linux-kernel - Re: [PATCH v2 6/9] rust: device: implement Bound device context

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Message-ID: <D96BDR2E04KF.1CSIS0TVB0YI6@proton.me>
Date: Mon, 14 Apr 2025 11:10:50 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: bhelgaas@...gle.com, kwilczynski@...nel.org, gregkh@...uxfoundation.org, rafael@...nel.org, abdiel.janulgue@...il.com, ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, daniel.almeida@...labora.com, robin.murphy@....com, linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/9] rust: device: implement Bound device context

On Mon Apr 14, 2025 at 12:56 PM CEST, Danilo Krummrich wrote:
> On Mon, Apr 14, 2025 at 10:49:49AM +0000, Benno Lossin wrote:
>> On Sun Apr 13, 2025 at 7:37 PM CEST, Danilo Krummrich wrote:
>> > The Bound device context indicates that a device is bound to a driver.
>> > It must be used for APIs that require the device to be bound, such as
>> > Devres or dma::CoherentAllocation.
>> >
>> > Implement Bound and add the corresponding Deref hierarchy, as well as the
>> > corresponding ARef conversion for this device context.
>> >
>> > Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>> > ---
>> >  rust/kernel/device.rs | 16 +++++++++++++++-
>> >  1 file changed, 15 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
>> > index 487211842f77..585a3fcfeea3 100644
>> > --- a/rust/kernel/device.rs
>> > +++ b/rust/kernel/device.rs
>> > @@ -232,13 +232,19 @@ pub trait DeviceContext: private::Sealed {}
>> >  /// any of the bus callbacks, such as `probe()`.
>> >  pub struct Core;
>> >  
>> > +/// The [`Bound`] context is the context of a bus specific device reference when it is guranteed to
>> > +/// be bound for the duration of its lifetime.
>> > +pub struct Bound;
>> 
>> One question about this: is it possible for me to
>> 1. have access to a `ARef<Device<Bound>>` (or `Core`) via some callback,
>> 2. store a clone of the `ARef` in some datastructure,
>> 3. wait for the device to become unbound,
>> 4. use a `Bound`-only context function and blow something up?
>
> You can never get an ARef<Device> that has a different device context than
> Normal.
>
> A device must only ever implement AlwaysRefCounted for Device (i.e.
> Device<Normal>).

Ah yes, I thought there was something there, that's perfect. Might also
be a good idea to write this down before it becomes tribal knowledge :)

---
Cheers,
Benno