Date: Thu, 30 May 2024 07:58:36 +0200
From: Dirk Behme <dirk.behme@...bosch.com>
To: Danilo Krummrich <dakr@...hat.com>, <gregkh@...uxfoundation.org>,
	<rafael@...nel.org>, <bhelgaas@...gle.com>, <ojeda@...nel.org>,
	<alex.gaynor@...il.com>, <wedsonaf@...il.com>, <boqun.feng@...il.com>,
	<gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <benno.lossin@...ton.me>,
	<a.hindborg@...sung.com>, <aliceryhl@...gle.com>, <airlied@...il.com>,
	<fujita.tomonori@...il.com>, <lina@...hilina.net>, <pstanner@...hat.com>,
	<ajanulgu@...hat.com>, <lyude@...hat.com>, Fabien Parent
	<fabien.parent@...aro.org>
CC: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-pci@...r.kernel.org>
Subject: Re: [RFC PATCH 02/11] rust: add driver abstraction

On 20.05.2024 19:25, Danilo Krummrich wrote:
> From: Wedson Almeida Filho <wedsonaf@...il.com>
> 
> This defines general functionality related to registering drivers with
> their respective subsystems, and registering modules that implement
> drivers.
> 
> Co-developed-by: Asahi Lina <lina@...hilina.net>
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> Co-developed-by: Andreas Hindborg <a.hindborg@...sung.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@...sung.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Signed-off-by: Danilo Krummrich <dakr@...hat.com>
> ---
>   rust/kernel/driver.rs        | 492 +++++++++++++++++++++++++++++++++++
>   rust/kernel/lib.rs           |   4 +-
>   rust/macros/module.rs        |   2 +-
>   samples/rust/rust_minimal.rs |   2 +-
>   samples/rust/rust_print.rs   |   2 +-
>   5 files changed, 498 insertions(+), 4 deletions(-)
>   create mode 100644 rust/kernel/driver.rs
> 
> diff --git a/rust/kernel/driver.rs b/rust/kernel/driver.rs
> new file mode 100644
> index 000000000000..e0cfc36d47ff
> --- /dev/null
> +++ b/rust/kernel/driver.rs
> @@ -0,0 +1,492 @@
> +// 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 [`DriverOps`], which allows drivers to register
> +//! using the [`Registration`] class.
> +
> +use crate::{
> +    alloc::{box_ext::BoxExt, flags::*},
> +    error::code::*,
> +    error::Result,
> +    str::CStr,
> +    sync::Arc,
> +    ThisModule,
> +};
> +use alloc::boxed::Box;
> +use core::{cell::UnsafeCell, marker::PhantomData, ops::Deref, pin::Pin};
> +
> +/// A subsystem (e.g., PCI, Platform, Amba, etc.) that allows drivers to be written for it.
> +pub trait DriverOps {
> +    /// 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.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `reg` must point to valid, initialised, and writable memory. It may be modified by this
> +    /// function to hold registration state.
> +    ///
> +    /// On success, `reg` must remain pinned and valid until the matching call to
> +    /// [`DriverOps::unregister`].
> +    unsafe fn register(
> +        reg: *mut Self::RegType,
> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result;
> +
> +    /// Unregisters a driver previously registered with [`DriverOps::register`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// `reg` must point to valid writable memory, initialised by a previous successful call to
> +    /// [`DriverOps::register`].
> +    unsafe fn unregister(reg: *mut Self::RegType);
> +}
> +
> +/// The registration of a driver.
> +pub struct Registration<T: DriverOps> {
> +    is_registered: bool,
> +    concrete_reg: UnsafeCell<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: DriverOps> Sync for Registration<T> {}
> +
> +impl<T: DriverOps> Registration<T> {
> +    /// Creates a new instance of the registration object.
> +    pub fn new() -> Self {
> +        Self {
> +            is_registered: false,
> +            concrete_reg: UnsafeCell::new(T::RegType::default()),
> +        }
> +    }
> +
> +    /// Allocates a pinned registration object and registers it.
> +    ///
> +    /// Returns a pinned heap-allocated representation of the registration.
> +    pub fn new_pinned(name: &'static CStr, module: &'static ThisModule) -> Result<Pin<Box<Self>>> {
> +        let mut reg = Pin::from(Box::new(Self::new(), GFP_KERNEL)?);
> +        reg.as_mut().register(name, module)?;
> +        Ok(reg)
> +    }
> +
> +    /// Registers a driver with its subsystem.
> +    ///
> +    /// It must be pinned because the memory block that represents the registration is potentially
> +    /// self-referential.
> +    pub fn register(
> +        self: Pin<&mut Self>,
> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result {
> +        // SAFETY: We never move out of `this`.
> +        let this = unsafe { self.get_unchecked_mut() };
> +        if this.is_registered {
> +            // Already registered.
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: `concrete_reg` was initialised via its default constructor. It is only freed
> +        // after `Self::drop` is called, which first calls `T::unregister`.
> +        unsafe { T::register(this.concrete_reg.get(), name, module) }?;
> +
> +        this.is_registered = true;
> +        Ok(())
> +    }
> +}
> +
> +impl<T: DriverOps> Default for Registration<T> {
> +    fn default() -> Self {
> +        Self::new()
> +    }
> +}
> +
> +impl<T: DriverOps> Drop for Registration<T> {
> +    fn drop(&mut self) {
> +        if self.is_registered {
> +            // SAFETY: This path only runs if a previous call to `T::register` completed
> +            // successfully.
> +            unsafe { T::unregister(self.concrete_reg.get()) };
> +        }
> +    }
> +}
> +
> +/// Conversion from a device id to a raw device id.
> +///
> +/// This is meant to be implemented by buses/subsystems so that they can use [`IdTable`] to
> +/// guarantee (at compile-time) zero-termination of device id tables provided by drivers.
> +///
> +/// Originally, RawDeviceId was implemented as a const trait. However, this unstable feature is
> +/// broken/gone in 1.73. To work around this, turn IdArray::new() into a macro such that it can use
> +/// concrete types (which can still have const associated functions) instead of a trait.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that:
> +///   - [`RawDeviceId::ZERO`] is actually a zeroed-out version of the raw device id.
> +///   - [`RawDeviceId::to_rawid`] stores `offset` in the context/data field of the raw device id so
> +///     that buses can recover the pointer to the data.

