Re: [PATCH v2 04/25] rust: driver: make Adapter trait lifetime-parameterized

["Gary Guo" <gary@garyguo.net>]

On Wed May 6, 2026 at 10:50 PM BST, Danilo Krummrich wrote:
> Parameterize the Adapter trait with a lifetime, changing the id_info(),
> of_id_info() and acpi_id_info() methods to take &'bound Device<Bound> and
> return &'bound Self::IdInfo instead of &'static.
>
> This is needed for the ForLt conversion of bus driver traits. Once
> Driver becomes lifetime-parameterized, its IdInfo associated type may
> depend on the lifetime parameter.

How? The `IdInfo` is part of the ID table which is static.

I feel that this is really just adding lifetime for the sake of adding it.
Nothing in `Adapter` is really tied to device lifetime.

> With Adapter<'bound>, the impl can set
> IdInfo = <F::Of<'bound> as Driver<'bound>>::IdInfo and the lifetime flows

Well, I do get that you're trying to avoid writing `Driver<'static>`. This is
needed because conceptually, a `Driver` trait have a lifetime-dependent part and
a lifetime independent part.

`IdInfo` and `ID_TABLE` are the lifetime-independent part, and `probe` and
`unbind` are lifetime dependent part. We really only have this issue because we
use `Self` instead of an associated data for driver's data.

The `Driver` could be more cleanly designed if expressed like this:

    trait Driver {
        type IdInfo;
        const ID_TABLE: IdTable<Self::IdInfo>;

        type Data<'bound>;

        fn probe<'bound>(
            dev: &'bound Device<device::Core>,
            id_info: &'bound Self::IdInfo,
        ) -> impl PinInit<Self::Data<'bound>, Error> + 'bound;

        ...
    }

Then there's no `Driver<'static>` at all.

But even if we don't go with that design, I'd prefer using `Driver<'static>` to
giving lifetime parameters to types that shouldn't have one.

> through naturally, avoiding the need for transmute.

I applied the following diff and it doesn't require
additional transmute?

diff --git a/rust/kernel/driver.rs b/rust/kernel/driver.rs
index 7c5148772697..4c5bc31fd71d 100644
--- a/rust/kernel/driver.rs
+++ b/rust/kernel/driver.rs
@@ -316,7 +316,7 @@ unsafe fn acpi_of_match_device(
 /// of a device and a driver.
 ///
 /// It provides bus independent functions for device / driver interactions.
-pub trait Adapter<'bound> {
+pub trait Adapter {
     /// The type holding driver private data about each device id supported by the driver.
     type IdInfo: 'static;
 
@@ -326,7 +326,7 @@ pub trait Adapter<'bound> {
     /// Returns the driver's private data from the matching entry in the [`acpi::IdTable`], if any.
     ///
     /// If this returns `None`, it means there is no match with an entry in the [`acpi::IdTable`].
-    fn acpi_id_info(dev: &'bound Device<Bound>) -> Option<&'bound Self::IdInfo> {
+    fn acpi_id_info(dev: &Device<Bound>) -> Option<&'static Self::IdInfo> {
         #[cfg(not(CONFIG_ACPI))]
         {
             let _ = dev;
@@ -360,7 +360,7 @@ fn acpi_id_info(dev: &'bound Device<Bound>) -> Option<&'bound Self::IdInfo> {
     /// Returns the driver's private data from the matching entry in the [`of::IdTable`], if any.
     ///
     /// If this returns `None`, it means there is no match with an entry in the [`of::IdTable`].
-    fn of_id_info(dev: &'bound Device<Bound>) -> Option<&'bound Self::IdInfo> {
+    fn of_id_info(dev: &Device<Bound>) -> Option<&'static Self::IdInfo> {
         let table = Self::of_id_table()?;
 
