On 20/03/2022 16:12, Jonathan Cameron wrote: > On Thu, 10 Mar 2022 22:24:03 +0100 > Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxx> 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). Different value of HW register within the same programming model does not make him incompatible. Quite contrary - it is compatible and to differentiate variants you do not need specific compatibles. Using arguments how driver behaves is wrong. Driver does not determine hardware/bindings. > >> 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. Driver implementation is not related to bindings, does not matter. You cannot use driver implementation as argument in discussion about bindings and compatibility. Implementation differs, is limited, can be changed. > 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. Still not argument about bindings and compatibility but about driver. > + There is code out there today doing this so inherently it is not > compatible. Still code of driver, not bindings/DTS/hardware. > > 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. There is no backwards compatibility of advanced path, so the DMP core. The device (not driver, we do not talk here about driver) is compatible with basic version fully. 100%. Only this part you need to keep always compatible between each other, > Extra functionality could only make use of fields in registers marked > reserved, or of new registers that didn't exist on the simpler device. Extra functionality is for new, extended compatible. See Documentation/devicetree/bindings/ABI.rst which exactly explains this case. Best regards, Krzysztof
