Re: [PATCH v2 3/3] iio: magnetometer: st_magn: honour st,fullscale-milligauss DT property

[Andy Shevchenko]

On Tue, Jun 23, 2026 at 08:29:16PM +0100, Jonathan Cameron wrote:
> On Tue, 16 Jun 2026 15:02:06 +0200
> Herman van Hazendonk <github.com@xxxxxxxxxx> wrote:
> 
> > The ST magnetometer core's common probe hardcodes fs_avl[0] -- the
> > highest-sensitivity full-scale supported by the chip -- as the
> > starting range. For the LSM303DLH that is +/-1.3 G; for the
> > LSM303DLHC and LSM303DLM it is +/-2 G; for the LIS3MDL it is +/-4 G.
> > 
> > That is the right default for "minimal noise floor at a desk", but
> > it leaves no margin for boards that pick up appreciable DC bias from
> > nearby PCB structures. On the HP TouchPad (apq8060 / tenderloin) the
> > LSM303DLH magnetometer is mounted close enough to the surrounding
> > power planes that X reads back as the chip's 0xF000 overflow
> > sentinel (== -4096 raw, the value the chip publishes when the ADC
> > saturates) on every sample at the chip-default range, while Y and Z
> > fall well within the +/-1.3 G window.
> > 
> > Parse the st,fullscale-milligauss device-tree property (documented
> > separately in dt-bindings/iio/st,st-sensors.yaml) in the
> > magnetometer common probe to select the initial fs_avl entry by its
> > mg value. The DT binding pins the accepted value set per compatible
> > via allOf/if-then enum clauses, so a malformed mg value fails
> > dt_binding_check rather than reaching the driver. Sensors with a
> > fixed full-scale (fs.addr == 0: LSM303AGR, LIS2MDL, IIS2MDC) have no
> > register to switch and the property is rejected outright for them
> > in the binding; the parse block is additionally gated on fs.addr as
> > defence in depth against stale DTBs.
> > 
> > Per-sensor mg ranges are listed in st_magn_sensors_settings[]. For
> > LSM303DLH and LSM303DLHC/DLM the valid values are 1300, 1900, 2500,
> > 4000, 4700, 5600 and 8100; for LIS3MDL, LSM9DS1-magn and LSM303C-magn
> > they are 4000, 8000, 12000, 16000.
> > 
> > Empirical scale sweep on the HP TouchPad confirmed that on this
> > board any fs_avl >= 1 produces non-saturated X readings:
> > 
> >     scale (0.001 G/LSB)  | X raw    Y raw    Z raw
> >     --------------------+-------------------------------
> >             1.100        | -4096    44       46    (X saturated)
> >             0.855        |  -547    37       37    (clean)
> >             0.670        |  -433    94      103    (clean)
> >             0.450        |  -266    44       71    (clean)
> >             0.400        |  -235    34       65    (clean)
> >             0.330        |  -196    27       56    (clean)
> >             0.230        |  -145    15       40    (clean)
> > 
> > 2500 mg is the natural choice for tenderloin: comfortably outside
> > the saturation regime while keeping useful precision for compass
> > applications.
> > 
> > Assisted-by: Claude:claude-opus-4-7 sparse smatch clang-analyzer coccinelle checkpatch
> > Assisted-by: Sashiko:claude-opus-4-7
> Hmm. First time I remember seeing Sashiko credited like this. Seems like pretty much
> every patch series of any complexity would end up crediting sashiko.
> Out of curiosity were you just looking at reports, or were you running it locally to
> help with development?

I believe it's the second one, because LKML version uses Gemini (as far as I
understand the case). At least that's why I haven't commented on this tag.

-- 
With Best Regards,
Andy Shevchenko