linux-kernel - Re: [PATCH v2] iio: light: Add support for ROHM RPR0521 sensor

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Message-Id: <20150611141849.6828A42CA3@saturn.retrosnub.co.uk>
Date:	Thu, 11 Jun 2015 15:18:49 +0100
From:	jic23@...23.retrosnub.co.uk
To:	Daniel Baluta <daniel.baluta@...el.com>
Cc:	Jonathan Cameron <jic23@...nel.org>, mranostay@...il.com,
	Peter Meerwald <pmeerw@...erw.net>,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	Vlad Dogaru <vlad.dogaru@...el.com>,
	Tiberiu Breana <tiberiu.a.breana@...el.com>
Subject: Re: [PATCH v2] iio: light: Add support for ROHM RPR0521 sensor

Daniel Baluta writes: 

> On Sun, Jun 7, 2015 at 12:08 AM, Jonathan Cameron <jic23@...nel.org> wrote:
>> 
>>
>> On 06/03/2015 09:56 AM, Daniel Baluta wrote:
>>>
>>> On Thu, May 28, 2015 at 5:17 PM, Daniel Baluta <daniel.baluta@...el.com>
>>> wrote:
>>>>
>>>> <snip> 
>>>>
>>>>>>>> +static const struct iio_chan_spec rpr0521_channels[] = {
>>>>>>>> +     {
>>>>>>>> +             .type = IIO_INTENSITY,
>>>>>>>> +             .modified = 1,
>>>>>>>> +             .address = RPR0521_CHAN_ALS_DATA0,
>>>>>>>> +             .channel2 = IIO_MOD_LIGHT_BOTH,
>>>>>>>> +             .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
>>>>>>>> +                     BIT(IIO_CHAN_INFO_CALIBSCALE),
>>>>>>> 
>>>>>>>
>>>>>>> why CALIBSCALE and not SCALE?
>>>>>> 
>>>>>>
>>>>>> Because this is used to set/get gain, which is used by the hardware
>>>>>> to do proper scaling. 
>>>>>>
>>>>>> AFAIK this should be calibscale.
>>>>> 
>>>>>
>>>>> in sysfs-bus-iiof on CALIBSCALE: Hardware applied calibration scale
>>>>> factor
>>>>> (assumed to fix production inaccuracies). 
>>>>>
>>>>> this doesn't seem applicable here, it is a gain factor controlling
>>>>> measurement resolution
>>>> 
>>>>
>>>> Ok, I see now and it makes sense :). 
>>>>
>>>> # echo 1 > in_intensity_ir_calibscale
>>>> # cat in_intensity_ir_raw
>>>> 79
>>>> # echo 64 > in_intensity_ir_calibscale
>>>> # cat in_intensity_ir_raw
>>>> 5084 
>>>>
>>>> The user should get the same value regardless of the gain :), and in the
>>>> above example for x64 gain it should have a 1/64 scale. 
>>>>
>>>>
>>>> <snip> 
>>>>
>>>>>> Or we can consider that the chan->type is always valid?
>>>>> 
>>>>>
>>>>> I'd think so; you also assume that chan->address is valid 
>>>>>
>>>>> I suggest to use chan->address to point to a table containing the
>>>>> address and the mask
>>>> 
>>>>
>>>> <snip> 
>>>>
>>>>>> Which sensors? It means they do not agree with the ABI: 
>>>>>>
>>>>>>
>>>>>> http://lxr.free-electrons.com/source/Documentation/ABI/testing/sysfs-bus-iio#L1131
>>>>> 
>>>>>
>>>>> that 'clarification' was added recently,
>>>>> 614e8842ddf5502f0e781f91695bfbc1e1e1d9b6 (with 3.18)
>>>>> "Proximity measurement .. by observing reflectivity" 
>>>>>
>>>>> high proximity <-> high reflectivity -- this is the reality of what most
>>>>> sensors output (including yours) 
>>>>>
>>>>> proximity and distance are opposite concepts;
>>>>> high proximity <-> low distance, and vice versa 
>>>>>
>>>>> the distance part doesn't make sense in the ABI description
>>>> 
>>>>
>>>> At least sx9500 uses this convention and userspace applications rely on
>>>> this.
>>> 
>>>
>>> OK, so wee need to agree on this part and to add a proper descriptor to
>>> the ABI. 
>>>
>>> Jonathan, what do you say? 
>>>
>> I agree that we need to agree one way or the other.  Proximity being higher
>> as you get closer seems slightly more logical to me
>> (I wish now that I'd argued in favour of just doing distance, but such
>> is hindsight).  Still I'm happy with whatever consensus forms.
> 
> + Matt. 
> 
> Ok, now I see where the ambiguity comes from. The ABI for proximity also
> covers AS3935 Franklin Lightning Sensor IC, where sensor's output provides
> an estimation on the distance to the head of a storm :). 
> 
> But, I don't think this is the type of proximity most of people think of :). 
> 
> I am not sure how to modify the ABI without breaking the AS3935, but I will
> think of a solution and send a RFC as soon as possible. 
> 
> thanks,
> Daniel.
Hmm. We could define new ABI and deprecate the old one I suppose. Or
take the view that ABI breakage fixes are aloud as long as we have both
options in the tree currently and hence are defining one of them to be
wrong.
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