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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Sat Apr 26, 2025 at 10:30 PM JST, 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 not clear whether the data has been revoked already due to the device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.
> +    ///
> +    /// # 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> {

Coming from `Revocable::try_access_with` (and the standard library in
general), the name of this method made me think that it would take a
closure to run while the resource is held. Maybe we should differenciate
the names a bit more? Maybe just `access` is fine, or
`access_with_device`?