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/* * Copyright (C) 2017 AltOS-Rust Team * * This program is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program. If not, see <http://www.gnu.org/licenses/>. */ #![no_std] #![feature(core_intrinsics)] #![feature(asm)] #![feature(unsize)] #![feature(coerce_unsized)] #![deny(trivial_casts, trivial_numeric_casts)] //! Volatile memory operations. //! //! This library contains wrappers around raw pointers to perform volatile memory operations. This //! is mainly useful for memory mapped I/O. Writing to the peripheral memory addresses will //! normally be optimized out by the compiler. The `Volatile` type wraps a memory address to //! perform volatile operations in order to force the compiler to keep these memory accesses and //! stores. //! //! The creation of a `Volatile` pointer is generally unsafe, but the actual operations that you //! can perform on it are considered safe by Rust's standards. This is because the actual memory //! operations are performed through the `Deref` and `DerefMut` traits, which are defined as safe //! methods. It is important to remember that a `Volatile` pointer is nearly identical to a //! primitive pointer, and so all dereferencing operations on one should be considered unsafe (even //! if not enforced by the compiler). //! //! # Examples //! //! ```rust,no_run //! use volatile::Volatile; //! //! const IO_ADDR: *const u32 = 0x4000_4400 as *const _; //! //! unsafe { //! let mut io_ptr = Volatile::new(IO_ADDR); //! // Some bit that we need to set for an IO operation //! *io_ptr |= 0b1 << 5; //! } //! ``` //! //! On some embedded devices you may want to do something like wait for a certain amount of time to //! pass measured by some amount of ticks. //! //! ```rust,no_run //! // Some tick counter that may be updated by a hardware interrupt //! static mut TICKS: usize = 0; //! //! while unsafe { TICKS < 10 } {/* wait for ticks to change */} //! ``` //! //! Normally, the Rust compilier would optimize this kind of operation into just an infinite //! `loop`, since the value of `TICKS` can't change in a single threaded environment, but `TICKS` //! could be updated by some hardware interrupt, so we want to keep reloading the value in order to //! check it. So to get around this we can use a `Volatile` pointer to force the compiler to reload //! the value every time through the loop. //! //! ```rust,no_run //! use volatile::Volatile; //! //! static mut TICKS: usize = 0; //! //! unsafe { //! let ticks_ptr = Volatile::new(&TICKS); //! while *ticks_ptr < 10 {/* wait for ticks to change */} //! } //! ``` //! //! Now the value of `TICKS` will be reloaded every time through the loop. //! //! Oftentimes when working with memory mapped peripherals, you will have a block of memory that //! you want to be working on that contains control, status, and data registers for some hardware //! peripheral. These are often best represented as structs with each of their registers as fields, //! but without volatile operations, loads and stores to these memory addresses often get optimized //! out by the compiler. To get around this you can use a `Volatile` pointer to point at the mapped //! address and have it be represented as a struct of the correct type. //! //! ```rust,no_run //! use volatile::Volatile; //! //! const USART_ADDR: *const Usart = 0x4000_4400 as *const _; //! // For transmitting and receiving data over serial //! #[repr(C)] //! struct Usart { //! control_reg: u32, //! status_reg: u32, //! tx_data_reg: u32, //! rx_data_reg: u32, //! } //! //! let recieved = unsafe { //! let mut usart_block = Volatile::new(USART_ADDR); //! // Set some bits, these will be hardware specific //! usart_block.control_reg |= 0b11 << 5; //! //! while usart_block.status_reg & 0b1 << 7 == 0 {/*wait for hardware to set a bit*/} //! //! // Transmit some data //! usart_block.tx_data_reg = 100; //! //! while usart_block.status_reg & 0b1 << 6 == 0 {/*wait for hardware to set some other