Re: [PATCH 2/3] rust: devres: implement Devres::access_with()

[Christian Schrefl <chrisi.schrefl@gmail.com>]

On 26.04.25 3:30 PM, Danilo Krummrich wrote:
> Implement a direct accessor for the data stored within the Devres for
> cases where we can proof that we own a reference to a Device<Bound>
> (i.e. a bound device) of the same device that was used to create the
> corresponding Devres container.
> 
> Usually, when accessing the data stored within a Devres container, it is
> not clear whether the data has been revoked already due to the device
> being unbound and, hence, we have to try whether the access is possible
> and subsequently keep holding the RCU read lock for the duration of the
> access.
> 
> However, when we can proof that we hold a reference to Device<Bound>
> matching the device the Devres container has been created with, we can
> guarantee that the device is not unbound for the duration of the
> lifetime of the Device<Bound> reference and, hence, it is not possible
> for the data within the Devres container to be revoked.
> 
> Therefore, in this case, we can bypass the atomic check and the RCU read
> lock, which is a great optimization and simplification for drivers.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/devres.rs | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 1e58f5d22044..ec2cd9cdda8b 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -181,6 +181,41 @@ pub fn new_foreign_owned(dev: &Device<Bound>, data: T, flags: Flags) -> Result {
>  
>          Ok(())
>      }
> +
> +    /// Obtain `&'a T`, bypassing the [`Revocable`].
> +    ///
> +    /// This method allows to directly obtain a `&'a T`, bypassing the [`Revocable`], by presenting
> +    /// a `&'a Device<Bound>` of the same [`Device`] this [`Devres`] instance has been created with.
> +    ///
> +    /// An error is returned if `dev` does not match the same [`Device`] this [`Devres`] instance
> +    /// has been created with.

I would prefer this as a `# Errors` section.

Also are there any cases where this is actually wanted as an error?
I'm not very familiar with the `Revocable` infrastructure,
but I would assume a mismatch here to be a bug in almost every case,
so a panic here might be reasonable.
(I would be fine with a reason for using an error here in the 
commit message or documentation/comments)

With that:

Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>

> +    ///
> +    /// # Example
> +    ///
> +    /// ```no_run
> +    /// # use kernel::{device::Core, devres::Devres, pci};
> +    ///
> +    /// fn from_core(dev: &pci::Device<Core>, devres: Devres<pci::Bar<0x4>>) -> Result<()> {
> +    ///     let bar = devres.access_with(dev.as_ref())?;
> +    ///
> +    ///     let _ = bar.read32(0x0);
> +    ///
> +    ///     // might_sleep()
> +    ///
> +    ///     bar.write32(0x42, 0x0);
> +    ///
> +    ///     Ok(())
> +    /// }
> +    pub fn access_with<'s, 'd: 's>(&'s self, dev: &'d Device<Bound>) -> Result<&'s T> {
> +        if self.0.dev.as_raw() != dev.as_raw() {
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: `dev` being the same device as the device this `Devres` has been created for
> +        // proofes that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as
> +        // long as `dev` lives; `dev` lives at least as long as `self`.
> +        Ok(unsafe { self.deref().access() })
> +    }
>  }
>  
>  impl<T> Deref for Devres<T> {