[PATCH v2 2/5] rust: Add bindings for reading device properties

Remo Senekowitsch
2025-04-14T15:26:59Z

The device property API is a firmware agnostic API for reading
properties from firmware (DT/ACPI) devices nodes and swnodes.

While the C API takes a pointer to a caller allocated variable/buffer,
the rust API is designed to return a value and can be used in struct
initialization. Rust generics are also utilized to support different
types of properties where appropriate.

The PropertyGuard is a way to force users to specify whether a property
is supposed to be required or not. This allows us to move error
logging of missing required properties into core, preventing a lot of
boilerplate in drivers.

Co-developed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
---
rust/kernel/property.rs | 385 +++++++++++++++++++++++++++++++++++++++-
1 file changed, 383 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/property.rs b/rust/kernel/property.rs
index f6e6c980d..0d4ea3168 100644
--- a/rust/kernel/property.rs
+++ b/rust/kernel/property.rs
@@ -4,9 +4,17 @@
//!
//! C header: [`include/linux/property.h`](srctree/include/linux/property.h)

-use core::ptr;
+use core::{mem::MaybeUninit, ptr};

-use crate::{bindings, device::Device, str::CStr, types::Opaque};
+use crate::{
+ alloc::KVec,
+ bindings, c_str,
+ device::Device,
+ error::{to_result, Result},
+ prelude::*,
+ str::{BStr, CStr, CString},
+ types::Opaque,
+};

impl Device {
/// Obtain the fwnode corresponding to the device.
@@ -28,6 +36,38 @@ fn fwnode(&self) -> &FwNode {
pub fn property_present(&self, name: &CStr) -> bool {
self.fwnode().property_present(name)
}
+
+ /// Returns firmware property `name` boolean value
+ pub fn property_read_bool(&self, name: &CStr) -> bool {
+ self.fwnode().property_read_bool(name)
+ }
+
+ /// Returns the index of matching string `match_str` for firmware string property `name`
+ pub fn property_match_string(&self, name: &CStr, match_str: &CStr) -> Result<usize> {
+ self.fwnode().property_match_string(name, match_str)
+ }
+
+ /// Returns firmware property `name` integer array values in a KVec
+ pub fn property_read_array_vec<'fwnode, 'name, T: PropertyInt>(
+ &'fwnode self,
+ name: &'name CStr,
+ len: usize,
+ ) -> Result<PropertyGuard<'fwnode, 'name, KVec<T>>> {
+ self.fwnode().property_read_array_vec(name, len)
+ }
+
+ /// Returns integer array length for firmware property `name`
+ pub fn property_count_elem<T: PropertyInt>(&self, name: &CStr) -> Result<usize> {
+ self.fwnode().property_count_elem::<T>(name)
+ }
+
+ /// Returns firmware property `name` integer scalar value
+ pub fn property_read<'fwnode, 'name, T: Property>(
+ &'fwnode self,
+ name: &'name CStr,
+ ) -> PropertyGuard<'fwnode, 'name, T> {
+ self.fwnode().property_read(name)
+ }
}

