On 25/07/2022 18:41, Krzysztof Kozlowski wrote: > On 25/07/2022 18:25, Doug Anderson wrote: >> Let's look specifically at the device tree file for the LTE board. One >> way to look at it is that the dts for the LTE board should have >> compatibles: >> compatible = "lte", "wifi-only" >> >> The above matches the normal device tree mentality. It says: "hey, if >> you've got a lte driver for this board then use it; otherwise use the >> wifi-only driver". >> >> However, the above is actually broken for the bootloader use case. The >> bootloader is trying to pick a device tree and, to the bootloader, the >> above says "you can use this dts for either an lte board or a >> wifi-only board". That's bad. If the bootloader picks this device tree >> for a wifi-only board then the OS will try to initialize lte and >> things will crash. To go further, if you think about it things >> actually work fine if the wifi-only device tree says it's compatible >> with the LTE board. This is why I say it's opposite... ;-) > > This is not specific to "bootloaders" but your specific implementation > of entire chain. How you described it, you have dependent pieces - > user-space must use the same DTB as bootloader chosen, but bootloader > makes different choices than user-space. It's perfectly fine to make > these choices different, but then user-space should not depend on > something which was/was not initialized in bootloader. > > IOW, if bootloader picked up generic WiFi compatible and user-space will > crash if picking up specific comaptible, you have a dependency and > user-space should probably bind to modified DTB, where LTE comaptible is > removed. > > Other systems - I would say most of them - are independent, IOW, we try > to make kernel and user-space independent of what bootloader did, > because we are never sure what bootloader actually did and what DTS it > received. You can BTW compare it nicely to Linux device driver binding. If a driver binds to more generic (WiFi) compatible, it is not allowed to use any features/code related to more specific compatible (LTE). Your case breaks this rule. Bootloader bound to generic (WiFi) compatible, but it passed entire DTB/FDT to kernel/user-space which can then run code for more specific compatible. Although I understand the point the board compatibles by themself provide little help for such use case. Best regards, Krzysztof
