Re: [PATCH 1/3] device: rust: documentation for DeviceContext

[Alexandre Courbot]

On Fri Jul 18, 2025 at 7:45 AM JST, Danilo Krummrich wrote:
> Expand the documentation around DeviceContext states and types, in order
> to provide detailed information about their purpose and relationship
> with each other.
>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>

Thanks, that's a welcome clarification and I think I finally understand
the Rust device model after going through this series!

A few minor nits/questions below.

> ---
>  rust/kernel/device.rs | 63 +++++++++++++++++++++++++++++++++++--------
>  1 file changed, 52 insertions(+), 11 deletions(-)
>
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index ca82926fd67f..d7ac56628fe5 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -311,28 +311,69 @@ unsafe impl Send for Device {}
>  // synchronization in `struct device`.
>  unsafe impl Sync for Device {}
>  
> -/// Marker trait for the context of a bus specific device.
> +/// Marker trait for the context or scope of a bus specific device.
>  ///
> -/// Some functions of a bus specific device should only be called from a certain context, i.e. bus
> -/// callbacks, such as `probe()`.
> +/// [`DeviceContext`] is a marker trait for structures representing the context of a bus specific
> +/// [`Device`].
>  ///
> -/// This is the marker trait for structures representing the context of a bus specific device.

Shall we say `types` instead of `structures`, since these are ZSTs?
`structures` carries the hint that they will contain data, when they
don't (but maybe that's only me :)).

> +/// The specific device context types are: [`CoreInternal`], [`Core`], [`Bound`] and [`Normal`].
> +///
> +/// [`DeviceContext`] types are hierarchical, which means that there is a strict hierarchy that
> +/// defines which [`DeviceContext`] type can be derived from another. For instance, any
> +/// [`Device<Core>`] can dereference to a [`Device<Bound>`].
> +///
> +/// The following enunumeration illustrates the dereference hierarchy of [`DeviceContext`] types.
> +///
> +/// - [`CoreInternal`] => [`Core`] => [`Bound`] => [`Normal`]
> +/// - [`Core`] => [`Bound`] => [`Normal`]
> +/// - [`Bound`] => [`Normal`]
> +/// - [`Normal`]

That graph is super helpful. The last 3 lines look redundant though,
since the graph can be followed from any node.

> +///
> +/// Bus devices can automatically implement the dereference hierarchy by using
> +/// [`impl_device_context_deref`](kernel::impl_device_context_deref).
>  pub trait DeviceContext: private::Sealed {}
>  
> -/// The [`Normal`] context is the context of a bus specific device when it is not an argument of
> -/// any bus callback.
> +/// The [`Normal`] context is the default [`DeviceContext`] of any [`Device`].
> +///
> +/// The normal context does not indicate any specific scope. Any `Device<Ctx>` is also a valid
> +/// [`Device<Normal>`]. It is the only [`DeviceContext`] for which it is valid to implement
> +/// [`AlwaysRefCounted`](kernel::types::AlwaysRefCounted) for.
>  pub struct Normal;

`Normal` as a name can be interpreted in many different ways, and in the
case of a device context it is not clear what the "normal" state is. I
think it would be helpful if we can elaborate a bit more on what this
implies (i.e. what concretely speaking are the limitations), and if
possible why this name has been chosen.