On 07/05/2022 19:04, Krzysztof Kozlowski wrote: > On 06/05/2022 23:33, Doug Anderson wrote: >> Hi, >> >> On Wed, May 4, 2022 at 12:04 AM Krzysztof Kozlowski >> <krzysztof.kozlowski@xxxxxxxxxx> wrote: >>> >>>>>>> The most specific compatible identifies or, like recently Rob confirmed >>>>>>> in case of Renesas, the list of compatibles: >>>>>>> https://lore.kernel.org/linux-devicetree/Yk2%2F0Jf151gLuCGz@xxxxxxxxxxxxxxxxxx/ >>>>>> >>>>>> I'm confused. If the device tree contains the compatibles: >>>>>> >>>>>> "google,lazor-rev4", "google,lazor-rev3", "google,lazor", "qualcomm,sc7180" >>>>>> >>>>>> You want to know what board you're on and you look at the compatible, >>>>>> right? You'll decide that you're on a "google,lazor-rev4" which is the >>>>>> most specific compatible. ...but you could have booted a >>>>>> "google,lazor-rev3". How do you know? >>>>> >>>>> Applying the wrong DTB on the wrong device will always give you the >>>>> wrong answer. You can try too boot google,lazor-rev3 on x86 PC and it >>>>> does not make it a google,lazor-rev3... >>>> >>>> I don't understand what you're saying here. If a device tree has the compatible: >>>> >>>> "google,lazor-rev4", "google,lazor-rev3", "google,lazor", "qualcomm,sc7180" >>>> >>>> You wouldn't expect to boot it on an x86 PC, but you would expect to >>>> boot it on either a "google,lazor-rev4" _or_ a "google,lazor-rev3". >>> >>> Yes, but booting it does not mean that the hardware is rev3 or rev4. >>> Booting it means only that we are running DTB on a compatible hardware. >>> The DTB determines what is accessible to user-space, not what *really* >>> the hardware is. The user-space (since we are going now to original >>> question) reads it and can understand that it is running on hardware >>> compatible with rev3 - either rev3 or rev4 - and act accordingly. >>> >>>> Correct? Now, after we've booted software wants to look at the >>>> compatible of the device tree that was booted. The most specific entry >>>> in that device tree is "google,lazor-rev4". ...but we could have >>>> booted it on a "google,lazor-rev3". How can you know? >>> >>> No, providing and loading a rev4 DTB on a rev3 board is not correct and >>> does not make any sense. rev3 boards are not compatible with rev4, it's >>> the other way. Not every fruit is an apple, but every apple is a fruit. >>> This is why I used that example - if you load rev4 DTB on rev3 hardware >>> then you have totally wrong booting process. >> >> I think this is the crux of the difference in opinion and there's no >> reasonable way I'm aware of to do what you're asking. If -rev3 and >> -rev4 are identical from a software point of view it would be silly >> not to share a device tree for the two of them. The number of device >> trees we'd have to land in the kernel tree would be multiplied by >> several times and we'd have many that are identical except for this >> compatible string. I see no benefit here and lots of downside. > > Wait, we agreed that you don't consider them identical, didn't we? If > they are identical, you do not need rev4 at all. So they are not > identical... > > If they are identical, just use rev3 and problem is gone. > If they are not identical or you need to assume there will be difference > (for future), then just go with rev3 without fallback to rev3 and also > problem is gone. This should be: If they are not identical or you need to assume there will be difference (for future), then just go with rev4 without fallback to rev3 and also problem is gone. > > Right now it's not possible to validate QCOM DTSes against DT bindings > because they throw big fat warnings about undocumented top compatibles. > This is a downside for us. > > Remember, you do not have to use Devicetree or Linux at all if it causes > you some downsides... No one is forced. :) If you choose to use it, > sorry, it comes with some requirements like being following Devicetree > specification or the binding guidelines. > > Best regards, > Krzysztof Best regards, Krzysztof
