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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-iio" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
