On Tue, 22 Mar 2022 11:23:18 +0100 Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote: > On 21/03/2022 18:42, Jonathan Cameron wrote: > > On Mon, 21 Mar 2022 16:22:38 +0100 > > Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote: > > > >> On 21/03/2022 16:04, Jonathan Cameron wrote: > >>> 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. > >>> > >> > >> The please explain me how this patch (the compatible set I proposed) > >> fails to work in such case? How a new board with icm20608 (not > >> icm20608d!) fails to work? > > > > I'm confused. An actual icm20608 would work. > > I guess you mean an icm20608d via compatible "invensense,icm20608"? > > In your example, new board with old kernel (so old kernel not supporting > icm20608d), icm20608d will work exactly the same. Meaning: not work. Old > kernel does not support it, new kernel will weirdly try to read WHOAMI > and return -EINVAL (or whatever is there). Same effect. 'work' that means 'not work' was the root of my confusion. With that in mind I now understand what you meant. + as suggested we should possibly 'fix' at least the kernels we can to relax to a warning in this case. Jonathan > > > > >> > >> To remind, the compatible has a format of: > >> comaptible = "new", "old" > >> e.g.: "invensense,icm20608d", "invensense,icm20608" > > > > Old kernel fails to match invensense,icm20608d, matches on invensense,icm20608. > > Checks the WHOAMI value and reports a missmatched value and fails the probe > > as it has no idea what the part was so no idea how to support it. > > And old kernel fails in your solution as well, because it does not know > the compatible and refuses to bind. > > > > > Obviously it wouldn't work anyway with an old kernel, but > > without the fallback compatible at least there would be no error message > > saying that the device is not the icm20608 we expected to see. > > You said before: > "...that won't work if someone uses..." > so still please explain how does this "will not work" happens. It does > not work with old kernel in both cases... > > Best regards, > Krzysztof
