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. Best regards, Krzysztof
