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//! # Cortex-M Interrupt Move //! //! It's the next best thing to moving to interrupt context. //! //! ## The goal //! //! The goal here is to replace usage of a mutex which may require an entire critical section, and instead model "Moving" of data to an interrupt context. //! //! This means that we don't need a critical section to access it, we just need to be in the interrupt we moved the data to. #![no_std] use bare_metal::Nr; use core::cell::UnsafeCell; use core::mem::MaybeUninit; use core::result::Result; use core::sync::atomic::AtomicUsize; use core::sync::atomic::Ordering; use cortex_m::interrupt::free; use cortex_m::peripheral::{scb::VectActive, SCB}; pub struct Move<T, I> { /// `data` contains the user data, which may or may not be initialized data: UnsafeCell<MaybeUninit<T>>, // `state` is a runtime tracking of our current state. state: AtomicUsize, // `inter` is the interrupt type. This type is unique to every chip // as it is generated by svd2rust, but all types implement the `Nr` // trait inter: I, } unsafe impl<T, I> Sync for Move<T, I> where T: Send + Sized, I: Nr, { } /// Move is a structure that is intended to be stored as a static variable, /// and represents a metaphorical "move" to an interrupt context. Data is moved /// to the interrupt context by calling `try_move` from thread (non-interrupt) /// context, and the data can be retrived within a selected interrupt using the /// `try_lock` method. impl<T, I> Move<T, I> { /// The data is uninitialized const UNINIT: usize = 0; /// The data is initialized and not currently locked const INIT_AND_IDLE: usize = 1; /// The data is initialized, but currently locked by an interrupt const LOCKED: usize = 2; /// Create a new `Move` structure without initializing the data contained by it. /// This is best used when the data cannot be initialized until runtime, such as /// a HAL peripheral, or the producer or consumer of a queue. /// /// Before using this in interrupt context, you must initialize it with the /// `try_move` function, or it will return errors upon access. /// /// You must provide the interrupt that is allowed to later access this data /// as the `inter` argument pub const fn new_uninitialized(inter: I) -> Self { Move { data: UnsafeCell::new(MaybeUninit::uninit()), inter, state: AtomicUsize::new(Self::UNINIT), } } /// Create a new `Move` structure, and initialize the data contained within it. /// This is best used when the data contained within is `const`, and doesn't require /// runtime initialization. /// /// This does not require further interaction before use in interrupt context. /// /// You must provide the interrupt that is allowed to later access this data /// as the `inter` argument pub const fn new(data: T, inter: I) -> Self { Move { data: UnsafeCell::new(MaybeUninit::new(data)), inter, state: AtomicUsize::new(Self::INIT_AND_IDLE), } } } impl<T, I> Move<T, I> where T: Send + Sized, I: Nr, { /// Attempt to initialize the data of the `Move` structure. /// This *MUST* be called from non-interrupt context, and a critical /// section will be in place while setting the data. /// /// Returns: /// /// * Ok(Some(T)): If we are in thread mode and the data was previously initialized /// * Ok(None): If we are in thread mode and the data was not previously initialized /// * Err(T): If we are not in thread mode (e.g. an interrupt is active), return the /// data that was going to be moved pub fn try_move(&self, data: T) -> Result<Option<T>, T> { free(|_cs| { // Check if we are in non-interrupt context match SCB::vect_active() { // TODO: Would it be reasonable to initialize this from a DIFFERENT // interrupt context? Basically anything but the destination interrupt? VectActive::ThreadMode => {} _ => { return Err(data); } } // Since we are in a critical section, it is not necessary to perform // an atomic compare and swap, as we cannot be pre-empted match self.state.load(Ordering::SeqCst) { Self::UNINIT => { unsafe { // Reference to an uninitialized MaybeUninit let mu_ref = &mut *self.data.get(); // Get a pointer to the data, and use ptr::write to avoid // viewing or creating a reference to uninitialized data let dat_ptr = mu_ref.as_mut_ptr(); dat_ptr.write(data); } self.state.store(Self::INIT_AND_IDLE, Ordering::SeqCst); Ok(None) } Self::INIT_AND_IDLE => { let old = unsafe { // Reference to an initialized MaybeUninit let mu_ref = &mut *self.data.get(); // Get a pointer to the data, and use ptr::replace, // a mem::swap is probably okay since this is initialized, // but use ptr methods anyway let dat_ptr = mu_ref.as_mut_ptr(); dat_ptr.replace(data) }; Ok(Some(old)) } Self::LOCKED | _ => Err(data), } }) } /// Attempt to recover the data from the `Move` structure. /// This *MUST* be called from non-interrupt context, and a critical /// section will be in place while receiving the data. /// /// Returns: /// /// * Ok(Some(T)): If we are in thread mode and the data was previously initialized /// * Ok(None): If we are in thread mode and the data was not previously initialized /// * Err(()): If we are not in thread mode (e.g. an interrupt is active) pub fn try_free(&self) -> Result<Option<T>, ()> { free(|_cs| { // Check if we are in non-interrupt context match SCB::vect_active() { // TODO: Would it be reasonable to free this from a DIFFERENT // interrupt context? Basically anything but the destination interrupt? VectActive::ThreadMode => {} _ => { return Err(()); } } // Since we are in a critical section, it is not necessary to perform // an atomic compare and swap, as we cannot be pre-empted match self.state.load(Ordering::SeqCst) { Self::UNINIT => Ok(None), Self::INIT_AND_IDLE => { let old = unsafe { // Get a pointer to the initialized data let mu_ptr = self.data.get(); // Replace it with an uninitialized field. I winder if this is // just a no-op, or if we should explicitly zero the memory here mu_ptr.replace(MaybeUninit::uninit()).assume_init() }; self.state.store(Self::UNINIT, Ordering::SeqCst); Ok(Some(old)) } Self::LOCKED | _ => Err(()), } }) } /// So, this isn't a classical mutex. It will *only* provide access if: /// /// * The selected interrupt is currently active /// * The mutex has not already been locked /// /// If these conditions are met, then you can access the variable from within /// a closure pub fn try_lock<R>(&self, f: impl FnOnce(&mut T) -> R) -> Result<R, ()> { match SCB::vect_active() { VectActive::Interrupt { irqn } if irqn == self.inter.nr() => { // Okay to go ahead } _ => return Err(()), }; // We know that the current interrupt is active, which means // that thread mode cannot resume until we exit this function. // We don't need to worry about compare and swap, because we // are now the only ones who can access this data match self.state.load(Ordering::SeqCst) { // The data is uninitialized. Don't provide access Self::UNINIT => Err(()), // The data is initialized, allow access within a closure // This prevents re-entrancy of re-calling lock within the // closure Self::INIT_AND_IDLE => { self.state.store(Self::LOCKED, Ordering::SeqCst); let dat_ref = unsafe { // Create a mutable reference to an initialized MaybeUninit let mu_ref = &mut *self.data.get(); // Create a mutable reference to the initialized data behind // the MaybeUninit. This is fine, because the scope of this // reference can only live to the end of this function, and // cannot be captured by the closure used below. // // Additionally we have a re-entrancy check above, to prevent // creating a duplicate &mut to the inner data let dat_ptr = mu_ref.as_mut_ptr(); &mut *dat_ptr }; // Call the user's closure, providing access to the data let ret = f(dat_ref); self.state.store(Self::INIT_AND_IDLE, Ordering::SeqCst); Ok(ret) } // The data is locked, or the status register is garbage. // Don't provide access Self::LOCKED | _ => Err(()), } } }