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Because in practice, APIs inside Linux do, in fact, change quite a bit -- and by itself maybe that wouldn't matter so much, but the nvidia driver has an insane amount of surface area on top of it. It's a massive driver. You can imagine then, that breaking it is actually easier than you might think.

There is no rule kernel interfaces can only change on major bumps. In reality, they change quite frequently, as new APIs and drivers are merged in, which requires generalization, refactoring, etc across API boundaries to keep things sane. Kernel developers specifically reject the notion of a "stable ABI" like this because they feel it would tie their hands, and lead them to design APIs and workarounds for things which would otherwise be fundamentally simple if you "just" break some function and its call sites. APIs in Linux tend to organically grow, and die, as they are needed, by this logic.

Why wait 5 years for a "major version bump" to delete an API call, you could just do it today and fix the callers, since they're all right there in the kernel tree? It's far easier and more straightforward to do this than attempting to work around "stable" systems for very long periods of time, which is likely to accumulate cruft.

Because they do not care about out-of-tree code, when an API changes, their obligations are to refactor the code using that API, inside the kernel, and nothing else. That means the person making the change also has to fix all the other drivers, too, even if they don't necessarily maintain them. Out of tree users will have to adapt on their own.

This also explains why they do not want a HAL. When a Linux driver interface changes, the person changing it is responsible for changing everything else and fixing other drivers. That means if AMD wants a large change, it may have to go and touch the Intel driver and refactor it to match the new API. If Intel wants something new, they may have to touch the AMD driver in turn. This, in effect, helps reduce the burden and share responsibilities among the affected people.

They don't want a HAL because a HAL is a massive impediment to exactly that workflow. If Intel wants to improve a DRM/DRI interface in the kernel for their GPUs, they could normally do so and touch all the other drivers. Out with the old, in with the new. But now, they'd have to also wade through like 50,000 lines of AMD abstraction code that no other system, no other driver, uses. It effectively makes life worse for every graphics subsystem maintainer when this happens, except for AMD I guess since they can pawn off some of the work. But if AMD plays by the rules -- Intel fixing their AMDGPU driver when they make a change shouldn't be that unusual, or any more difficult compared any other graphics driver. And likewise -- AMD making a change and having to fix Intel's driver? That's just par for the course.

Obviously Linux isn't perfect here and they do, and have, accepted questionable things in the past, or have rejected seemingly reasonable API changes out of stability fear (while simultaneously not wanting a stable ABI -- which is fair). But the logic is basically something like the above, as to why this is all happening.