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iio: consumers: Fix handling of negative channel scale in iio_convert_raw_to_processed()

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There is an issue with the handling of negative channel scales
in iio_convert_raw_to_processed_unlocked() when the channel-scale
is of the IIO_VAL_INT_PLUS_[MICRO|NANO] type:

Things work for channel-scale values > -1.0 and < 0.0 because of
the use of signed values in:

	*processed += div_s64(raw64 * (s64)scale_val2 * scale, 1000000LL);

Things will break however for scale values < -1.0. Lets for example say
that raw = 2, (caller-provided)scale = 10 and (channel)scale_val = -1.5.

The result should then be 2 * 10 * -1.5 = -30.

channel-scale = -1.5 means scale_val = -1 and scale_val2 = 500000,
now lets see what gets stored in processed:

1. *processed = raw64 * scale_val * scale;
2. *processed += raw64 * scale_val2 * scale / 1000000LL;

1. Sets processed to 2 * -1 * 10 = -20
2. Adds 2 * 500000 * 10 / 1000000 = 10 to processed

And the end result is processed = -20 + 10 = -10, which is not correct.

Fix this by always using the abs value of both scale_val and scale_val2
and if either is negative multiply the end-result by -1.

Note there seems to be an unwritten rule about negative
IIO_VAL_INT_PLUS_[MICRO|NANO] values that:

i.   values > -1.0 and < 0.0 are written as val=0 val2=-xxx
ii.  values <= -1.0 are written as val=-xxx val2=xxx

But iio_format_value() will also correctly display a third option:

iii. values <= -1.0 written as val=-xxx val2=-xxx

Since iio_format_value() uses abs(val) when val2 < 0.

This fix also makes iio_convert_raw_to_processed() properly handle
channel-scales using this third option.

Fixes: 48e44ce0f8 ("iio:inkern: Add function to read the processed value")
Cc: Matteo Martelli <matteomartelli3@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://patch.msgid.link/20250831104825.15097-2-hansg@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Hans de Goede
2025-08-31 12:48:20 +02:00
committed by Jonathan Cameron
parent 0f2aeee584
commit 0f85406bf8
1 changed files with 14 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

28
drivers/iio/inkern.c
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@@ -11,6 +11,7 @@
#include <linux/mutex.h>
#include <linux/property.h>
#include <linux/slab.h>
#include <linux/units.h>
#include <linux/iio/iio.h>
#include <linux/iio/iio-opaque.h>
@@ -604,7 +605,7 @@ static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
{
int scale_type, scale_val, scale_val2;
int offset_type, offset_val, offset_val2;
s64 raw64 = raw;
s64 denominator, raw64 = raw;
offset_type = iio_channel_read(chan, &offset_val, &offset_val2,
IIO_CHAN_INFO_OFFSET);
@@ -648,20 +649,19 @@ static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
*processed = raw64 * scale_val * scale;
break;
case IIO_VAL_INT_PLUS_MICRO:
if (scale_val2 < 0)
*processed = -raw64 * scale_val * scale;
else
*processed = raw64 * scale_val * scale;
*processed += div_s64(raw64 * (s64)scale_val2 * scale,
1000000LL);
break;
case IIO_VAL_INT_PLUS_NANO:
if (scale_val2 < 0)
*processed = -raw64 * scale_val * scale;
else
*processed = raw64 * scale_val * scale;
*processed += div_s64(raw64 * (s64)scale_val2 * scale,
1000000000LL);
switch (scale_type) {
case IIO_VAL_INT_PLUS_MICRO:
denominator = MICRO;
break;
case IIO_VAL_INT_PLUS_NANO:
denominator = NANO;
break;
}
*processed = raw64 * scale * abs(scale_val);
*processed += div_s64(raw64 * scale * abs(scale_val2), denominator);
if (scale_val < 0 || scale_val2 < 0)
*processed *= -1;
break;
case IIO_VAL_FRACTIONAL:
*processed = div_s64(raw64 * (s64)scale_val * scale,