Re: iio: accel: dmard09: in_accel_scale always returns -EINVAL

[Mert Seftali]

On Mon, 29 Jun 2026 21:06:..., Jelle van der Waa wrote:
> Thanks for digging into this, I no longer have the device in question.
> But the vendor driver can be found on GitHub:
>
> https://github.com/minstrelsy/mediatek/blob/.../dmard09/dmard09.c

Thanks Jelle, got this from the repository;

It has the sensitivity, and it's used in the driver's own conversion:

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

with sensitivity = 32 and GRAVITY_EARTH_1000 = 9807 ("about
(9.80665)*1000" in the MTK header), so 32 counts == 1 g.

It also clears up that bit-twiddling i was worried about. The vendor does
'data >>= 3; data &= 0x1ff' plus a sign-extend of bit 8 on the 16-bit
read, i.e. a signed 9-bit value (register bits [11:3]), and our '<<4 >>7'
dance lands on the exact same number. So the 32 applies to what the driver
already returns as raw. Self consistent too: 256 / 32 = 8g, which matches
the +/-8g.

So this time i think we can implement it instead of dropping it:

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

happy to send that Jonathan, it's the vendor's 32, so same
no-datasheet situation as before. or just drop the scale declaration if
you'd rather not. Whichever way you point.


Thanks,
Mert


Am Mo., 29. Juni 2026 um 21:06 Uhr schrieb Jelle van der Waa <jelle@xxxxxxxx>:
>
> Hi,
>
> On 17/06/2026 17:53, Mert Seftali wrote:
> > Hi,
> >
> > I've been digging into IIO to learn my way around it, and the dmard09
> > accelerometer driver caught my eye as I think its scale attribute has
> > been broken since day one, so I wanted to flag it.
> >
> > The channels advertise scale:
> >
> >      .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
> >
> > which makes the core create in_accel_scale. The trouble is
> > dmard09_read_raw() never handles IIO_CHAN_INFO_SCALE, it only does
> > RAW, and SCALE drops straight into 'default: return -EINVAL'. So
> > reading the scale just errors out:
> >
> >      $ cat .../iio:deviceX/in_accel_scale
> >      cat: in_accel_scale: Invalid argument
> >
> > and userspace is left with raw counts it can't turn into m/s^2.
> >
> > To be clear about how I found this: I don't have the hardware, I traced
> > it through the core (iio_read_channel_info() -> read_raw() with the
> > SCALE mask) rather than seeing it fail on a device. But the path looks
> > unambiguous.
> >
> > I went looking to fix it properly; add a SCALE case returning the
> > sensitivity, the way dmard06 and dmard10 do, but I couldn't find a
> > DMARD09 datasheet anywhere. Digging through the list archives, it looks
> > like the scale was declared in the original 2016 submission but never
> > implemented, and the driver was written from a vendor source without a
> > datasheet to begin with. So I don't have a real number to use, and I'd
> > rather not make one up.
> Thanks for digging into this, I no longer have the device in question.
> But the vendor driver can be found on GitHub:
>
> https://github.com/minstrelsy/mediatek/blob/1f49d8c87b839651bc89afc870277e8e0f2e2d55/custom/common/kernel/accelerometer/dmard09/dmard09.c
>
> Greetings,
>
> Jelle van der Waa