Add support for LTC2688

The ABI defined for this driver has some subtleties that were previously
discussed in this RFC [1]. This might not be the final state but,
hopefully, we are close to it:

toggle mode channels:

 * out_voltageY_toggle_en
 * out_voltageY_raw0
 * out_voltageY_raw1
 * out_voltageY_symbol

dither mode channels:

 * out_voltageY_dither_en
 * out_voltageY_dither_raw
 * out_voltageY_dither_raw_available
 * out_voltageY_dither_offset
 * out_voltageY_dither_frequency
 * out_voltageY_dither_frequency_available
 * out_voltageY_dither_phase
 * out_voltageY_dither_phase_available

Default channels won't have any of the above ABIs. A channel is toggle
capable if the devicetree 'adi,toggle-mode' flag is set. For dither, the
assumption is more silent. If 'adi,toggle-mode' is not given and a
channel is associated with a TGPx pin through 'adi,toggle-dither-input',
then the channel is assumed to be dither capable (there's no point in
having a dither capable channel without an input clock).

changes in v2:

 ltc2688:
  * Use local buffer for regmap read. Do not assume that reg is part of
larger buffer;
  * Renamed GPIO to "clr" so that is consistent with the datasheet;
  * Renamed 'mask' and 'm' to info. 'mask' is a thing from the past;
  * Removed 'LTC2688_CHAN_TOGGLE()' and defined to static ext_info arrays;
  * Use 'regmap_set_bits' to set external ref;
  * Use FIELD_{PREP|GET} for dither amplitude and channel calibbias where
only 13bits are used;
  * Use 'regmap_write()' instead of update_bits for channels settings;
  * Init 'val' at the beginning of the channel configuration loop
(and drop mask);
  * Comment 'ltc2688_reg_writable()' to account for the special condition;
  * Kmemdup default channels so that it can be safely changed per probed
device;
  * Replace extended info multiplexer functions by individual functions;
  * Use raw0 ABI for toggle channels;
  * Use dedicated offset ABI for dither channels;
  * Misc changes (spell fixes, blank lines...);
  * Have a clock property per channel. Note that we this I moved to OF
since we now have to use 'devm_get_clk_from_child()' which is using
device_node. Note that I could use 'to_of_node()' but mixing of.h and
property.h does not feel like a good idea.

 ABI:
  * Added out_voltageY_raw0 ABI for toggle mode;
  * Added out_voltageY_dither_offset.

 Bindings:
  * Use standard microvolt unit;
  * Added constrains for adi,output-range-microvolt and removed negative
values from the dts example;
  * Moved clocks to the channel object;
  * Dropped clock-names;
  * Add a dependency between 'adi,toggle-dither-input' and 'clocks'.

Changes in v3:

 ltc2688:
  * Fix mismatch between functions and function pointers detected by kernel
test bot; 
  * Always use if (ret) when ret > 0 has no meaning;
  * Rename ltc2688_bulk_disable -> ltc2688_disable_regulators;
  * Report dither phase in radians rather than degrees.

 ABI:
  * Specify units for dither_phase and dither_freqency; 
  * Say why its useful to have dither_en and toggle_en;
  * Combine out_voltageY_raw0 and out_voltageY_raw1;
  * Fix some description issues in out_voltageY_raw{0|1} and
out_voltageY_symbol.

 Bindings:
  * Remove mentions to ABI (linux specifix);
  * Slightly rephrased VREF and adi,toggle-dither-input properties and
suggested.
   
changes in v4:

 ltc2688:
  * Use reg_size + val_size instead of plain 3 in regmap;
  * Use out_unlock instead of unlock in goto labels;
  * Add comma to LTC2688_CHANNEL(), ltc2688_regmap_bus and
ltc2688_regmap_bus;
  * Use __clear_bit() instead of clear_bit();
  * Flip the logic in vref regulator so that error condition is handled
first;
  * Change to device API. With this, we need to_of_node()
for devm_get_clk_from_child().

 ABI:
  * Update kernel version.

 Bindings:
  * Add Rob's Rb tag.

[1]: https://marc.info/?l=linux-iio&m=163662843603265&w=2

Nuno Sá (3):
  iio: dac: add support for ltc2688
  iio: ABI: add ABI file for the LTC2688 DAC
  dt-bindings: iio: Add ltc2688 documentation

 .../ABI/testing/sysfs-bus-iio-dac-ltc2688     |   86 ++
 .../bindings/iio/dac/adi,ltc2688.yaml         |  146 +++
 MAINTAINERS                                   |    9 +
 drivers/iio/dac/Kconfig                       |   11 +
 drivers/iio/dac/Makefile                      |    1 +
 drivers/iio/dac/ltc2688.c                     | 1071 +++++++++++++++++
 6 files changed, 1324 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2688
 create mode 100644 Documentation/devicetree/bindings/iio/dac/adi,ltc2688.yaml
 create mode 100644 drivers/iio/dac/ltc2688.c

-- 
2.34.1