Re: [PATCH v3] iio: temperature: add support for Maxim thermocouple chips

[Jonathan Cameron <jic23@kernel.org>]

On 22/06/16 09:02, Lars-Peter Clausen wrote:
> On 06/11/2016 06:48 PM, Jonathan Cameron wrote:
>> On 03/06/16 13:33, Jonathan Cameron wrote:
>>> On 30/05/16 02:37, Matt Ranostay wrote:
>>>> Add initial driver support for MAX6675, and MAX31885 thermocouple chips.
>>>>
>>>> Cc: Marek Vasut <marex@denx.de>
>>>> Cc: Matt Porter <mporter@konsulko.com>
>>>> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
>>> I'm going to let this sit for a sort while as I'd like some discussion
>>> of the invalidate buffer bit.
>>>
>>> Cc'd a few more people for views.
>> Hmm. Deadly silence.
>>
>> Daniel, Lars, Peter - this is a fairly fundamental abi question.
>>
>> What do we do to signify an 'invalid reading' in the buffer.
>>
>> Here the part is driven by a software trigger - and if we skip
>> a reading we are obviously out of sync.
>>
>> Old and nasty trick we used in some (possibly only one)
>> early driver was to set an invalid state for these cases.
>>
>> Anyone have a better idea?
> 
> Ideally the driver would leave the data including the status bit intact and
> not replace it with a magic constant that way an application that is aware
> of the way the chip behaves could handle that.

In response to what I'd misunderstood this as:

It's tricky as the status bit is not in general in the same register as the
value.  We would need to add a meta data element to the buffer to handle this.

I'm inclined to go with the magic value as the best 'general purpose' option
we have right now...  I don't like it, but adding meta data is also somewhat
hideous as most usecases and devices wouldn't use it.

Lets keep in mind that we might want to 'clean' this up at some point.

I'll let this sit a few more days for other inputs.

And what is actually going on :
Actually reading the datasheets on this I think I'd missunderstood.

These are low not if a conversion is in progress, but rather if
the thermocouple is physically open (i.e. open circuit).
This should be reported out of band to my mind rather than in the buffer at all...

The buffer can have whatever rubbish in it as long as there is an easy way
of detecting the thermocouple is broken...

How to report broken hardware is always unclear in kernel, but
here I think a simple count of observed instances of it being open
would do the job nicely.

What do you think?

Jonathan
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