sensor.h API semantics and consistency.

Marcus Shawcroft <marcus.shawcroft@...>


The sensor.h API provides various SENSOR_CHAN_*_ANY channels. The
intent of these, and so far as I can see all current use of these ANY
channels is to represent a the tuple of all of a group of related X,
Y, Z channels. Given that it always refers to *all* of the related
channels rather than any individual one of the related set I wonder
whether it would be less confusing if we rename then as

Thoughts ?

The sensor attributes provide a mechanism to set an OFFSET on any
specific channel. This would appear to be intended as a mechanism to
exploit a hardware capability provided by some devices to configure an
arbitrary measurement offset. We have ~ 2 drivers that support this
attribute in the tree.

Given the current API, an application (or other driver user) who might
want to use the OFFSET mechanism must either:
1) Assume no driver implements OFFSET and post process results itself.
2) Hardwire knowledge that it is using the driver for a specific
device that does, or does not suport the OFFSET feature.
3) Attempt to use OFFSET, detect the ENOTSUP (or other error code used
for the same reason) and fallback to post processing results itself.

Number 1 essentially means the OFFSET mechanism serves no purpose
unless it is ubiquitous.
Number 2 undermines the device abstraction property of a platform OS
Number 3 means increased application logic and to a certain extent
also undermines the device abstraction.

From an applications / device driver users perspective the usability
of the API would be much improved if the OFFSET attribute feature was
either mandatory for a specific channel type across all drivers (or
removed completely). A driver for hardware with no offset feature
can easily emulate the behaviour within the driver itself.

I think we should give consideration to mandating driver support for
OFFSET attributes either for all channels or for specific channels (ie
those where we already have driver support for OFFSET).

I think this argument for consistency applies equally to other attribute types.

Thoughts ?

The SENSOR_ATTR_CALIB_TARGET, is I believe intended as a mechanism to
allow an application to provide a driver with a "right answer" at the
current point in time, and tell it to calibrate itself such that its
results match the calib target ?

The ATTR_OFFSET mechanism and the ATTR_CALIB_TARGET mechanism have
potential to interact on a channel where both are supported, I think
we should clarify in the API how they are intended to interact.
Notably if an application issues andOFFSET followed by a CALIB_TARGET
for the same channel, we should define how the current offset affects
the behaviour of the CALIB operation. I can imagine two alternatives:
1) CALIB has the effect of zeroing any current OFFSET.
2) Any current OFFSET is added to the CALIB target before calibration
and the current OFFSET remains in force after the CALIB operation.

I think 1) is intuitively more obvious and I can think of no benefits
in choosing 2).



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