kernel/deferred_call.rs
1// Licensed under the Apache License, Version 2.0 or the MIT License.
2// SPDX-License-Identifier: Apache-2.0 OR MIT
3// Copyright Tock Contributors 2022.
4
5//! Hardware-independent kernel interface for deferred calls.
6//!
7//! This allows any struct in the kernel which implements [`DeferredCallClient`]
8//! to set and receive deferred calls, Tock's version of software interrupts.
9//!
10//! These can be used to implement long-running in-kernel algorithms or software
11//! devices that are supposed to work like hardware devices. Essentially, this
12//! allows the chip to handle more important interrupts, and lets a kernel
13//! component return the function call stack up to the scheduler, automatically
14//! being called again.
15//!
16//! Usage
17//! -----
18//!
19//! The `DEFCALLS` array size determines how many [`DeferredCall`]s may be
20//! registered. By default this is set to 32. To support more deferred calls,
21//! this file would need to be modified to use a larger variable for `BITMASK`
22//! (e.g. `BITMASK` could be a u64 and the array size increased to 64). If more
23//! than 32 deferred calls are created, the kernel will panic at the beginning
24//! of the kernel loop.
25//!
26//! ```rust
27//! use kernel::deferred_call::{DeferredCall, DeferredCallClient};
28//! use kernel::static_init;
29//!
30//! struct SomeCapsule {
31//! deferred_call: DeferredCall
32//! }
33//! impl SomeCapsule {
34//! pub fn new() -> Self {
35//! Self {
36//! deferred_call: DeferredCall::new(),
37//! }
38//! }
39//! }
40//! impl DeferredCallClient for SomeCapsule {
41//! fn handle_deferred_call(&self) {
42//! // Your action here
43//! }
44//!
45//! fn register(&'static self) {
46//! self.deferred_call.register(self);
47//! }
48//! }
49//!
50//! // main.rs or your component must register the capsule with its deferred
51//! // call. This should look like:
52//! let some_capsule = unsafe { static_init!(SomeCapsule, SomeCapsule::new()) };
53//! some_capsule.register();
54//! ```
55
56use crate::utilities::cells::OptionalCell;
57use core::cell::Cell;
58use core::marker::Copy;
59use core::marker::PhantomData;
60use core::ptr::addr_of;
61
62/// This trait should be implemented by clients which need to receive
63/// [`DeferredCall`]s.
64// This trait is not intended to be used as a trait object; e.g. you should not
65// create a `&dyn DeferredCallClient`. The `Sized` supertrait prevents this.
66pub trait DeferredCallClient: Sized {
67 /// Software interrupt function that is called when the deferred call is
68 /// triggered.
69 fn handle_deferred_call(&self);
70
71 // This function should be implemented as
72 // `self.deferred_call.register(&self);`.
73 fn register(&'static self);
74}
75
76/// This struct serves as a lightweight alternative to the use of trait objects
77/// (e.g. `&dyn DeferredCall`). Using a trait object will include a 20 byte
78/// vtable per instance, but this alternative stores only the data and function
79/// pointers, 8 bytes per instance.
80#[derive(Copy, Clone)]
81struct DynDefCallRef<'a> {
82 data: *const (),
83 callback: fn(*const ()),
84 _lifetime: PhantomData<&'a ()>,
85}
86
87impl<'a> DynDefCallRef<'a> {
88 // SAFETY: We define the callback function as being a closure which casts
89 // the passed pointer to be the appropriate type (a pointer to `T`) and then
90 // calls `T::handle_deferred_call()`. In practice, the closure is optimized
91 // away by LLVM when the ABI of the closure and the underlying function are
92 // identical, making this zero-cost, but saving us from having to trust that
93 // `fn(*const ())` and `fn handle_deferred_call(&self)` will always have the
94 // same calling convention for any type.
95 fn new<T: DeferredCallClient>(x: &'a T) -> Self {
96 Self {
97 data: core::ptr::from_ref(x) as *const (),
98 callback: |p| unsafe { T::handle_deferred_call(&*p.cast()) },
99 _lifetime: PhantomData,
100 }
101 }
102}
103
104impl DynDefCallRef<'_> {
105 // More efficient to pass by `self` if we don't have to implement
106 // `DeferredCallClient` directly.
107 fn handle_deferred_call(self) {
108 (self.callback)(self.data)
109 }
110}
111
112/// Counter for the number of deferred calls that have been created, this is
113/// used to track that no more than 32 deferred calls have been created.
114// All 3 of the below global statics are accessed only in this file, and all
115// accesses are via immutable references. Tock is single threaded, so each will
116// only ever be accessed via an immutable reference from the single kernel
117// thread. TODO: Once Tock decides on an approach to replace `static mut` with
118// some sort of `SyncCell`, migrate all three of these to that approach
119// (https://github.com/tock/tock/issues/1545).
120static mut CTR: Cell<usize> = Cell::new(0);
121
122/// This bitmask tracks which of the up to 32 existing deferred calls have been
123/// scheduled. Any bit that is set in that mask indicates the deferred call with
124/// its [`DeferredCall::idx`] field set to the index of that bit has been
125/// scheduled and not yet serviced.
126static mut BITMASK: Cell<u32> = Cell::new(0);
127
128/// An array that stores references to up to 32 `DeferredCall`s via the low-cost
129/// [`DynDefCallRef`].
130// This is a 256 byte array, but at least resides in `.bss`.
131static mut DEFCALLS: [OptionalCell<DynDefCallRef<'static>>; 32] =
132 [const { OptionalCell::empty() }; 32];
133
134pub struct DeferredCall {
135 idx: usize,
136}
137
138impl DeferredCall {
139 /// Create a new deferred call with a unique ID.
