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

[Mert Seftali <mertsftl@gmail.com>]

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>
---
Changes in v2:
- Reword the opening around reading the attribute and merge the duplicated
  explanation into it.
- Describe the sample as a signed 9-bit value in plain language instead of
  the register bit operations (the vendor code is in the link).
- Turn the vendor-source URL into a Link: tag.
- Express the scale via the IIO_G_TO_M_S_2() helper instead of a
  precomputed constant.
- Add a blank line before the return in the SCALE case.

 drivers/iio/accel/dmard09.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/iio/accel/dmard09.c b/drivers/iio/accel/dmard09.c
index fe35a1270786..6f0497ab6133 100644
--- a/drivers/iio/accel/dmard09.c
+++ b/drivers/iio/accel/dmard09.c
@@ -8,6 +8,7 @@
 #include <linux/unaligned.h>
 #include <linux/module.h>
 #include <linux/i2c.h>
+#include <linux/units.h>
 #include <linux/iio/iio.h>
 
 #define DMARD09_DRV_NAME	"dmard09"
@@ -79,6 +80,12 @@ static int dmard09_read_raw(struct iio_dev *indio_dev,
 		*val = accel;
 
 		return IIO_VAL_INT;
+	case IIO_CHAN_INFO_SCALE:
+		*val = 0;
+		/* 1 g / 32 LSB, in m/s^2 */
+		*val2 = IIO_G_TO_M_S_2(NANO / 32);
+
+		return IIO_VAL_INT_PLUS_NANO;
 	default:
 		return -EINVAL;
 	}
-- 
2.55.0