linux-kernel - Re: [PATCH v2] ARM: zynq: Add #io-channel-cells to (x)adc node for iio-hwmon

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Date:   Mon, 20 Mar 2017 16:33:28 +0100
From:   Michal Simek <michal.simek@...inx.com>
To:     Michal Simek <michal.simek@...inx.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Moritz Fischer <moritz.fischer.private@...il.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        Sören Brinkmann <soren.brinkmann@...inx.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Julia Cartwright <julia@...com>
Subject: Re: [PATCH v2] ARM: zynq: Add #io-channel-cells to (x)adc node for
 iio-hwmon

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@...inx.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@...07100
>>>>>>>> OF: /iio_hwmon: could not get #io-channel-cells for
>>>>>>>> /amba/adc@...07100
>>>>>>>> OF: /iio_hwmon: could not get #io-channel-cells for
>>>>>>>> /amba/adc@...07100
>>>>>>>>
>>>>>>>> by adding the #io-channel-cells property.
>>>>>>>>
>>>>>>>> Signed-off-by: Moritz Fischer <mdf@...nel.org>
>>>>>>>> Cc: Michal Simek <michal.simek@...inx.com>
>>>>>>>> Cc: Sören Brinkmann <soren.brinkmann@...inx.com>
>>>>>>>> Cc: Julia Cartwright <julia@...com>
>>>>>>>> Cc: linux-kernel@...r.kernel.org
>>>>>>>> Cc: devicetree@...r.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@...08000 {
>>>>>>>>
>>>>>>>
>>>>>>> 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?

Thanks,
Michal