Why was dm->cached_state storing -12 instead of a pointer? Most likely this happened because earlier during suspend, dm_suspend() assigned dm.cached_state = drm_atomic_helper_suspend(adev_to_drm(adev)). The callee drm_atomic_helper_suspend() could return either a valid pointer, or ERR_PTR(err) which encoded errors as negative pointers. But the caller function assigned the return value directly to a pointer which gets dereferenced upon resume, instead of testing the return value for an error.
One more point for rust in the kernel. Just can't happen if you're required to handle a Result type.
But of course defaults matter and the kernel’s rich history of not modernizing coding practices is going to work against improvements in C land. Ironically, it’s that same resistance that frustrates the Rust devs so much because their resistant to even cleaning up their own subsystems or putting down markers documenting how the subsystems are supposed to work.
Maybe https://github.com/llvm/llvm-project/issues/74205 would help once it trickles down into the kernel, but I suspect that people are still going to choose to do this manual overloading of the pointer instead of using types for safety.
One more point for rust in the kernel. Just can't happen if you're required to handle a Result type.