bit*/} //! //! // Receive some data //! usart_block.rx_data_reg //! }; //! ``` //! //! Every field access to a pointed at struct will be considered volatile and so will not be //! optimized out by the compiler. //! //! Just as with primitive pointers, `Volatile` pointers can be created from valid references //! safely, though their use should still be considered unsafe. //! //! ```rust //! # #![allow(dead_code)] //! use volatile::Volatile; //! //! let x: u32 = 0; //! let ptr = Volatile::from(&x); //! ``` mod tests; use core::fmt; use core::hash; use core::ops::*; use core::marker::Unsize; use core::intrinsics::{volatile_load, volatile_store}; /// A volatile pointer. /// /// This type acts as a pointer that only uses volatile operations. Pointer arithmetic can be /// performed on it, and it can be dereferenced to its raw type in order to perform volatile memory /// operations. This is especially useful for I/O operations where writing and reading from memory /// mapped I/O registers would normally be optimized out by the compiler. /// /// The `Volatile` type has the same syntax as a primitive pointer, so all functions that you can /// expect to use with a primitive pointer like `*const` or `*mut` can also be used with a /// `Volatile` pointer. /// /// # Examples /// /// ```rust /// use volatile::Volatile; /// /// let value: i32 = 0; /// unsafe { /// let mut ptr = Volatile::new(&value); /// let mask = 0x0F0F; /// *ptr |= mask; /// } /// assert_eq!(value, 0x0F0F); /// ``` /// /// `Volatile` pointers can also be used to point at whole structs, and any memory accesses into /// that struct will also be considered volatile /// /// ```rust /// use volatile::Volatile; /// use std::mem; /// /// struct IODevice { /// reg1: u32, /// reg2: u32, /// reg3: u32, /// } /// /// let io_device: IODevice = unsafe { mem::uninitialized() }; /// /// unsafe { /// let mut ptr = Volatile::new(&io_device); /// ptr.reg1 = 1; /// ptr.reg2 = 2; /// ptr.reg3 = 3; /// assert_eq!(ptr.reg1, 1); /// assert_eq!(ptr.reg2, 2); /// assert_eq!(ptr.reg3, 3); /// } /// ``` /// /// # Safety /// /// Because `Volatile` pointers are often used for memory mapped peripherals, the compiler can not /// guarantee that the address pointed at is valid, so the creation of a `Volatile` pointer is /// unsafe. If you have a valid reference to some object, you can safely create a `Volatile` /// pointer to that object through the `From` trait implemented for references. /// /// ```rust /// use volatile::Volatile; /// /// let x: u32 = 0; /// let ptr = Volatile::from(&x); /// ``` /// /// Be wary however that a `Volatile` pointer provides interior mutability, so creating a pointer /// from a shared reference may break immutability guarantees if used improperly. #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug)] pub struct Volatile<T: ?Sized>(*const T); impl<T: ?Sized> Volatile<T> { /// Create a new `Volatile` pointer. /// /// # Examples /// /// ```rust /// use volatile::Volatile; /// /// const IO_ADDR: *const u32 = 0x4100_2000 as *const _; /// /// unsafe { /// let ptr = Volatile::new(IO_ADDR); /// } /// ``` /// /// # Safety /// /// Because `Volatile` pointers are often used to point at essentially arbitrary memory /// addresses, the compiler can not guarantee that the pointed at address is valid, so the /// creation of a `Volatile` pointer is considered unsafe. pub unsafe fn new(ptr: *const T) -> Self { Volatile(ptr) } /// Return true if the pointer is null, false otherwise pub fn is_null(self) -> bool where T: Sized { (self.0).is_null() } /// Return the inner pointer. /// /// Future accesses to this inner pointer will not be volatile. pub fn as_ptr(self) -> *const T { self.0 } /// Returns the inner pointer mutably. /// /// Future accesses to this inner pointer will not be volatile. pub fn as_mut(self) -> *mut T { self.0 as *mut T } /// Calculate the offset from the inner pointer, returning a new Volatile pointer. /// /// `count` is in units of T; e.g. a `count` of 3 represents a pointer