/// A reference-counted fwnode_handle.
@@ -59,6 +99,150 @@ pub fn property_present(&self, name: &CStr) -> bool {
// SAFETY: By the invariant of `CStr`, `name` is null-terminated.
unsafe { bindings::fwnode_property_present(self.as_raw().cast_const(), name.as_char_ptr()) }
}
+
+ /// Returns firmware property `name` boolean value
+ pub fn property_read_bool(&self, name: &CStr) -> bool {
+ // SAFETY: `name` is non-null and null-terminated. `self.as_raw()` is valid
+ // because `self` is valid.
+ unsafe { bindings::fwnode_property_read_bool(self.as_raw(), name.as_char_ptr()) }
+ }
+
+ /// Returns the index of matching string `match_str` for firmware string property `name`
+ pub fn property_match_string(&self, name: &CStr, match_str: &CStr) -> Result<usize> {
+ // SAFETY: `name` and `match_str` are non-null and null-terminated. `self.as_raw` is
+ // valid because `self` is valid.
+ let ret = unsafe {
+ bindings::fwnode_property_match_string(
+ self.as_raw(),
+ name.as_char_ptr(),
+ match_str.as_char_ptr(),
+ )
+ };
+ to_result(ret)?;
+ Ok(ret as usize)
+ }
+
+ /// Returns firmware property `name` integer array values in a KVec
+ pub fn property_read_array_vec<'fwnode, 'name, T: PropertyInt>(
+ &'fwnode self,
+ name: &'name CStr,
+ len: usize,
+ ) -> Result<PropertyGuard<'fwnode, 'name, KVec<T>>> {
+ let mut val: KVec<T> = KVec::with_capacity(len, GFP_KERNEL)?;
+
+ // SAFETY: `val.as_mut_ptr()` is valid because `KVec::with_capacity`
+ // didn't return an error and it has at least space for `len` number
+ // of elements.
+ let err = unsafe { read_array_out_param::<T>(self, name, val.as_mut_ptr(), len) };
+ let res = if err < 0 {
+ Err(Error::from_errno(err))
+ } else {
+ // SAFETY: fwnode_property_read_int_array() writes exactly `len` entries on success
+ unsafe { val.set_len(len) }
+ Ok(val)
+ };
+ Ok(PropertyGuard {
+ inner: res,
+ fwnode: self,
+ name,
+ })
+ }
+
+ /// Returns integer array length for firmware property `name`
+ pub fn property_count_elem<T: PropertyInt>(&self, name: &CStr) -> Result<usize> {
+ // SAFETY: `out_param` is allowed to be null because `len` is zero.
+ let ret = unsafe { read_array_out_param::<T>(self, name, ptr::null_mut(), 0) };
+ to_result(ret)?;
+ Ok(ret as usize)
+ }
+
+ /// Returns the value of firmware property `name`.
+ ///
+ /// This method is generic over the type of value to read. Informally,
+ /// the types that can be read are booleans, strings, unsigned integers and
+ /// arrays of unsigned integers.
+ ///
+ /// Reading a `KVec` of integers is done with the separate
+ /// method [`Self::property_read_array_vec`], because it takes an
+ /// additional `len` argument.
+ ///
+ /// When reading a boolean, this method never fails. A missing property
+ /// is interpreted as `false`, whereas a present property is interpreted
+ /// as `true`.
+ ///
+ /// For more precise documentation about what types can be read, see
+ /// the [implementors of Property][Property#implementors] and [its
+ /// implementations on foreign types][Property#foreign-impls].
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// # use crate::{device::Device, types::CString};
+ /// fn examples(dev: &Device) -> Result {
+ /// let fwnode = dev.fwnode();
+ /// let b: bool = fwnode.property_read("some-bool").required()?;
+ /// if let Some(s) = fwnode.property_read::<CString>("some-str").optional() {
+ /// // ...
+ /// }
+ /// }
+ /// ```
+ pub fn property_read<'fwnode, 'name, T: Property>(
+ &'fwnode self,
+ name: &'name CStr,
+ ) -> PropertyGuard<'fwnode, 'name, T> {
+ PropertyGuard {
+ inner: T::read(self, name),
+ fwnode: self,
+ name,
+ }
+ }
+
+ /// helper used to display name or path of a fwnode
+ ///
+ /// # Safety
+ ///
+ /// Callers must provide a valid format string for a fwnode.
+ unsafe fn fmt(&self, f: &mut core::fmt::Formatter<'_>, fmt_str: &CStr) -> core::fmt::Result {
+ let mut buf = [0; 256];
+ // SAFETY: `buf` is valid and `buf.len()` is its length. `self.as_raw()` is
+ // valid because `self` is valid.
+ let written = unsafe {
+ bindings::scnprintf(buf.as_mut_ptr(), buf.len(), fmt_str.as_ptr(), self.as_raw())
+ };
+ // SAFETY: `written` is smaller or equal to `buf.len()`.
+ let b: &[u8] = unsafe { core::slice::from_raw_parts(buf.as_ptr(), written as usize) };
+ write!(f, "{}", BStr::from_bytes(b))
+ }
+
+ /// Returns an object that implements [`Display`](core::fmt::Display) for
+ /// printing the name of a node.
+ pub fn display_name(&self) -> impl core::fmt::Display + use<'_> {
+ struct FwNodeDisplayName<'a>(&'a FwNode);
+
+ impl core::fmt::Display for FwNodeDisplayName<'_> {
+ fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+ // SAFETY: "%pfwP" is a valid format string for fwnode
+ unsafe { self.0.fmt(f, c_str!("%pfwP")) }
+ }
+ }
+
+ FwNodeDisplayName(self)
+ }
+
+ /// Returns an object that implements [`Display`](core::fmt::Display) for
+ /// printing the full path of a node.
+ pub fn display_path(&self) -> impl core::fmt::Display + use<'_> {
+ struct FwNodeDisplayPath<'a>(&'a FwNode);
+
+ impl core::fmt::Display for FwNodeDisplayPath<'_> {
+ fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+ // SAFETY: "%pfwf" is a valid format string for fwnode
+ unsafe { self.0.fmt(f, c_str!("%pfwf")) }
+ }
+ }
+
+ FwNodeDisplayPath(self)
+ }
}

