rust: implement generic driver registration

Implement the generic `Registration` type and the `RegistrationOps`
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 `RegistrationOps` trait is provided to subsystems, which have
to implement `RegistrationOps::register` and
`RegistrationOps::unregister`. Subsystems 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
`RegistrationOps::register` and pci_unregister_driver() from
`DrvierOps::unregister`.

Co-developed-by: Wedson Almeida Filho <[email protected]>
Signed-off-by: Wedson Almeida Filho <[email protected]>
Signed-off-by: Danilo Krummrich <[email protected]>

Author: Danilo Krummrich

MAINTAINERS

@@ -7033,6 +7033,7 @@ F:	include/linux/kobj*
 F:	include/linux/property.h
 F:	lib/kobj*
 F:	rust/kernel/device.rs
+F:	rust/kernel/driver.rs
 
 DRIVERS FOR OMAP ADAPTIVE VOLTAGE SCALING (AVS)
 M:	Nishanth Menon <[email protected]>

rust/kernel/driver.rs

@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Generic support for drivers of different buses (e.g., PCI, Platform, Amba, etc.).
+//!
+//! Each bus / subsystem is expected to implement [`RegistrationOps`], which allows drivers to
+//! register using the [`Registration`] class.
+
+use crate::error::{Error, Result};
+use crate::{init::PinInit, str::CStr, try_pin_init, types::Opaque, ThisModule};
+use core::pin::Pin;
+use macros::{pin_data, pinned_drop};
+
+/// 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;
+
+    /// Registers a driver.
+    ///
+    /// On success, `reg` must remain pinned and valid until the matching call to
+    /// [`RegistrationOps::unregister`].
+    fn register(
+        reg: &Opaque<Self::RegType>,
+        name: &'static CStr,
+        module: &'static ThisModule,
+    ) -> Result;
+
+    /// Unregisters a driver previously registered with [`RegistrationOps::register`].
+    fn unregister(reg: &Opaque<Self::RegType>);
+}
+
+/// A [`Registration`] is a generic type that represents the registration of some driver type (e.g.
+/// `bindings::pci_driver`). Therefore a [`Registration`] must be initialized with a type that
+/// implements the [`RegistrationOps`] trait, such that the generic `T::register` and
+/// `T::unregister` calls result in the subsystem specific registration calls.
+///
+///Once the `Registration` structure is dropped, the driver is unregistered.
+#[pin_data(PinnedDrop)]
+pub struct Registration<T: RegistrationOps> {
+    #[pin]
+    reg: Opaque<T::RegType>,
+}
+
+// SAFETY: `Registration` has no fields or methods accessible via `&Registration`, so it is safe to
+// share references to it with multiple threads as nothing can be done.
+unsafe impl<T: RegistrationOps> Sync for Registration<T> {}
+
+// SAFETY: Both registration and unregistration are implemented in C and safe to be performed from
+// any thread, so `Registration` is `Send`.
+unsafe impl<T: RegistrationOps> Send for Registration<T> {}
+
+impl<T: RegistrationOps> Registration<T> {
+    /// Creates a new instance of the registration object.
+    pub fn new(name: &'static CStr, module: &'static ThisModule) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            reg <- Opaque::try_ffi_init(|ptr: *mut T::RegType| {
+                // SAFETY: `try_ffi_init` guarantees that `ptr` is valid for write.
+                unsafe { ptr.write(T::RegType::default()) };
+
+                // SAFETY: `try_ffi_init` guarantees that `ptr` is valid for write, and it has
+                // just been initialised above, so it's also valid for read.
+                let drv = unsafe { &*(ptr as *const Opaque<T::RegType>) };
+
+                T::register(drv, name, module)
+            }),
+        })
+    }
+}
+
+#[pinned_drop]
+impl<T: RegistrationOps> PinnedDrop for Registration<T> {
+    fn drop(self: Pin<&mut Self>) {
+        T::unregister(&self.reg);
+    }
+}
+
+/// Declares a kernel module that exposes a single driver.
+///
+/// It is meant to be used as a helper by other subsystems so they can more easily expose their own
+/// macros.
+#[macro_export]
+macro_rules! module_driver {
+    (<$gen_type:ident>, $driver_ops:ty, { type: $type:ty, $($f:tt)* }) => {
+        type Ops<$gen_type> = $driver_ops;
+
+        #[$crate::prelude::pin_data]
+        struct DriverModule {
+            #[pin]
+            _driver: $crate::driver::Registration<Ops<$type>>,
+        }
+
+        impl $crate::InPlaceModule for DriverModule {
+            fn init(
+                module: &'static $crate::ThisModule
+            ) -> impl $crate::init::PinInit<Self, $crate::error::Error> {
+                $crate::try_pin_init!(Self {
+                    _driver <- $crate::driver::Registration::new(
+                        <Self as $crate::ModuleMetadata>::NAME,
+                        module,
+                    ),
+                })
+            }
+        }
+
+        $crate::prelude::module! {
+            type: DriverModule,
+            $($f)*
+        }
+    }
+}

rust/kernel/lib.rs

@@ -37,6 +37,7 @@ pub mod block;
 pub mod build_assert;
 pub mod cred;
 pub mod device;
+pub mod driver;
 pub mod error;
 #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
 pub mod firmware;