offset of `3 * /// size_of::<T>()` bytes. /// /// # Safety /// /// Both the starting and resulting pointer must be either in bounds or one byte past the end /// of an allocated object. If either pointer is out of bounds or arithmetic overflow occurs /// then any further use of the returned value will result in undefined behavior. pub unsafe fn offset(self, count: isize) -> Self where T: Sized { Volatile::new((self.0).offset(count)) } /// Store a value into the memory address pointed at. /// /// This operation is guaranteed to not be optimized out by the compiler, even if it believes /// that it will have no effect on the program. /// /// # Examples /// /// ```rust /// use volatile::Volatile; /// /// let x: u32 = 0; /// unsafe { /// let mut ptr = Volatile::new(&x); /// ptr.store(0x1234); /// } /// /// assert_eq!(x, 0x1234); /// ``` /// /// # Safety /// /// Storing to a `Volatile` pointer is equivalent to storing a value to a primitive raw pointer /// so the operation could potentially be performed on some shared data or even an invalid /// address. pub unsafe fn store(&mut self, rhs: T) where T: Sized { volatile_store(self.0 as *mut T, rhs); } /// Load a value from the memory address pointed at. /// /// This operation is guaranteed to not be optimized out by the compiler, even if it believes /// that it will have no effect on the program. /// /// # Examples /// /// ```rust /// use volatile::Volatile; /// /// let x: u32 = 0xAAAA; /// unsafe { /// let ptr = Volatile::new(&x); /// assert_eq!(ptr.load(), 0xAAAA); /// } /// ``` /// /// # Safety /// /// Loading from a `Volatile` pointer is equivalent to loading a value from a primitive raw /// pointer so the operation could potentially be performed on an invalid address. pub unsafe fn load(&self) -> T where T: Sized { volatile_load(self.0) } /// Perform a read-modify-write operation on the memory address pointed at. /// /// This operation is guaranteed to not be optimized out by the compiler, even if it believes /// that it will have no effect on the program. The value stored in the address pointed at will /// be loaded and passed into the given function before being written back out to memory. /// /// # Examples /// ```rust /// use volatile::Volatile; /// /// let x: u32 = 0; /// unsafe { /// let mut ptr = Volatile::new(&x); /// ptr.modify(|x| { /// let old = *x; /// if old == 0 { /// *x = 100; /// } /// else { /// *x = 200; /// } /// }); /// } /// assert_eq!(x, 100); /// ``` /// /// # Safety /// /// Modifying a `Volatile` pointer's contents involves both a load and store of the underlying /// raw pointer so it could be performed on some shared data or even on an invalid address. pub unsafe fn modify<F>(&mut self, f: F) where T: Sized, F: FnOnce(&mut T) { let mut value = volatile_load(self.0); f(&mut value); volatile_store(self.0 as *mut T, value); } } impl<'a, T: ?Sized + 'a> From<&'a T> for Volatile<T> { fn from(reference: &'a T) -> Self { unsafe { Volatile::new(reference) } } } impl<'a, T: ?Sized + 'a> From<&'a mut T> for Volatile<T> { fn from(reference: &'a mut T) -> Self { unsafe { Volatile::new(reference) } } } impl<T: ?Sized> Deref for Volatile<T> { type Target = T; fn deref(&self) -> &Self::Target { unsafe { // A bit of a hack to forcibly get Rust to reload the value from memory, we mark this // empty assemby block here as clobbering memory, so the compiler thinks that the // pointer we're about to load from may have been touched asm!("" ::: "memory" : "volatile"); &*(self.0) } } } impl<T: ?Sized> DerefMut for Volatile<T> { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { asm!("" ::: "memory" : "volatile"); &mut *((self.0) as *mut _) } } } impl<T: ?Sized> hash::Hash for Volatile<T> where T: Sized { fn hash<H: hash::Hasher>(&self, state: &mut H) { (self.0).hash(state); } } impl<T> fmt::Pointer for Volatile<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Pointer::fmt(&self.0, f) } } impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Volatile<U>> for Volatile<T> {}