In his I2C branch Fabien has a patch [1] [2] to remove the 
RawDeviceId::to_rawid part above. Maybe it could be aligned that that 
patch isn't required any more?

Best regards

Dirk

[1] 
https://github.com/Fabo/linux/commit/4b65b8d7ffe07057672b8eb89d376759d67bf060

[2]

 From 4b65b8d7ffe07057672b8eb89d376759d67bf060 Mon Sep 17 00:00:00 2001
From: Fabien Parent <fabien.parent@...aro.org>
Date: Sun, 28 Apr 2024 11:12:46 -0700
Subject: [PATCH] fixup! rust: add driver abstraction

RawDeviceId::to_rawid is not part of the RawDeviceId trait. Nonetheless
this function must be defined by the type that will implement
RawDeviceId, but to keep `rustdoc` from throwing a warning, let's just
remove it from the docs.

	warning: unresolved link to `RawDeviceId::to_rawid`
	   --> rust/kernel/driver.rs:120:11
	    |
	120 | ///   - [`RawDeviceId::to_rawid`] stores `offset` in the 
context/data field of the raw device id so
	    |           ^^^^^^^^^^^^^^^^^^^^^ the trait `RawDeviceId` has no 
associated item named `to_rawid`
	    |
	    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

	warning: 1 warning emitted
---
  rust/kernel/driver.rs | 2 --
  1 file changed, 2 deletions(-)

diff --git a/rust/kernel/driver.rs b/rust/kernel/driver.rs
index c1258ce0d041af..d141e23224d3db 100644
--- a/rust/kernel/driver.rs
+++ b/rust/kernel/driver.rs
@@ -128,8 +128,6 @@ impl<T: DriverOps> Drop for Registration<T> {
  ///
  /// Implementers must ensure that:
  ///   - [`RawDeviceId::ZERO`] is actually a zeroed-out version of the 
raw device id.
-///   - [`RawDeviceId::to_rawid`] stores `offset` in the context/data 
field of the raw device id so
-///     that buses can recover the pointer to the data.
  pub unsafe trait RawDeviceId {
      /// The raw type that holds the device id.
      ///

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