6.15-stable review patch. If anyone has any objections, please let me know.
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From: Danilo Krummrich dakr@kernel.org
[ Upstream commit f301cb978c068faa8fcd630be2cb317a2d0ec063 ]
Implement a direct accessor for the data stored within the Devres for cases where we can prove 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Usually, when accessing the data stored within a Devres container, it is not clear whether the data has been revoked already due to the device being unbound and, hence, we have to try whether the access is possible and subsequently keep holding the RCU read lock for the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 prove 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.
Reviewed-by: Christian Schrefl chrisi.schrefl@gmail.com Reviewed-by: Alexandre Courbot acourbot@nvidia.com Acked-by: Boqun Feng boqun.feng@gmail.com Reviewed-by: Joel Fernandes joelagnelf@nvidia.com Link: https://lore.kernel.org/r/20250428140137.468709-3-dakr@kernel.org Signed-off-by: Danilo Krummrich dakr@kernel.org Stable-dep-of: 20c96ed278e3 ("rust: devres: do not dereference to the internal Revocable") Signed-off-by: Sasha Levin sashal@kernel.org --- rust/kernel/devres.rs | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+)
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs index f3a4e3383b8d2..acb8e1d13ddd9 100644 --- a/rust/kernel/devres.rs +++ b/rust/kernel/devres.rs @@ -196,6 +196,44 @@ pub fn new_foreign_owned(dev: &Device, 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. + /// + /// # Errors + /// + /// 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(dev.as_ref())?; + /// + /// let _ = bar.read32(0x0); + /// + /// // might_sleep() + /// + /// bar.write32(0x42, 0x0); + /// + /// Ok(()) + /// } + /// ``` + pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a 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 + // proves 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> {