Re: [PATCH v3] rust: device: change the from_raw() function

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Oct 01, 2024 at 05:56:03PM -0300, Guilherme Giacomo Simoes wrote:
> The function Device::from_raw() increments a refcount by a call to
> bindings::get_device(ptr). This can be confused because usually
> from_raw() functions don't increment a refcount.
> Hence, rename Device::from_raw() to avoid confuion with other "from_raw"
> semantics.
> 
> The new name of function should be "get_device" to be consistent with
> the function get_device() already exist in .c files.
> 
> This function body also changed, because the `into()` will convert the
> `&'a Device` into `ARef<Device>` and also call `inc_ref` from the
> `AlwaysRefCounted` trait implemented for Device.
> 
> Signed-off-by: Guilherme Giacomo Simoes <trintaeoitogc@gmail.com>

Acked-by: Danilo Krummrich <dakr@kernel.org>

> ---
> differences from v1 to v2:
>  - remove the 0/1 patch
>  - refactor get_device() function
> 
> differences from v2:
> - fix the place of changelog.
> 
> The motivation from this change was will discussion in:
> https://rust-for-linux.zulipchat.com/#narrow/stream/291566-Library/topic/Inconsistency.20of.20.60from_raw.60.2E
> 
> I would like to thanks for Greg <gregkh@linuxfoundation.org>, Danilo
> <dakr@kernel.org> and Alice <aliceryhl@google.com> for help me with this
> patch.
> ---
>  rust/kernel/device.rs   | 15 +++------------
>  rust/kernel/firmware.rs |  2 +-
>  2 files changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 851018eef885..c8199ee079ef 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -51,18 +51,9 @@ impl Device {
>      ///
>      /// It must also be ensured that `bindings::device::release` can be called from any thread.
>      /// While not officially documented, this should be the case for any `struct device`.
> -    pub unsafe fn from_raw(ptr: *mut bindings::device) -> ARef<Self> {
> -        // SAFETY: By the safety requirements, ptr is valid.
> -        // Initially increase the reference count by one to compensate for the final decrement once
> -        // this newly created `ARef<Device>` instance is dropped.
> -        unsafe { bindings::get_device(ptr) };
> -
> -        // CAST: `Self` is a `repr(transparent)` wrapper around `bindings::device`.
> -        let ptr = ptr.cast::<Self>();
> -
> -        // SAFETY: `ptr` is valid by the safety requirements of this function. By the above call to
> -        // `bindings::get_device` we also own a reference to the underlying `struct device`.
> -        unsafe { ARef::from_raw(ptr::NonNull::new_unchecked(ptr)) }
> +    pub unsafe fn get_device(ptr: *mut bindings::device) -> ARef<Self> {
> +        // SAFETY: By the safety requirements ptr is valid
> +        unsafe { Self::as_ref(ptr) }.into()
>      }
>  
>      /// Obtain the raw `struct device *`.
> diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> index dee5b4b18aec..13a374a5cdb7 100644
> --- a/rust/kernel/firmware.rs
> +++ b/rust/kernel/firmware.rs
> @@ -44,7 +44,7 @@ fn request_nowarn() -> Self {
>  ///
>  /// # fn no_run() -> Result<(), Error> {
>  /// # // SAFETY: *NOT* safe, just for the example to get an `ARef<Device>` instance
> -/// # let dev = unsafe { Device::from_raw(core::ptr::null_mut()) };
> +/// # let dev = unsafe { Device::get_device(core::ptr::null_mut()) };
>  ///
>  /// let fw = Firmware::request(c_str!("path/to/firmware.bin"), &dev)?;
>  /// let blob = fw.data();
> -- 
> 2.46.2
>