Re: [PATCH v2 1/3] device: rust: expand documentation for DeviceContext

[Daniel Almeida <daniel.almeida@collabora.com>]

Hi Danilo,

> On 22 Jul 2025, at 11:59, Danilo Krummrich <dakr@kernel.org> wrote:
> 
> Expand the documentation around DeviceContext states and types, in order
> to provide detailed information about their purpose and relationship
> with each other.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
> rust/kernel/device.rs | 69 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 58 insertions(+), 11 deletions(-)
> 
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index ca82926fd67f..f5d1db568f00 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -311,28 +311,75 @@ 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 types representing the context of a bus specific
> +/// [`Device`].
> ///
> -/// This is the marker trait for structures representing the context of a bus specific device.
> +/// 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`]
> +///
> +/// Bus devices can automatically implement the dereference hierarchy by using
> +/// [`impl_device_context_deref`].
> +///
> +/// Note that the guarantee for a [`Device`] reference to have a certain [`DeviceContext`] comes
> +/// from the specific scope the [`Device`] reference is valid in.
> +///
> +/// [`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 context. Any `Device<Ctx>` is also a valid
> +/// [`Device<Normal>`]. It is the only [`DeviceContext`] for which it is valid to implement
> +/// [`AlwaysRefCounted`] for.
> +///
> +/// [`AlwaysRefCounted`]: kernel::types::AlwaysRefCounted
> pub struct Normal;
> 
> -/// The [`Core`] context is the context of a bus specific device when it is supplied as argument of
> -/// any of the bus callbacks, such as `probe()`.
> +/// The [`Core`] context is the context of a bus specific device when it appears as argument of
> +/// any bus specific callback, such as `probe()`.
> +///
> +/// The core context indicates that the [`Device<Core>`] reference's scope is limited to the bus
> +/// callback it appears in. It is intended to be used for synchronization purposes. Bus device
> +/// implementations can implement methods for [`Device<Core>`], such that they can only be called
> +/// from bus callbacks.
> pub struct Core;
> 
> -/// Semantically the same as [`Core`] but reserved for internal usage of the corresponding bus
> +/// Semantically the same as [`Core`], but reserved for internal usage of the corresponding bus
> +/// abstraction.
> +///
> +/// The internal core context is intended to be used in exactly the same way as the [Core] context,
> +/// with the difference that this [`DeviceContext`] is internal to the corresponding bus
> /// abstraction.
> +///
> +/// This context mainly exists to share generic [`Device`] infrastructure that should only be called
> +/// from bus callbacks with bus abstractions, but without making them accessible for drivers.
> pub struct CoreInternal;
> 
> -/// The [`Bound`] context is the context of a bus specific device reference when it is guaranteed to
> -/// be bound for the duration of its lifetime.
> +/// The [`Bound`] context is the [`DeviceContext`] of a bus specific device when it is guaranteed to
> +/// be bound to a driver.
> +///
> +/// The bound context indicates that for the entire duration of the lifetime of a [`Device<Bound>`]
> +/// reference, the [`Device`] is guaranteed to be bound to a driver.
> +///
> +/// Some APIs, such as [`dma::CoherentAllocation`] or [`Devres`] rely on the [`Device`] to be bound,
> +/// which can be proven with the [`Bound`] device context.
> +///
> +/// Any abstraction that can guarantee a scope where the corresponding bus device is bound, should
> +/// provide a [`Device<Bound>`] reference to its users for this scope. This allows users to benefit
> +/// from optimizations for accessing device resources, see also [`Devres::access`].
> +///
> +/// [`Devres`]: kernel::devres::Devres
> +/// [`Devres::access`]: kernel::devres::Devres::access
> +/// [`dma::CoherentAllocation`]: kernel::dma::CoherentAllocation
> pub struct Bound;
> 
> mod private {
> -- 
> 2.50.0
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>