolaus