kernel/device_id.rs
1// SPDX-License-Identifier: GPL-2.0
2
3//! Generic implementation of device IDs.
4//!
5//! Each bus / subsystem that matches device and driver through a bus / subsystem specific ID is
6//! expected to implement [`RawDeviceId`].
7
8use core::mem::MaybeUninit;
9
10/// Marker trait to indicate a Rust device ID type represents a corresponding C device ID type.
11///
12/// This is meant to be implemented by buses/subsystems so that they can use [`IdTable`] to
13/// guarantee (at compile-time) zero-termination of device id tables provided by drivers.
14///
15/// # Safety
16///
17/// Implementers must ensure that `Self` is layout-compatible with [`RawDeviceId::RawType`];
18/// i.e. it's safe to transmute to `RawDeviceId`.
19///
20/// This requirement is needed so `IdArray::new` can convert `Self` to `RawType` when building
21/// the ID table.
22///
23/// Ideally, this should be achieved using a const function that does conversion instead of
24/// transmute; however, const trait functions relies on `const_trait_impl` unstable feature,
25/// which is broken/gone in Rust 1.73.
26pub unsafe trait RawDeviceId {
27 /// The raw type that holds the device id.
28 ///
29 /// Id tables created from [`Self`] are going to hold this type in its zero-terminated array.
30 type RawType: Copy;
31}
32
33/// Extension trait for [`RawDeviceId`] for devices that embed an index or context value.
34///
35/// This is typically used when the device ID struct includes a field like `driver_data`
36/// that is used to store a pointer-sized value (e.g., an index or context pointer).
37///
38/// # Safety
39///
40/// Implementers must ensure that `DRIVER_DATA_OFFSET` is the correct offset (in bytes) to
41/// the context/data field (e.g., the `driver_data` field) within the raw device ID structure.
42/// This field must be correctly sized to hold a `usize`.
43///
44/// Ideally, the data should be added during `Self` to `RawType` conversion,
45/// but there's currently no way to do it when using traits in const.
46pub unsafe trait RawDeviceIdIndex: RawDeviceId {
47 /// The offset (in bytes) to the context/data field in the raw device ID.
48 const DRIVER_DATA_OFFSET: usize;
49
50 /// The index stored at `DRIVER_DATA_OFFSET` of the implementor of the [`RawDeviceIdIndex`]
51 /// trait.
52 fn index(&self) -> usize;
53}
54
55/// A zero-terminated device id array.
56#[repr(C)]
57pub struct RawIdArray<T: RawDeviceId, const N: usize> {
58 ids: [T::RawType; N],
59 sentinel: MaybeUninit<T::RawType>,
60}
61
62impl<T: RawDeviceId, const N: usize> RawIdArray<T, N> {
63 #[doc(hidden)]
64 pub const fn size(&self) -> usize {
65 core::mem::size_of::<Self>()
66 }
67}
68
69/// A zero-terminated device id array, followed by context data.
70#[repr(C)]
71pub struct IdArray<T: RawDeviceId, U, const N: usize> {
72 raw_ids: RawIdArray<T, N>,
73 id_infos: [U; N],
74}
75
76impl<T: RawDeviceId, U, const N: usize> IdArray<T, U, N> {
77 /// Creates a new instance of the array.
78 ///
79 /// The contents are derived from the given identifiers and context information.
80 ///
81 /// # Safety
82 ///
83 /// `data_offset` as `None` is always safe.
84 /// If `data_offset` is `Some(data_offset)`, then:
85 /// - `data_offset` must be the correct offset (in bytes) to the context/data field
86 /// (e.g., the `driver_data` field) within the raw device ID structure.
87 /// - The field at `data_offset` must be correctly sized to hold a `usize`.
88 const unsafe fn build(ids: [(T, U); N], data_offset: Option<usize>) -> Self {
89 let mut raw_ids = [const { MaybeUninit::<T::RawType>::uninit() }; N];
90 let mut infos = [const { MaybeUninit::uninit() }; N];
91
92 let mut i = 0usize;
93 while i < N {
94 // SAFETY: by the safety requirement of `RawDeviceId`, we're guaranteed that `T` is
95 // layout-wise compatible with `RawType`.
96 raw_ids[i] = unsafe { core::mem::transmute_copy(&ids[i].0) };
97 if let Some(data_offset) = data_offset {
98 // SAFETY: by the safety requirement of this function, this would be effectively
99 // `raw_ids[i].driver_data = i;`.
