Re: Why is only one int returned in iio_read_channel_processed?

[Jonathan Cameron <jic23@kernel.org>]

On 05/23/2013 02:28 PM, Lars-Peter Clausen wrote:
> On 05/23/2013 03:18 PM, Guillaume Ballet wrote:
>>>>
>>>> - if IIO_INT_VAL_PLUS_NANO is returned (common when dealing with
>>>> current sources), 32 bits is a bit tight - which is why the read_raw
>>>> function pointer in iio_info has (val, val2) in the first place.
>>>> - People like me who do not use the iio_convert_raw_to_processed
>>>> path() but need to support IIO_CHAN_INFO_PROCESSED directly in their
>>>> driver have an issue: we would need to be passed the scale in the
>>>> read_raw function of iio_info. That would impact _all_ IIO drivers.
>>>
>>> IIO_CHAN_INFO_PROCESSED is by definition supposed to return the value in the
>>> proper unit. If that doesn't work for you use IIO_CHAN_INFO_RAW +
>>> IIO_CHAN_INFO_SCALE. Think of IIO_CHAN_INFO_PROCESSED as IIO_CHAN_INFO_RAW
>>> with the scale set to 1.0
>>
>> This isn't a unit problem, this is a precision problem: if I want to
>> return a current in Ampères, for instance 5.000000001, I can't get
>> that by calling iio_read_channel_processed() (or
>> iio_read_channel_raw() for that matter) as the precision is too big to
>> fit in only one integer. The issue is that the callback that handles
>> IIO_CHAN_INFO_PROCESSED and IIO_CHAN_INFO_RAW does allow to return
>> such a value. There's an inconsistency in the interface.
> 
> I doubt anybody actually cares about the 0.000000001 in that case.
> 
>>
>>>
>>>> - The scale parameter to iio_convert_raw_to_processed() itself is an
>>>> int, and IIO_CHAN_INFO_SCALE can return a scale in the
>>>> IIO_VAL_INT_PLUS_NANO scheme. It means somewhere along the road,
>>>> precision is lost.
>>>
>>> The scale would be passed in by the consumer, so the consumer is able to
>>> specify the amount of precision it wants.
>>
>> Not if they want a precision as high as the IIO driver is able to
>> deliver: the scale returned by a IIO_CHAN_INFO_SCALE is a 64-bits
>> fixed point integer. The scaled passed to
>> iio_convert_raw_to_processed() is a 32 bit integer. If one needs great
>> precision on big numbers, it won't fit.
> 
> The problem is that there is no in kernel user who can actually make use of
> anything but a 32bit integer. If we need a larger range we should change the
> return type to a 64bit integer rather than adopting the val1, val2 scheme
> for the in-kernel API.

Just to make things clear....

If an in kernel user were proposed that actually did make good use
of this it might be a different matter. It's just that no one has suggested
one as yet. I can contrive of complex cases where it might make sense, but
until we have a real world usecase, let us leave this.
> 
> - Lars
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