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Danilo Krummrich <dakr@kernel.org> says:
Currently, Rust device drivers access device resources such as PCI BAR mappings
and I/O memory regions through Devres<T>.
Devres::access() provides zero-overhead access by taking a &Device<Bound>
reference as proof that the device is still bound. Since a &Device<Bound> is
available in almost all contexts by design, Devres is mostly a type-system level
proof that the resource is valid, but it can also be used from scopes without
this guarantee through its try_access() accessor.
This works well in general, but has a few limitations:
- Every access to a device resource goes through Devres::access(), which
despite zero cost, adds boilerplate to every access site.
- Destructors do not receive a &Device<Bound>, so they must use try_access(),
which can fail. In practice the access succeeds if teardown ordering is
correct, but the type system can't express this, forcing drivers to handle a
failure path that should never be taken.
- Sharing a resource across components (e.g. passing a BAR to a sub-component)
requires Arc<Devres<T>>.
- Device references must be stored as ARef<Device> rather than plain &Device
borrows.
These limitations stem from the driver's bus device private data being 'static
-- the driver struct cannot borrow from the device reference it receives in
probe(), even though it structurally cannot outlive the device binding.
This series introduces Higher-Ranked Lifetime Types (HRT) for Rust device
drivers. An HRT is a type that is generic over a lifetime -- it does not have a
fixed lifetime, but can be instantiated with any lifetime chosen by the caller.
Bus driver traits use a Generic Associated Type (GAT) type Data<'bound> to
introduce the lifetime on the private data, rather than parameterizing the
Driver trait itself. This avoids a driver trait global lifetime and avoids the
need for ForLt for bus device private data, making the bus implementations much
simpler. ForLt is only needed for auxiliary registration data, where the
lifetime is not introduced by a trait callback but must be threaded through
Registration.
With HRT, driver structs carry a lifetime parameter tied to the device binding
scope -- the interval of a bus device being bound to a driver. Device resources
like pci::Bar<'bound> and IoMem<'bound> are handed out with this lifetime, so
the compiler enforces at build time that they do not escape the binding scope.
Before:
struct MyDriver {
pdev: ARef<pci::Device>,
bar: Devres<pci::Bar<BAR_SIZE>>,
}
let io = self.bar.access(dev)?;
io.read32(OFFSET);
After:
struct MyDriver<'bound> {
pdev: &'bound pci::Device,
bar: pci::Bar<'bound, BAR_SIZE>,
}
self.bar.read32(OFFSET);
Lifetime-parameterized device resources can be put into a Devres at any point
via Bar::into_devres() / IoMem::into_devres(), providing the exact same
semantics as before. This is useful for resources shared across subsystem
boundaries where revocation is needed.
This also synergizes with the upcoming self-referential initialization support
in pin-init, which allows one field of the driver struct to borrow another
during initialization without unsafe code.
The same pattern is applied to auxiliary device registration data as a first
example beyond bus device private data. Registration<F: ForLt> can hold
lifetime-parameterized data tied to the parent driver's binding scope. Since the
auxiliary bus guarantees that the parent remains bound while the auxiliary
device is registered, the registration data can safely borrow the parent's
device resources.
More generally, binding resource lifetimes to a registration scope applies to
every registration that is scoped to a driver binding -- auxiliary devices,
class devices, IRQ handlers, workqueues.
A follow-up series extends this to class device registrations, starting with
DRM, so that class device callbacks (IOCTLs, etc.) can safely access device
resources through the separate registration data bound to the registration's
lifetime without Devres indirection.
Thanks to Gary for coming up with the ForLt implementation; thanks to Alice for
the early discussions around lifetime-parameterized private data that helped
shape the direction of this work.
Link: https://patch.msgid.link/20260525202921.124698-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
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| .. | ||
| alloc | ||
| block | ||
| debugfs | ||
| device | ||
| drm | ||
| fs | ||
| gpu | ||
| interop | ||
| io | ||
| iommu | ||
| irq | ||
| list | ||
| mm | ||
| net | ||
| num | ||
| pci | ||
| ptr | ||
| str | ||
| sync | ||
| time | ||
| types | ||
| .gitignore | ||
| acpi.rs | ||
| alloc.rs | ||
| auxiliary.rs | ||
| bitmap.rs | ||
| bits.rs | ||
| block.rs | ||
| bug.rs | ||
| build_assert.rs | ||
| clk.rs | ||
| configfs.rs | ||
| cpu.rs | ||
| cpufreq.rs | ||
| cpumask.rs | ||
| cred.rs | ||
| debugfs.rs | ||
| device_id.rs | ||
| device.rs | ||
| devres.rs | ||
| dma.rs | ||
| driver.rs | ||
| error.rs | ||
| faux.rs | ||
| firmware.rs | ||
| fmt.rs | ||
| fs.rs | ||
| generated_arch_reachable_asm.rs.S | ||
| generated_arch_static_branch_asm.rs.S | ||
| generated_arch_warn_asm.rs.S | ||
| gpu.rs | ||
| i2c.rs | ||
| id_pool.rs | ||
| impl_flags.rs | ||
| init.rs | ||
| interop.rs | ||
| io.rs | ||
| ioctl.rs | ||
| iov.rs | ||
| irq.rs | ||
| jump_label.rs | ||
| kunit.rs | ||
| lib.rs | ||
| list.rs | ||
| maple_tree.rs | ||
| miscdevice.rs | ||
| mm.rs | ||
| module_param.rs | ||
| net.rs | ||
| num.rs | ||
| of.rs | ||
| opp.rs | ||
| page.rs | ||
| pci.rs | ||
| pid_namespace.rs | ||
| platform.rs | ||
| prelude.rs | ||
| print.rs | ||
| processor.rs | ||
| ptr.rs | ||
| pwm.rs | ||
| rbtree.rs | ||
| regulator.rs | ||
| revocable.rs | ||
| safety.rs | ||
| scatterlist.rs | ||
| security.rs | ||
| seq_file.rs | ||
| sizes.rs | ||
| soc.rs | ||
| std_vendor.rs | ||
| str.rs | ||
| sync.rs | ||
| task.rs | ||
| time.rs | ||
| tracepoint.rs | ||
| transmute.rs | ||
| types.rs | ||
| uaccess.rs | ||
| usb.rs | ||
| workqueue.rs | ||
| xarray.rs | ||