Re: [PATCH 0/3] dt-bindings: arm: qcom: define schema, not devices

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

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