Re: [PATCH v2] iio: accel: dmard09: Implement IIO_CHAN_INFO_SCALE

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri, 10 Jul 2026 10:51:09 +0200
Joshua Crofts <joshua.crofts1@gmail.com> wrote:

> On Fri, 10 Jul 2026 10:36:23 +0200
> Mert Seftali <mertsftl@gmail.com> wrote:
> 
> > 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: https://github.com/minstrelsy/mediatek/blob/1f49d8c87b839651bc89afc870277e8e0f2e2d55/custom/common/kernel/accelerometer/dmard09/dmard09.c [1]
> > Fixes: a4fa6509dda4 ("iio: accel: add support for the Domintech DMARD09 3-axis accelerometer")
> > Signed-off-by: Mert Seftali <mertsftl@gmail.com>
> > ---  
> 
> Good thing that Jelle sent the vendor driver, LGTM.
Indeed
> 
> Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
> 
Applied to the testing branch of iio.git.  It's been broken
from the start, so whilst it is a fix, I'm not rushing to get
it upstream!

Thanks,

Jonathan