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Comment by aseipp | Hacker News Reader

Someone from Qualcomm posted a "DT-ACPI Hybrid Mode" set of patches a while back that let you call ACPI table functions, etc even while booting with the device tree, allowing you to control some things like keyboard lightning/power management independently. That would be a good first step but I'm not sure it went anywhere in the meantime. In theory supporting Device Tree by including the blob in a ROM somewhere shouldn't be much more work (if any) than including working ACPI tables, but we all know how that works out in practice!

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> supporting Device Tree by including the blob in a ROM somewhere shouldn't be much more work (if any) than including working ACPI tables, but we all know how that works out in practice!

This is definitely a thing in some embedded systems I’ve used. There are some Marvell boards which do this. They all use U-boot though.

The problem is that the content of the device tree isn't really stable -- if a new kernel version has a driver with a new required parameter for instance, suddenly your ROM device tree is out of date and much less useful.

Oh, that's a good point I forgot about; I suppose DT interfaces aren't considered part of the stability contract for users... Really Device Tree is somewhat "Linux-specific"-enough and low level enough that issues like this aren't ideal. But if we could just get to the point that was a problem, I'd be happier than now, I suspect!

In theory device trees are supposed to be a OS-neutral stable ABI. Being able to put a device tree into unchangeable mask ROM is an often mentioned design goal.

However, there is a fatal flaw in the process that makes this a pipe dream in reality. In Linux, these rules only apply to reviewed and merged drivers. And during review any new DT interface will almost certainly be bikeshedded in some way.

So unless your chip vendor has upstreamed ALL their drivers before a device is manufactured, the device's firmware-provided DT will very likely work only against a vendor kernel and requires updating.

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