Re: [PATCH v2 5/5] rust: firmware: implement `Ownable` for Firmware

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Oct 23, 2024 at 01:44:49AM +0300, Abdiel Janulgue wrote:
> For consistency, wrap the firmware as an `Owned` smart pointer in the
> constructor.
> 
> Cc: Danilo Krummrich <dakr@redhat.com>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
> ---
>  rust/kernel/firmware.rs | 31 ++++++++++++++++++-------------
>  1 file changed, 18 insertions(+), 13 deletions(-)
> 
> diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> index dee5b4b18aec..6da834b37455 100644
> --- a/rust/kernel/firmware.rs
> +++ b/rust/kernel/firmware.rs
> @@ -4,8 +4,8 @@
>  //!
>  //! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h)
>  
> -use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
> -use core::ptr::NonNull;
> +use crate::{bindings, device::Device, error::Error, error::Result, str::CStr,
> +            types::{Opaque, Owned, Ownable}};
>  
>  /// # Invariants
>  ///
> @@ -52,10 +52,11 @@ fn request_nowarn() -> Self {
>  /// # Ok(())
>  /// # }
>  /// ```
> -pub struct Firmware(NonNull<bindings::firmware>);
> + #[repr(transparent)]
> +pub struct Firmware(Opaque<bindings::firmware>);
>  
>  impl Firmware {
> -    fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
> +    fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Owned<Self>> {

I think it's fine to implement this for consistency, but I'm not sure I like
that drivers have to refer to it as `Owned<Firmware>`.

Anyway, if we keep it this way the patch also needs the following change.

diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
index 6da834b37455..1db854eb2422 100644
--- a/rust/kernel/firmware.rs
+++ b/rust/kernel/firmware.rs
@@ -115,8 +115,8 @@ unsafe fn ptr_drop(ptr: *mut Self) {

 // SAFETY: `Firmware` only holds a pointer to a C `struct firmware`, which is safe to be used from
 // any thread.
-unsafe impl Send for Firmware {}
+unsafe impl Send for Owned<Firmware> {}

 // SAFETY: `Firmware` only holds a pointer to a C `struct firmware`, references to which are safe to
 // be used from any thread.
-unsafe impl Sync for Firmware {}
+unsafe impl Sync for Owned<Firmware> {}

>          let mut fw: *mut bindings::firmware = core::ptr::null_mut();
>          let pfw: *mut *mut bindings::firmware = &mut fw;
>  
> @@ -65,25 +66,26 @@ fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
>          if ret != 0 {
>              return Err(Error::from_errno(ret));
>          }
> -
> +        // CAST: Self` is a `repr(transparent)` wrapper around `bindings::firmware`.
> +        let ptr = fw.cast::<Self>();
>          // SAFETY: `func` not bailing out with a non-zero error code, guarantees that `fw` is a
>          // valid pointer to `bindings::firmware`.
> -        Ok(Firmware(unsafe { NonNull::new_unchecked(fw) }))
> +        Ok(unsafe { Owned::to_owned(ptr) })
>      }
>  
>      /// Send a firmware request and wait for it. See also `bindings::request_firmware`.
> -    pub fn request(name: &CStr, dev: &Device) -> Result<Self> {
> +    pub fn request(name: &CStr, dev: &Device) -> Result<Owned<Self>> {
>          Self::request_internal(name, dev, FwFunc::request())
>      }
>  
>      /// Send a request for an optional firmware module. See also
>      /// `bindings::firmware_request_nowarn`.
> -    pub fn request_nowarn(name: &CStr, dev: &Device) -> Result<Self> {
> +    pub fn request_nowarn(name: &CStr, dev: &Device) -> Result<Owned<Self>> {
>          Self::request_internal(name, dev, FwFunc::request_nowarn())
>      }
>  
>      fn as_raw(&self) -> *mut bindings::firmware {
> -        self.0.as_ptr()
> +        self.0.get()
>      }
>  
>      /// Returns the size of the requested firmware in bytes.
> @@ -101,10 +103,13 @@ pub fn data(&self) -> &[u8] {
>      }
>  }
>  
> -impl Drop for Firmware {
> -    fn drop(&mut self) {
> -        // SAFETY: `self.as_raw()` is valid by the type invariant.
> -        unsafe { bindings::release_firmware(self.as_raw()) };
> +unsafe impl Ownable for Firmware {
> +    unsafe fn ptr_drop(ptr: *mut Self) {
> +        // SAFETY:
> +        // - By the type invariants, we have ownership of the ptr and can free it.
> +        // - Per function safety, this is called in Owned::drop(), so `ptr` is a
> +        //   unique pointer to object, it's safe to release the firmware.
> +        unsafe { bindings::release_firmware(ptr.cast()) };
>      }
>  }
>  
> -- 
> 2.43.0
>