Re: [PATCH 1/2] dt-bindings: iio: imu: mpu6050: Document invensense,icm20608d

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 10 Mar 2022 22:24:03 +0100
Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com> wrote:

> On 10/03/2022 19:56, Michael Srba wrote:
> > Hi,
> > the thing is, the only reason the different compatible is needed at all
> > is that the chip presents a different WHOAMI, and the invensense,icm20608
> > compatible seems to imply the non-D WHOAMI value.  
> 
> But this is a driver implementation issue, not related to bindings.
> Bindings describe the hardware.

Indeed, but the key thing here is the WHOAMI register is hardware.

> 
> > I'm not sure how the driver would react to both compatibles being present,
> > and looking at the driver code, it seems that icm20608d is not the only
> > fully icm20608-compatible (to the extent of features supported by
> > the driver, and excluding the WHOAMI value) invensense IC, yet none
> > of these other ICs add the invensense,icm20608 compatible, so I guess I
> > don't see a good reason to do something different.  
> 
> Probably my question should be asked earlier, when these other
> compatibles were added in such way.
> 
> Skipping the DMP core, the new device is fully backwards compatible with
> icm20608.

No. It is 'nearly' compatible...  The different WHOAMI value (used
to check the chip is the one we expect) makes it incompatible.  Now we
could change the driver to allow for that bit of incompatibility and
some other drivers do (often warning when the whoami is wrong but continuing
anyway). 

> Therefore extending the compatible makes sense. This is not
> only correct from devicetree point of view, but also is friendly towards
> out of tree users of bindings.
> 
> The Linux driver behavior about whoami register does not matter here.
> Not mentioning that it would be easy for driver to accept multiple
> values of whoami.

I disagree entirely. Any driver that makes use of the whoami will not
be compatible with this new part.  It's a driver design choice on whether
to make use of that, but it's a perfectly valid one to refuse to probe
if it doesn't detect that the device is the one it expects.
+ There is code out there today doing this so inherently it is not
compatible.

So no, a fall back compatible is not suitable here because it simply
is not compatible.

Now, if intent was to provide a backwards compatible path from this
more advanced part then the behaviour of every register defined for
the simpler part, must be identical on the more advanced part.
Extra functionality could only make use of fields in registers marked
reserved, or of new registers that didn't exist on the simpler device.

There are other ways of handling backwards compatibility but they all
require statements in the simpler device spec about how you can tell
for future more complicated devices that they are compatible with this
spec. E.g. Feature registers, version registers etc.

Jonathan
> 
> 
> Best regards,
> Krzysztof