Subject: Re: [PATCH v2] ARM: zynq: Add #io-channel-cells to (x)adc node for
 iio-hwmon
To: Lars-Peter Clausen <lars@metafoo.de>,
        Michal Simek <michal.simek@xilinx.com>,
        Moritz Fischer <moritz.fischer.private@gmail.com>
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CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Devicetree List <devicetree@vger.kernel.org>,
        =?UTF-8?Q?S=c3=b6ren_Brinkmann?= <soren.brinkmann@xilinx.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Julia Cartwright <julia@ni.com>
From: Michal Simek <michal.simek@xilinx.com>
Message-ID: <c7b5cf05-fb8d-e3d9-b15e-593064daba62@xilinx.com>
Date: Fri, 7 Apr 2017 09:11:37 +0200
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On 20.3.2017 16:34, Lars-Peter Clausen wrote:
> On 03/20/2017 04:33 PM, Michal Simek wrote:
>> On 17.3.2017 07:46, Michal Simek wrote:
>>> On 16.3.2017 22:20, Lars-Peter Clausen wrote:
>>>> On 03/16/2017 07:06 PM, Michal Simek wrote:
>>>>> On 16.3.2017 17:51, Lars-Peter Clausen wrote:
>>>>>> On 03/16/2017 05:45 PM, Michal Simek wrote:
>>>>>>> On 16.3.2017 17:39, Moritz Fischer wrote:
>>>>>>>> On Thu, Mar 16, 2017 at 9:16 AM, Michal Simek <michal.simek@xilinx.com> wrote:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> On 8.3.2017 21:11, Moritz Fischer wrote:
>>>>>>>>>> Fix
>>>>>>>>>>
>>>>>>>>>> OF: /iio_hwmon: could not get #io-channel-cells for
>>>>>>>>>> /amba/adc@f8007100
>>>>>>>>>> OF: /iio_hwmon: could not get #io-channel-cells for
>>>>>>>>>> /amba/adc@f8007100
>>>>>>>>>> OF: /iio_hwmon: could not get #io-channel-cells for
>>>>>>>>>> /amba/adc@f8007100
>>>>>>>>>>
>>>>>>>>>> by adding the #io-channel-cells property.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Moritz Fischer <mdf@kernel.org>
>>>>>>>>>> Cc: Michal Simek <michal.simek@xilinx.com>
>>>>>>>>>> Cc: Sören Brinkmann <soren.brinkmann@xilinx.com>
>>>>>>>>>> Cc: Julia Cartwright <julia@ni.com>
>>>>>>>>>> Cc: linux-kernel@vger.kernel.org
>>>>>>>>>> Cc: devicetree@vger.kernel.org
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>> Changes from v1:
>>>>>>>>>> - fix messed up commit message
>>>>>>>>>> ---
>>>>>>>>>>  arch/arm/boot/dts/zynq-7000.dtsi | 1 +
>>>>>>>>>>  1 file changed, 1 insertion(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/arch/arm/boot/dts/zynq-7000.dtsi b/arch/arm/boot/dts/zynq-7000.dtsi
>>>>>>>>>> index f3ac9bf..98233a8 100644
>>>>>>>>>> --- a/arch/arm/boot/dts/zynq-7000.dtsi
>>>>>>>>>> +++ b/arch/arm/boot/dts/zynq-7000.dtsi
>>>>>>>>>> @@ -72,6 +72,7 @@
>>>>>>>>>>                       interrupts = <0 7 4>;
>>>>>>>>>>                       interrupt-parent = <&intc>;
>>>>>>>>>>                       clocks = <&clkc 12>;
>>>>>>>>>> +                     #io-channel-cells = <1>;
>>>>>>>>>>               };
>>>>>>>>>>
>>>>>>>>>>               can0: can@e0008000 {
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I think it will be good to the next step too.
>>>>>>>>> It means also add iio-hwmon node too.
>>>>>>>>>
>>>>>>>>> What do you think?
>>>>>>>>
>>>>>>>> I hadn't put it in there since dts is supposed to describe hw,
>>>>>>>> but obviously putting the actual hwmon in there makes it more useful.
>>>>>>>
>>>>>>> I had one discussion about this with Grant in past and it is common
>>>>>>> mistake. It is simplification of purpose of dts.
>>>>>>>
>>>>>>
>>>>>> If the iio-hwmon binding had gone through review it would have been rejected.
>>>>>
>>>>> Isn't it a time to deprecate it?
>>>>
>>>> Well, it's ABI now and has to stay forever. Deprecating it makes only sense
>>>> if there is a replacement, which there is not. The iio-hwmon bridge has its
>>>> usecases it's just instantiating it via devicetree which is not so nice.
>>>>
>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> I can resubmit with the hwmon node in there.
>>>>>>>
>>>>>>> If you grep kernel tree you will see that others are using it too.
>>>>>>> Also there is accepted binding for that that's why I can't see big
>>>>>>> problem with it.
>>>>>>
>>>>>> Since this is an application specific binding I wouldn't put it in the
>>>>>> generic DT include file. It's a bit like adding a gpio-key binding for each
>>>>>> of the GPIOs just in case somebody wants to use it.
>>>>>
>>>>> psci is system specific too.
>>>>>
>>>>> IIRC this driver for zynq was written by ADI or with ADI help that's why
>>>>> you know much better than I what's the correct configuration.
>>>>>
>>>>> This targets PS IP which should be present in the hw all the time.
>>>>> Not sure if for all configuration but I expect at least the part of it
>>>>> is there all them time.
>>>>>
>>>>> If binding is incorrect then please remove it with removing from all
>>>>> dts/dtsi files which have this. The same is for of probe function in the
>>>>> driver itself.
>>>>> If this is not done then this is just +1 case.
>>>>>
>>>>> If you still insist that we shouldn't do it then please at least extend
>>>>> commit message and put there example how to wire it on zynq.
>>>>
>>>> There is a IIO driver for the XADC, this driver has a userspace interface
>>>> that exposes the measurements provided by the hardware. Using the hwmon
>>>> bridge will expose the same information just through a hwmon interface.
>>>>
>>>> One reason for using the iio-hwmon bridge is because you have a legacy
>>>> application that expects the a hwmon interface. But new applications that
>>>> want to access the XADC should really use the IIO interface if possible.
>>>>
>>>> In my opinion instantiating the hwmon bridge by default will only cause
>>>> confusion. There are now redundant interfaces and users will wonder what is
>>>> the difference between the two. Is it the same data, is it different data?
>>>> Which is the preferred interface? Which one is 'better'?
>>>
>>> IMHO this should be covered by documentation. One paragraph in iio-hwmon
>>> binding can have answers for this and it will be very clear what people
>>> should use.
>>
>> Any comment on this one?
> 
> Send a patch? :)
> 

I don't feel that I know iio enough to be good candidate to describe
this properly.

Thanks,
Michal