100 unsafe {
101 raw_ids[i]
102 .as_mut_ptr()
103 .byte_add(data_offset)
104 .cast::<usize>()
105 .write(i);
106 }
107 }
108
109 // SAFETY: this is effectively a move: `infos[i] = ids[i].1`. We make a copy here but
110 // later forget `ids`.
111 infos[i] = MaybeUninit::new(unsafe { core::ptr::read(&ids[i].1) });
112 i += 1;
113 }
114
115 core::mem::forget(ids);
116
117 Self {
118 raw_ids: RawIdArray {
119 // SAFETY: this is effectively `array_assume_init`, which is unstable, so we use
120 // `transmute_copy` instead. We have initialized all elements of `raw_ids` so this
121 // `array_assume_init` is safe.
122 ids: unsafe { core::mem::transmute_copy(&raw_ids) },
123 sentinel: MaybeUninit::zeroed(),
124 },
125 // SAFETY: We have initialized all elements of `infos` so this `array_assume_init` is
126 // safe.
127 id_infos: unsafe { core::mem::transmute_copy(&infos) },
128 }
129 }
130
131 /// Creates a new instance of the array without writing index values.
132 ///
133 /// The contents are derived from the given identifiers and context information.
134 /// If the device implements [`RawDeviceIdIndex`], consider using [`IdArray::new`] instead.
135 pub const fn new_without_index(ids: [(T, U); N]) -> Self {
136 // SAFETY: Calling `Self::build` with `offset = None` is always safe,
137 // because no raw memory writes are performed in this case.
138 unsafe { Self::build(ids, None) }
139 }
140
141 /// Reference to the contained [`RawIdArray`].
142 pub const fn raw_ids(&self) -> &RawIdArray<T, N> {
143 &self.raw_ids
144 }
145}
146
147impl<T: RawDeviceId + RawDeviceIdIndex, U, const N: usize> IdArray<T, U, N> {
148 /// Creates a new instance of the array.
149 ///
150 /// The contents are derived from the given identifiers and context information.
151 pub const fn new(ids: [(T, U); N]) -> Self {
152 // SAFETY: by the safety requirement of `RawDeviceIdIndex`,
153 // `T::DRIVER_DATA_OFFSET` is guaranteed to be the correct offset (in bytes) to
154 // a field within `T::RawType`.
155 unsafe { Self::build(ids, Some(T::DRIVER_DATA_OFFSET)) }
156 }
157}
158
159/// A device id table.
160///
161/// This trait is only implemented by `IdArray`.
162///
163/// The purpose of this trait is to allow `&'static dyn IdArray<T, U>` to be in context when `N` in
164/// `IdArray` doesn't matter.
165pub trait IdTable<T: RawDeviceId, U> {
166 /// Obtain the pointer to the ID table.
167 fn as_ptr(&self) -> *const T::RawType;
168
169 /// Obtain the pointer to the bus specific device ID from an index.
170 fn id(&self, index: usize) -> &T::RawType;
171
172 /// Obtain the pointer to the driver-specific information from an index.
173 fn info(&self, index: usize) -> &U;
174}
175
176impl<T: RawDeviceId, U, const N: usize> IdTable<T, U> for IdArray<T, U, N> {
177 fn as_ptr(&self) -> *const T::RawType {
178 // This cannot be `self.ids.as_ptr()`, as the return pointer must have correct provenance
179 // to access the sentinel.
180 core::ptr::from_ref(self).cast()
181 }
182
183 fn id(&self, index: usize) -> &T::RawType {
184 &self.raw_ids.ids[index]
185 }
186
187 fn info(&self, index: usize) -> &U {
188 &self.id_infos[index]
189 }
190}
191
192/// Create device table alias for modpost.
193#[macro_export]
194macro_rules! module_device_table {
195 ($table_type: literal, $module_table_name:ident, $table_name:ident) => {
196 #[rustfmt::skip]
197 #[export_name =
198 concat!("__mod_device_table__", $table_type,
199 "__", module_path!(),
200 "_", line!(),
201 "_", stringify!($table_name))
202 ]
203 static $module_table_name: [::core::mem::MaybeUninit<u8>; $table_name.raw_ids().size()] =
204 unsafe { ::core::mem::transmute_copy($table_name.raw_ids()) };
205 };
206}