On Mon, 21 Mar 2022 09:04:11 +0100 Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote: > 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. Whilst I don't personally agree with the definition of "compatible" and think you are making false distinctions between hardware and software... I'll accept Rob's statement of best practice. However we can't just add a compatible that won't work if someone uses it on a new board that happens to run an old kernel. Jonathan > > 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