// SAFETY: Instances of `FwNode` are always reference-counted.
@@ -73,3 +257,200 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
unsafe { bindings::fwnode_handle_put(obj.cast().as_ptr()) }
}
}
+
+/// Implemented for several types that can be read as properties.
+///
+/// Informally, this is implemented for strings, integers and arrays of
+/// integers. It's used to make [`FwNode::property_read`] generic over the
+/// type of property being read. There are also two dedicated methods to read
+/// other types, because they require more specialized function signatures:
+/// - [`property_read_bool`](Device::property_read_bool)
+/// - [`property_read_array_vec`](Device::property_read_array_vec)
+pub trait Property: Sized {
+ /// Used to make [`FwNode::property_read`] generic.
+ fn read(fwnode: &FwNode, name: &CStr) -> Result<Self>;
+}
+
+impl Property for CString {
+ fn read(fwnode: &FwNode, name: &CStr) -> Result<Self> {
+ let mut str: *mut u8 = ptr::null_mut();
+ let pstr: *mut _ = &mut str;
+
+ // SAFETY: `name` is non-null and null-terminated. `fwnode.as_raw` is
+ // valid because `fwnode` is valid.
+ let ret = unsafe {
+ bindings::fwnode_property_read_string(fwnode.as_raw(), name.as_char_ptr(), pstr.cast())
+ };
+ to_result(ret)?;
+
+ // SAFETY: `pstr` contains a non-null ptr on success
+ let str = unsafe { CStr::from_char_ptr(*pstr) };
+ Ok(str.try_into()?)
+ }
+}
+/// Implemented for all integers that can be read as properties.
+///
+/// This helper trait is needed on top of the existing [`Property`]
+/// trait to associate the integer types of various sizes with their
+/// corresponding `fwnode_property_read_*_array` functions.
+pub trait PropertyInt: Copy {
+ /// # Safety
+ ///
+ /// Callers must uphold the same safety invariants as for the various
+ /// `fwnode_property_read_*_array` functions.
+ unsafe fn read_array(
+ fwnode: *const bindings::fwnode_handle,
+ propname: *const ffi::c_char,
+ val: *mut Self,
+ nval: usize,
+ ) -> ffi::c_int;
+}
+// This macro generates implementations of the traits `Property` and
+// `PropertyInt` for integers of various sizes. Its input is a list
+// of pairs separated by commas. The first element of the pair is the
+// type of the integer, the second one is the name of its corresponding
+// `fwnode_property_read_*_array` function.
+macro_rules! impl_property_for_int {
+ ($($int:ty: $f:ident),* $(,)?) => { $(
+ impl PropertyInt for $int {
+ unsafe fn read_array(
+ fwnode: *const bindings::fwnode_handle,
+ propname: *const ffi::c_char,
+ val: *mut Self,
+ nval: usize,
+ ) -> ffi::c_int {
+ // SAFETY: The safety invariants on the trait require
+ // callers to uphold the invariants of the functions
+ // this macro is called with.
+ unsafe {
+ bindings::$f(fwnode, propname, val.cast(), nval)
+ }
+ }
+ }
+ )* };
+}
+impl_property_for_int! {
+ u8: fwnode_property_read_u8_array,
+ u16: fwnode_property_read_u16_array,
+ u32: fwnode_property_read_u32_array,
+ u64: fwnode_property_read_u64_array,
+ i8: fwnode_property_read_u8_array,
+ i16: fwnode_property_read_u16_array,
+ i32: fwnode_property_read_u32_array,
+ i64: fwnode_property_read_u64_array,
+}
+/// # Safety
+///
+/// Callers must ensure that if `len` is non-zero, `out_param` must be
+/// valid and point to memory that has enough space to hold at least
+/// `len` number of elements.
+unsafe fn read_array_out_param<T: PropertyInt>(
+ fwnode: &FwNode,
+ name: &CStr,
+ out_param: *mut T,
+ len: usize,
+) -> ffi::c_int {
+ // SAFETY: `name` is non-null and null-terminated.
