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iio: accel: dmard09: Implement IIO_CHAN_INFO_SCALE

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Reading the in_accel_scale attribute on the DMARD09 has always returned
-EINVAL: the channels advertise scale via info_mask_shared_by_type so the
IIO core exposes the attribute, but dmard09_read_raw() only handles
IIO_CHAN_INFO_RAW, so a SCALE read falls through to 'default: return
-EINVAL':

    $ cat .../iio:deviceX/in_accel_scale
    cat: in_accel_scale: Invalid argument

leaving userspace with raw counts it cannot convert to m/s^2.

The driver was written from a vendor source [1] without a datasheet, and
the scale was declared but never implemented. The vendor source carries
the sensitivity: its conversion is

    acc = raw * GRAVITY_EARTH_1000 / sensitivity   (then / 1000 -> m/s^2)

with sensitivity = 32 and GRAVITY_EARTH_1000 = 9807 ("about
(9.80665)*1000"), i.e. 32 counts correspond to 1 g.

That sensitivity applies to the value this driver already reports as raw:
the vendor reduces each 16-bit sample to a signed 9-bit value, and the
preparation in dmard09_read_raw() yields the same value. It is
self-consistent: 256 counts / 32 = 8 g full scale, matching the +/-8g
range.

Implement the scale derived from that sensitivity using standard gravity:

    scale = 9.80665 / 32 = 0.3064578125 m/s^2 per LSB

Link: 1f49d8c87b/custom/common/kernel/accelerometer/dmard09/dmard09.c [1]
Fixes: a4fa6509dd ("iio: accel: add support for the Domintech DMARD09 3-axis accelerometer")
Signed-off-by: Mert Seftali <mertsftl@gmail.com>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
This commit is contained in:
Mert Seftali 2026-07-10 10:36:23 +02:00 • committed by Jonathan Cameron
commit aa58ecc734
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