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    iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs · 3adbf342
    Martin Blumenstingl authored
    This adds support for the SAR (Successive Approximation Register) ADC
    on the Amlogic Meson SoCs.
    
    The code is based on the public S805 (Meson8b) and S905 (GXBB)
    datasheets (see [0] and [1]), as well as by reading (various versions
    of) the vendor driver and by inspecting the registers on the vendor
    kernels of my testing-hardware.
    
    Currently the GXBB, GXL and GXM SoCs are supported. GXBB hardware has
    10-bit ADC resolution, while GXL and GXM have 12-bit ADC resolution.
    The code was written to support older SoCs (Meson8 and Meson8b) as well,
    but due to lack of actual testing-hardware no of_device_id was added for
    these.
    
    Two "features" from the vendor driver are currently missing:
    - the vendor driver uses channel #7 for calibration (this improves the
      accuracy of the results - in my tests the results were less than 3%
      off without calibration compared to the vendor driver). Adding support
      for this should be easy, but is not required for most applications.
    - channel #6 is connected to the SoCs internal temperature sensor.
      Adding support for this is probably not so easy since (based on the
      u-boot sources) most SoC versions are using different registers and
      algorithms for the conversion from "ADC value" to temperature.
    
    Supported by the hardware but currently not supported by the driver:
    - reading multiple channels at the same time (the hardware has a FIFO
      buffer which stores multiple results)
    - continuous sampling (this would require a way to enable this
      individually because otherwise the ADC would be drawing power
      constantly)
    - interrupt support (similar to the vendor driver this new driver is
      polling the results. It is unclear if the IRQ-mode is supported on
      older (Meson6 or Meson8) hardware as well or if there are any errata)
    
    [0]
    http://dn.odroid.com/S805/Datasheet/S805_Datasheet%20V0.8%2020150126.pdf
    [1] http://dn.odroid.com/S905/DataSheet/S905_Public_Datasheet_V1.1.4.pdf
    
    
    
    Signed-off-by: default avatarMartin Blumenstingl <martin.blumenstingl@googlemail.com>
    Tested-by: default avatarNeil Armstrong <narmstrong@baylibre.com>
    Signed-off-by: default avatarJonathan Cameron <jic23@kernel.org>
    3adbf342