Message-ID: <Zx68k94GrHb3Kz3-@Boquns-Mac-mini.local>
Date: Sun, 27 Oct 2024 15:20:03 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>,
	rust-for-linux@...r.kernel.org, aliceryhl@...gle.com,
	dakr@...hat.com, linux-kernel@...r.kernel.org, airlied@...hat.com,
	miguel.ojeda.sandonis@...il.com
Subject: Re: [PATCH v2 5/5] rust: firmware: implement `Ownable` for Firmware

On Wed, Oct 23, 2024 at 11:35:15AM +0200, Danilo Krummrich wrote:
> 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@...hat.com>
> > Suggested-by: Boqun Feng <boqun.feng@...il.com>
> > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.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>`.
> 

May I ask why not? ;-)

Ideally, we should not wrap a pointer to particular type, instead we
should wrap the type and then combine it with a meaningful pointer type,
e.g. Box<>, ARef<>, Owned<> ... in this way, we de-couple how the
lifetime of object is maintained (described by the pointer type) and
what operations are available on the object (described by the wrapper
type).

If later on, a firmware object creation is doable in pure Rust code for
some condition, we can then have a function that returns a
`KBox<Firmware>` (assume using kmalloc for the object), and it will just
work (tm).

Regards,
Boqun

> 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
> > 
> 

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