+ // `fwnode.as_raw` is valid because `fwnode` is valid.
+ // `out_param` is valid and has enough space for at least
+ // `len` number of elements as per the safety requirement.
+ unsafe { T::read_array(fwnode.as_raw(), name.as_char_ptr(), out_param, len) }
+}
+impl<T: PropertyInt, const N: usize> Property for [T; N] {
+ fn read(fwnode: &FwNode, name: &CStr) -> Result<Self> {
+ let mut val: [MaybeUninit<T>; N] = [const { MaybeUninit::uninit() }; N];
+
+ // SAFETY: `val.as_mut_ptr()` is valid and points to enough space for
+ // `N` elements. Casting from `*mut MaybeUninit<T>` to `*mut T` is safe
+ // because `MaybeUninit<T>` has the same memory layout as `T`.
+ let ret = unsafe { read_array_out_param::<T>(fwnode, name, val.as_mut_ptr().cast(), N) };
+ to_result(ret)?;
+
+ // SAFETY: `val` is always initialized when
+ // fwnode_property_read_<T>_array is successful.
+ Ok(val.map(|v| unsafe { v.assume_init() }))
+ }
+}
+impl<T: PropertyInt> Property for T {
+ fn read(fwnode: &FwNode, name: &CStr) -> Result<Self> {
+ let val: [_; 1] = <[T; 1] as Property>::read(fwnode, name)?;
+ Ok(val[0])
+ }
+}
+
+/// A helper for reading device properties.
+///
+/// Use [`Self::required`] if a missing property is considered a bug and
+/// [`Self::optional`] otherwise.
+///
+/// For convenience, [`Self::or`] and [`Self::or_default`] are provided.
+pub struct PropertyGuard<'fwnode, 'name, T> {
+ /// The result of reading the property.
+ inner: Result<T>,
+ /// The fwnode of the property, used for logging in the "required" case.
+ fwnode: &'fwnode FwNode,
+ /// The name of the property, used for logging in the "required" case.
+ name: &'name CStr,
+}
+
+impl<T> PropertyGuard<'_, '_, T> {
+ /// Access the property, indicating it is required.
+ ///
+ /// If the property is not present, the error is automatically logged. If a
+ /// missing property is not an error, use [`Self::optional`] instead.
+ pub fn required(self) -> Result<T> {
+ if self.inner.is_err() {
+ // Get the device associated with the fwnode for device-associated
+ // logging.
+ // TODO: Are we allowed to do this? The field `fwnode_handle.dev`
+ // has a somewhat vague comment, which could mean we're not
+ // supposed to access it:
+ // https://elixir.bootlin.com/linux/v6.13.6/source/include/linux/fwnode.h#L51
+ // SAFETY: According to the invariant of FwNode, it is valid.
+ let dev = unsafe { (*self.fwnode.as_raw()).dev };
+
+ if dev.is_null() {
+ pr_err!(
+ "{}: property '{}' is missing\n",
+ self.fwnode.display_path(),
+ self.name
+ );
+ } else {
+ // SAFETY: If dev is not null, it points to a valid device.
+ let dev: &Device = unsafe { &*dev.cast() };
+ dev_err!(
+ dev,
+ "{}: property '{}' is missing\n",
+ self.fwnode.display_path(),
+ self.name
+ );
+ };
+ }
+ self.inner
+ }
+
+ /// Access the property, indicating it is optional.
+ ///
+ /// In contrast to [`Self::required`], no error message is logged if
+ /// the property is not present.
+ pub fn optional(self) -> Option<T> {
+ self.inner.ok()
+ }
+
+ /// Access the property or the specified default value.
+ ///
+ /// Do not pass a sentinel value as default to detect a missing property.
+ /// Use [`Self::required`] or [`Self::optional`] instead.
+ pub fn or(self, default: T) -> T {
+ self.inner.unwrap_or(default)
+ }
+}
+
+impl<T: Default> PropertyGuard<'_, '_, T> {
+ /// Access the property or a default value.
+ ///
+ /// Use [`Self::or`] to specify a custom default value.
+ pub fn or_default(self) -> T {
+ self.inner.unwrap_or_default()
+ }
+}
--
2.49.0