[PATCH 0/4] Add QST QMC5883P magnetometer driver

[Hardik Phalet <hardik.phalet@pm.me>]

This series adds initial Linux support for the QST QMC5883P, a 3-axis
anisotropic magneto-resistive (AMR) magnetometer with a 16-bit ADC that
communicates over I2C. To my knowledge there is no existing upstream
driver for this device.

The driver supports:
  - Raw magnetic field readings on X, Y, and Z axes
  - Four selectable full-scale ranges (±2 G, ±8 G, ±12 G, ±30 G)
  - Configurable output data rate (10, 50, 100, 200 Hz)
  - Configurable oversampling ratio (1, 2, 4, 8)
  - Configurable downsampling ratio (1, 2, 4, 8) via a custom sysfs
    attribute
  - Runtime PM with a 2 s autosuspend delay
  - System suspend/resume via pm_runtime_force_suspend/resume

Regmap with an rbtree cache is used throughout. CTRL_1 and CTRL_2
bit fields are accessed via regmap_field to avoid read-modify-write
races. The STATUS register is marked precious so regmap never reads
it speculatively and clears the DRDY/OVFL bits unexpectedly.

The init sequence on probe is: soft reset → wait 1 ms → deassert
reset → configure SET/RESET control → apply default ODR/OSR/DSR/RNG
→ enter normal mode. This ordering was determined empirically on
hardware to produce reliable, non-zero axis readings.

The driver is placed under drivers/staging/iio/magnetometer/ with a
TODO file tracking the remaining work before it can graduate:
  - Triggered buffer support (iio_triggered_buffer_setup)
  - DRDY interrupt support
  - Self-test implementation

Patches:
  1/4 - dt-bindings: vendor-prefixes: Add 'qst' for QST Corporation
  2/4 - dt-bindings: iio: magnetometer: Add binding for QST QMC5883P
  3/4 - staging: iio: magnetometer: Add QST QMC5883P driver
  4/4 - MAINTAINERS: Add entry for QST QMC5883P magnetometer driver

Testing
-------
Tested on a Raspberry Pi 4B running a mainline kernel (aarch64) with a
GY-271 HM-246 board connected via I2C bus 1. The chip was confirmed to
enumerate at address 0x2C via i2cdetect.

The driver was cross-compiled from Fedora (x86_64) targeting aarch64
and loaded as a module (qmc5883p.ko) with the Device Tree overlay
pointing at i2c1:0x2c.

Verification steps performed:
  - Chip ID register (0x00) reads back 0x80 on probe, confirming the
    correct device is present
  - All three axes (in_magn_x_raw, in_magn_y_raw, in_magn_z_raw) return
    non-zero, stable values when the board is held still and change
    appropriately when the board is rotated
  - in_magn_x_scale (and Y, Z) returns the expected fractional value for
    the default ±8 G range (1/37500000)
  - in_magn_sampling_frequency / _available, in_magn_oversampling_ratio /
    _available, and downsampling_ratio / downsampling_ratio_available all
    read and write correctly; the chip responds without error to each
    valid setting
  - Runtime PM: after 2 s of inactivity the device enters suspend mode
    (MODE = 0x00 confirmed via i2cdump); the next sysfs read correctly
    resumes the device and returns valid data
  - System suspend/resume (echo mem > /sys/power/state) leaves the
    driver in a consistent state; readings remain valid after resume
  - dt_binding_check passes for patch 2/4
  - Kernel builds cleanly with W=1 and no new warnings

Hardik Phalet (4):
  dt-bindings: vendor-prefixes: Add QST Corporation
  dt-bindings: iio: magnetometer: Add binding for QST QMC5883P
  Subject: [PATCH 3/4] staging: iio: magnetometer: Add QST QMC5883P
    driver
  MAINTAINERS: Add entry for QST QMC5883P magnetometer driver

 .../iio/magnetometer/qst,qmc5883p.yaml        |  48 +
 .../devicetree/bindings/vendor-prefixes.yaml  |   2 +
 MAINTAINERS                                   |   7 +
 drivers/staging/iio/Kconfig                   |   1 +
 drivers/staging/iio/Makefile                  |   1 +
 drivers/staging/iio/magnetometer/Kconfig      |  20 +
 drivers/staging/iio/magnetometer/Makefile     |   7 +
 drivers/staging/iio/magnetometer/TODO         |   5 +
 drivers/staging/iio/magnetometer/qmc5883p.c   | 819 ++++++++++++++++++
 9 files changed, 910 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/magnetometer/qst,qmc5883p.yaml
 create mode 100644 drivers/staging/iio/magnetometer/Kconfig
 create mode 100644 drivers/staging/iio/magnetometer/Makefile
 create mode 100644 drivers/staging/iio/magnetometer/TODO
 create mode 100644 drivers/staging/iio/magnetometer/qmc5883p.c

-- 
2.53.0