linux-kernel - Re: [PATCH v4 02/13] rust: implement generic driver registration

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Message-ID: <CAH5fLghRVFAb06YYfUbuyuR1pOK0cHzGk6A25c5hX3CyvMm+sw@mail.gmail.com>
Date: Fri, 6 Dec 2024 14:57:19 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com, 
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
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	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org, devicetree@...r.kernel.org, 
	Wedson Almeida Filho <wedsonaf@...il.com>
Subject: Re: [PATCH v4 02/13] rust: implement generic driver registration

On Thu, Dec 5, 2024 at 3:16 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> Implement the generic `Registration` type and the `DriverOps` trait.
>
> The `Registration` structure is the common type that represents a driver
> registration and is typically bound to the lifetime of a module. However,
> it doesn't implement actual calls to the kernel's driver core to register
> drivers itself.
>
> Instead the `DriverOps` trait is provided to subsystems, which have to
> implement `DriverOps::register` and `DrvierOps::unregister`. Subsystems

typo

> have to provide an implementation for both of those methods where the
> subsystem specific variants to register / unregister a driver have to
> implemented.
>
> For instance, the PCI subsystem would call __pci_register_driver() from
> `DriverOps::register` and pci_unregister_driver() from
> `DrvierOps::unregister`.
>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

[...]

> +/// The [`RegistrationOps`] trait serves as generic interface for subsystems (e.g., PCI, Platform,
> +/// Amba, etc.) to provide the corresponding subsystem specific implementation to register /
> +/// unregister a driver of the particular type (`RegType`).
> +///
> +/// For instance, the PCI subsystem would set `RegType` to `bindings::pci_driver` and call
> +/// `bindings::__pci_register_driver` from `RegistrationOps::register` and
> +/// `bindings::pci_unregister_driver` from `RegistrationOps::unregister`.
> +pub trait RegistrationOps {
> +    /// The type that holds information about the registration. This is typically a struct defined
> +    /// by the C portion of the kernel.
> +    type RegType: Default;

This Default implementation doesn't seem useful. You initialize it and
then `register` calls a C function to initialize it. Having `register`
return an `impl PinInit` seems like it would work better here.

> +    /// Registers a driver.
> +    ///
> +    /// On success, `reg` must remain pinned and valid until the matching call to
> +    /// [`RegistrationOps::unregister`].
> +    fn register(
> +        reg: &mut Self::RegType,

If the intent is that RegType is going to be the raw bindings:: type,
then this isn't going to work because you're creating &mut references
to the raw type without a Opaque wrapper in between.

> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result;
> +
> +    /// Unregisters a driver previously registered with [`RegistrationOps::register`].
> +    fn unregister(reg: &mut Self::RegType);

I believe this handles pinning incorrectly. You can't hand out &mut
references to pinned values.

Alice