         #[cfg(not(any(CONFIG_OF, CONFIG_ACPI)))]
@@ -424,7 +424,7 @@ fn of_id_info(dev: &'bound Device<Bound>) -> Option<&'bound Self::IdInfo> {
     ///
     /// If this returns `None`, it means that there is no match in any of the ID tables directly
     /// associated with a [`device::Device`].
-    fn id_info(dev: &'bound Device<Bound>) -> Option<&'bound Self::IdInfo> {
+    fn id_info(dev: &Device<Bound>) -> Option<&'static Self::IdInfo> {
         let id = Self::acpi_id_info(dev);
         if id.is_some() {
             return id;
diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
index 5a601cba7dc0..739b97c9917d 100644
--- a/rust/kernel/i2c.rs
+++ b/rust/kernel/i2c.rs
@@ -183,7 +183,7 @@ extern "C" fn probe_callback(idev: *mut bindings::i2c_client) -> kernel::ffi::c_
 
         from_result(|| {
             let info = Self::i2c_id_info(idev)
-                .or_else(|| <Self as driver::Adapter<'_>>::id_info(idev.as_ref()));
+                .or_else(|| <Self as driver::Adapter>::id_info(idev.as_ref()));
             let data = <F::Of<'_> as Driver<'_>>::probe(idev, info);
 
             idev.as_ref().set_drvdata::<F>(data)?;
@@ -243,19 +243,19 @@ fn i2c_id_info<'bound>(
     }
 }
 
-impl<'bound, F> driver::Adapter<'bound> for Adapter<F>
+impl<F> driver::Adapter for Adapter<F>
 where
     F: ForLt + 'static,
-    F::Of<'bound>: Driver<'bound>,
+    for<'bound> F::Of<'bound>: Driver<'bound>,
 {
-    type IdInfo = <F::Of<'bound> as Driver<'bound>>::IdInfo;
+    type IdInfo = <F::Of<'static> as Driver<'static>>::IdInfo;
 
     fn of_id_table() -> Option<of::IdTable<Self::IdInfo>> {
-        <F::Of<'bound> as Driver<'bound>>::OF_ID_TABLE
+        <F::Of<'static> as Driver<'static>>::OF_ID_TABLE
     }
 
     fn acpi_id_table() -> Option<acpi::IdTable<Self::IdInfo>> {
-        <F::Of<'bound> as Driver<'bound>>::ACPI_ID_TABLE
+        <F::Of<'static> as Driver<'static>>::ACPI_ID_TABLE
     }
 }
 
diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
index f59f22283c53..f4df2e474ca0 100644
--- a/rust/kernel/platform.rs
+++ b/rust/kernel/platform.rs
@@ -120,7 +120,7 @@ extern "C" fn probe_callback(pdev: *mut bindings::platform_device) -> kernel::ff
         let pdev = unsafe { &*pdev.cast::<Device<device::CoreInternal>>() };
 
         from_result(|| {
-            let info = <Self as driver::Adapter<'_>>::id_info(pdev.as_ref());
+            let info = <Self as driver::Adapter>::id_info(pdev.as_ref());
             let data = <F::Of<'_> as Driver<'_>>::probe(pdev, info);
 
             pdev.as_ref().set_drvdata::<F>(data)?;
@@ -143,19 +143,19 @@ extern "C" fn remove_callback(pdev: *mut bindings::platform_device) {
     }
 }
 
-impl<'bound, F> driver::Adapter<'bound> for Adapter<F>
+impl<F> driver::Adapter for Adapter<F>
 where
     F: ForLt + 'static,
     for<'b> F::Of<'b>: Driver<'b>,
 {
-    type IdInfo = <F::Of<'bound> as Driver<'bound>>::IdInfo;
+    type IdInfo = <F::Of<'static> as Driver<'static>>::IdInfo;
 
     fn of_id_table() -> Option<of::IdTable<Self::IdInfo>> {
-        <F::Of<'bound> as Driver<'bound>>::OF_ID_TABLE
+        <F::Of<'static> as Driver<'static>>::OF_ID_TABLE
     }
 
     fn acpi_id_table() -> Option<acpi::IdTable<Self::IdInfo>> {
-        <F::Of<'bound> as Driver<'bound>>::ACPI_ID_TABLE
+        <F::Of<'static> as Driver<'static>>::ACPI_ID_TABLE
     }
 }