140 pub fn new() -> Self {
141 // SAFETY: No accesses to CTR are via an &mut, and the Tock kernel is
142 // single-threaded so all accesses will occur from this thread.
143 let ctr = unsafe { &*addr_of!(CTR) };
144 let idx = ctr.get();
145 ctr.set(idx + 1);
146 DeferredCall { idx }
147 }
148
149 // To reduce monomorphization bloat, the non-generic portion of register is
150 // moved into this function without generic parameters.
151 #[inline(never)]
152 fn register_internal_non_generic(&self, handler: DynDefCallRef<'static>) {
153 // SAFETY: No accesses to DEFCALLS are via an &mut, and the Tock kernel
154 // is single-threaded so all accesses will occur from this thread.
155 let defcalls = unsafe { &*addr_of!(DEFCALLS) };
156 if self.idx >= defcalls.len() {
157 // This error will be caught by the scheduler at the beginning of
158 // the kernel loop, which is much better than panicking here, before
159 // the debug writer is setup. Also allows a single panic for
160 // creating too many deferred calls instead of NUM_DCS panics (this
161 // function is monomorphized).
162 return;
163 }
164 defcalls[self.idx].set(handler);
165 }
166
167 /// This function registers the passed client with this deferred call, such
168 /// that calls to [`DeferredCall::set()`] will schedule a callback on the
169 /// [`handle_deferred_call()`](DeferredCallClient::handle_deferred_call)
170 /// method of the passed client.
171 pub fn register<DC: DeferredCallClient>(&self, client: &'static DC) {
172 let handler = DynDefCallRef::new(client);
173 self.register_internal_non_generic(handler);
174 }
175
176 /// Schedule a deferred callback on the client associated with this deferred
177 /// call.
178 pub fn set(&self) {
179 // SAFETY: No accesses to BITMASK are via an &mut, and the Tock kernel
180 // is single-threaded so all accesses will occur from this thread.
181 let bitmask = unsafe { &*addr_of!(BITMASK) };
182 bitmask.set(bitmask.get() | (1 << self.idx));
183 }
184
185 /// Check if a deferred callback has been set and not yet serviced on this
186 /// deferred call.
187 pub fn is_pending(&self) -> bool {
188 // SAFETY: No accesses to BITMASK are via an &mut, and the Tock kernel
189 // is single-threaded so all accesses will occur from this thread.
190 let bitmask = unsafe { &*addr_of!(BITMASK) };
191 bitmask.get() & (1 << self.idx) == 1
192 }
193
194 /// Services and clears the next pending [`DeferredCall`], returns which
195 /// index was serviced.
196 pub fn service_next_pending() -> Option<usize> {
197 // SAFETY: No accesses to BITMASK/DEFCALLS are via an &mut, and the Tock
198 // kernel is single-threaded so all accesses will occur from this
199 // thread.
200 let bitmask = unsafe { &*addr_of!(BITMASK) };
201 let defcalls = unsafe { &*addr_of!(DEFCALLS) };
202 let val = bitmask.get();
203 if val == 0 {
204 None
205 } else {
206 let bit = val.trailing_zeros() as usize;
207 let new_val = val & !(1 << bit);
208 bitmask.set(new_val);
209 defcalls[bit].map(|dc| {
210 dc.handle_deferred_call();
211 bit
212 })
213 }
214 }
215
216 /// Returns true if any deferred calls are waiting to be serviced, false
217 /// otherwise.
218 pub fn has_tasks() -> bool {
219 // SAFETY: No accesses to BITMASK are via an &mut, and the Tock kernel
220 // is single-threaded so all accesses will occur from this thread.
221 let bitmask = unsafe { &*addr_of!(BITMASK) };
222 bitmask.get() != 0
223 }
224
225 /// This function should be called at the beginning of the kernel loop to
226 /// verify that deferred calls have been correctly initialized. This
227 /// function verifies two things:
228 ///
229 /// 1. That <= [`DEFCALLS.len()`] deferred calls have been created, which is
230 /// the maximum this interface supports.
231 ///
232 /// 2. That exactly as many deferred calls were registered as were created,
233 /// which helps to catch bugs if board maintainers forget to call
234 /// [`register()`](DeferredCall::register) on a created [`DeferredCall`].
235 ///
236 /// Neither of these checks are necessary for soundness, but they are
237 /// necessary for confirming that [`DeferredCall`]s will actually be
238 /// delivered as expected. This function costs about 300 bytes, so you can
239 /// remove it if you are confident your setup will not exceed 32 deferred
240 /// calls, and that all of your components register their deferred calls.
241 // Ignore the clippy warning for using `.filter(|opt| opt.is_some())` since
242 // we don't actually have an Option (we have an OptionalCell) and
243 // IntoIterator is not implemented for OptionalCell.
244 #[allow(clippy::iter_filter_is_some)]
245 pub fn verify_setup() {
246 // SAFETY: No accesses to CTR/DEFCALLS are via an &mut, and the Tock
247 // kernel is single-threaded so all accesses will occur from this
248 // thread.
249 let ctr = unsafe { &*addr_of!(CTR) };
250 let defcalls = unsafe { &*addr_of!(DEFCALLS) };
251 let num_deferred_calls = ctr.get();
252 let num_registered_calls = defcalls.iter().filter(|opt| opt.is_some()).count();
253 if num_deferred_calls > defcalls.len() {
254 panic!("ERROR: too many deferred calls: {}", num_deferred_calls);
255 } else if num_deferred_calls != num_registered_calls {
256 panic!(
257 "ERROR: {} deferred calls, {} registered. A component may have forgotten to register a deferred call.",
258 num_deferred_calls, num_registered_calls
259 );
260 }
261 }
262}