[PATCH v1 0/3] Support ROHM BU27008 RGB sensor

[Matti Vaittinen]

Add support for ROHM BU27008 RGB sensor.

The ROHM BU27008 is a sensor with 5 photodiodes (red, green, blue, clear
and IR) with four configurable channels. Red and green being always
available and two out of the rest three (blue, clear, IR) can be
selected to be simultaneously measured. Typical application is adjusting
LCD backlight of TVs, mobile phones and tablet PCs.

This series supports reading the RGBC and IR channels using IIO
frameeork. However, only two of the BC+IR can be enabled at the same
time. Series adds also support for scale and integration time
configuration, where scale consists of impact of both the integration
time and hardware gain. The gain and time support is backed by the newly
introduced IIO GTS helper. This series depends on GTS helper patches
added in BU27034 support series which is already merged in iio/togreg
which this series is based on.

The hardware allows configuring gain setting by writing a 5-bit gain
selector value to a register. Part of the gain setting is common for all
channels (RGBC + IR) but part of the selector value can be set
separately for RGBC and IR:

MODE_CONTROL2 REG:
bit 7	    6	    5	    4	    3	    2	    1	    0
-----------------------------------------------------------------
|	RGB	selector		|
+---------------------------------------+
-----------------------------------------------------------------
| high bits IR	|			| low bits IR selector	|
+---------------+			+-----------------------+

In theory it would be possible to set certain separate gain values for
RGBC and IR channels, but this gets pretty confusing because there are a
few 'unsupported' selector values. If only RGBC or IR was set, some
extra handling should be done to prevent the other channel from getting
unsupported value due to change in high-bits. Furthermore, allowing the
channels to be set different gain values (in some cases when gains are
such the HW supports it) would make the cases where also integration
time is changed to achieve correct scale ... interesting. It might also
be confusing for user to try predicting when setting different scales
succeeds and when it does not. Furthermore, if for example the scale
setting for RGBC caused IR selector to be invalid - it could also cause
the IR scale to "jump" very far from previous value.

To make the code simpler and more predictable for users, the current
logic is as follows:

1. Prevent setting IR scale. (My assumption is IR is less used than
RGBC)
2. When RGBC scale is set, set also the IR-selector to the same value.
This prevents unsupported selector values and makes the IR scale changes
predictable.

The 2) could mean we effectively have the same scale for all channels.
Unfortunately, the HW design is slightly peculiar and selector 0 means
gain 1X on RGBC but gain 2X on IR. Rest of the selectors equal same gain
values on RGBC and IR. The result is that while changin selector from 0
=> 1 causes RGBC gain to go from 1X => 4X, it causes IR gain to go from
2X => 4X.

So, the driver provides separate scale entries for all channels (also
RGB and C will have separate gain entries because these channels are of
same type as IR channel). This makes it possible for user applications
to go read the scales for all channels after setting scale for one (in
order to detect the IR scale difference).

Having the separate IR scale entry which applications can read to detect
"arbitrary scale changes" makes it possible for applications to be
written so they can cope if we need to implement the 'allow setting some
different gains for IR and RGBC' - later.

Finally, the scales_available is also provided for all other channels
except the IR channel, which does not allow the scale to be changed.

The sensor provides a data-ready IRQ and the driver implements a
triggered buffer mode using this IRQ as a trigger.

---

Matti Vaittinen (3):
  dt-bindings: iio: light: ROHM BU27008
  iio: light: ROHM BU27008 color sensor
  MAINTAINERS: Add ROHM BU27008

 .../bindings/iio/light/rohm-bu27008.yaml      |   49 +
 MAINTAINERS                                   |    3 +-
 drivers/iio/light/Kconfig                     |   14 +
 drivers/iio/light/Makefile                    |    1 +
 drivers/iio/light/rohm-bu27008.c              | 1028 +++++++++++++++++
 5 files changed, 1094 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/devicetree/bindings/iio/light/rohm-bu27008.yaml
 create mode 100644 drivers/iio/light/rohm-bu27008.c


base-commit: 52cc189b4fc6af6accc45fe7b7053d76d8724059
-- 
2.40.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =]

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