rust/kernel: chrdev: remove unnecessary pinning

[TheSven73]

chrdev's Registration and RegistrationInner do not contain any
self-referential or borrowed-with-kernel structures. Therefore they
do not require pinning.

Remove all pinning. This simplifies the code greatly, and eliminates
an unsafe block, and a fallible Box allocation.

As part of this commit, also update chrdev client drivers.

Signed-off-by: Sven Van Asbroeck thesven73@gmail.com

[ksquirrel]

Review of aae24e980c83:

• ✔️ Commit aae24e9: Looks fine!

[nbdd0121]

I think we should keep the pinning, and use cdev_init instead of cdev_alloc. This removes an indirection and an allocation.

[TheSven73]

I'm in the process of learning about pinning. Can folks confirm that the pinning is indeed redundant here? I'm not 100% sure myself.

If so, then pinning can be removed. Next, the question arises whether it should be removed. That's a different question altogether :)

[nbdd0121]

Pinning essentially means that a stable address is wanted and the object cannot be freely moved. So miscdev requires pinning because it contains a bindings::miscdevice. chrdev does not currently requires pinning, because it only contains pointers to bindings::cdev. So yes, it can be removed.

But I think we probably want to remove this indirection, and let RegistrationInner contains bindings::cdev instead. If we do that, then pinning would be required. So I think we probably shouldn't remove pinning for now.

[TheSven73]

Very simple to do - just revert #207

[nbdd0121]

Ah, sorry, I am not aware of the PR.

We could make RegistrationInner itself a kobject and the parent of cdev it contains though? I am just pointing out reasons that we might want it pinned, not that we should or should not make the change.

[TheSven73]

@ojeda How can I assign a "Prio: low" label to this PR?

[ojeda]

Let me give you Triage permissions :)

[TheSven73]

No consensus that this is worthwhile.