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. - 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
