Re: [PATCH 5/6] IIO:hwmon interface client driver.

[Guenter Roeck <guenter.roeck@ericsson.com>]

On Mon, 2011-10-24 at 11:58 -0400, Jonathan Cameron wrote:
> On 10/24/11 16:39, Guenter Roeck wrote:
> > On Mon, 2011-10-24 at 06:09 -0400, Jonathan Cameron wrote:
> > [ ... ]
> >>>>> +/*
> >>>>> + * Assumes that IIO and hwmon operate in the same base units.
> >>>>> + * This is supposed to be true, but needs verification for
> >>>>> + * new channel types.
> >>>>> + */
> >>>>> +static ssize_t iio_hwmon_read_val(struct device *dev,
> >>>>> +				  struct device_attribute *attr,
> >>>>> +				  char *buf)
> >>>>> +{
> >>>>> +	long result;
> >>>>> +	int val, ret, scaleint, scalepart;
> >>>>> +	struct sensor_device_attribute *sattr = to_sensor_dev_attr(attr);
> >>>>> +	struct iio_hwmon_state *state = dev_get_drvdata(dev);
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * No locking between this pair, so theoretically possible
> >>>>> +	 * the scale has changed.
> >>>>> +	 */
> >>>>> +	ret = iio_read_channel_raw(state->channels[sattr->index],
> >>>>> +				   &val);
> >>>>> +	if (ret < 0)
> >>>>> +		return ret;
> >>>>> +
> >>>>> +	ret = iio_read_channel_scale(state->channels[sattr->index],
> >>>>> +				     &scaleint, &scalepart);
> >>>>> +	if (ret < 0)
> >>>>> +		return ret;
> >>>>> +	switch (ret) {
> >>>>> +	case IIO_VAL_INT:
> >>>>> +		result = val * scaleint;
> >>>>> +		break;
> >>>>> +	case IIO_VAL_INT_PLUS_MICRO:
> >>>>> +		result = (long)val * (long)scaleint +
> >>>>> +			(long)val * (long)scalepart / 1000000L;
> >>>>> +		break;
> >>>>> +	case IIO_VAL_INT_PLUS_NANO:
> >>>>> +		result = (long)val * (long)scaleint +
> >>>>> +			(long)val * (long)scalepart / 1000000000L;
> >>>>> +		break;
> >>>>
> >>>> Still easy to imagine that val * scalepart gets larger than 2147483647L
> >>>> (on machines where sizeof(long) = 4) ... it will already happen if the
> >>>> result of (val * scalepart / 1000000000) is larger than 2. 
> >>> Good point.  I really ought to have done the calcs.
> >>> If we have maximum possible value in here things will be ugly.
> >>>
> >>> Worst case is scalepart is 9999999999. (could be done as 1 - 0.000000001
> >>> which would be nicer, but we don't specify a preference - from this
> >>> discussion I am suspecting we should!)
> >>>
> >>> Looks like 64 bits is going to be a requirement as you say.
> >>>>
> >>>> What value range do you expect to see here ?
> >>>>
> >>>> If (val * scaleint) is already the milli-unit, scalepart would possibly
> >>>> only address fractions of milli-units. If so, the result of (val *
> >>>> scalepart / 1000000000L) might always be smaller than 1, ie 0. 
> >>> It certainly should be.
> >>>> If so, for the calculation to have any value, you might be better off using
> >>>> DIV_ROUND_CLOSEST(val * scalepart, 1000000000L).
> >>> Good idea.
> >>>>
> >>>> I am a bit confused by this anyway. Since hwmon in general reports
> >>>> milli-units, VAL_INT appears to reflect milli-units, VAL_INT_PLUS_MICRO
> >>>> really means nano-units, and IIO_VAL_INT_PLUS_NANO really means
> >>>> pico-units. Is this correct ?
> >>> Micro units of the scale factor.
> >>>
> >>> Take my test part a max1363...
> >>> Scale is actually 0.5 so each adc count (e.g. raw value) is 0.5millivolts.
> >>>
> >>> scale int here is 0,
> >>> scale part is 500,000 (so 0.5) and it returns IIO_VAL_INT_PLUS_MICRO.
> >>
> >> How about the following?  It'll be extremely costly, but this isn't exactly
> >> a fast path!
> >>
> >> 	case IIO_VAL_INT_PLUS_MICRO:
> >> 		result = (s64)val * (s64)scaleint +
> >> 			div_s64((s64)val * (s64)scalepart, 1000000LL);
> >> 		break;
> >> 	case IIO_VAL_INT_PLUS_NANO:
> >> 		result = (s64)val * (s64)scaleint +
> >> 			div_s64((s64)val * (s64)scalepart, 1000000000LL);
> >> 		break;
> > 
> > Is div_s64 really necessary, or would
> > 
> > 		result = (long)val * (long)scaleint +
> > 			DIV_ROUND_CLOSEST((s64)val * (s64)scalepart,
> > 					 1000000000LL);
> > 
> > work as well ?
> Not if you want it to compile on arm v5 by the look of it.
> 
> ERROR: "__aeabi_ldivmod" [drivers/staging/iio/iio_hwmon.ko] undefined!
> 
Annoying. Ok, I don't have a better idea than using div_s64. You don't
need s64 for the first part of the operation (val * scaleint), though,
since the result is a long.

Thanks,
Guenter