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#![cfg_attr(feature="const_fn", feature(const_fn))]
//! Provides wrapper types `Volatile`, `ReadOnly`, `WriteOnly`, `ReadWrite`, which wrap any copy-able type and allows for
//! volatile memory access to wrapped value. Volatile memory accesses are never optimized away by
//! the compiler, and are useful in many low-level systems programming and concurrent contexts.
//!
//! The wrapper types *do not* enforce any atomicity guarantees; to also get atomicity, consider
//! looking at the `Atomic` wrapper type found in `libcore` or `libstd`.
//!
//! These wrappers do not depend on the standard library and never panic.
//!
//! # Dealing with Volatile Pointers
//!
//! Frequently, one may have to deal with volatile pointers, eg, writes to specific memory
//! locations. The canonical way to solve this is to cast the pointer to a volatile wrapper
//! directly, eg:
//!
//! ```rust
//! use volatile::Volatile;
//!
//! let mut_ptr = 0xFEE00000 as *mut u32;
//!
//! let volatile_ptr = mut_ptr as *mut Volatile<u32>;
//! ```
//!
//! and then perform operations on the pointer as usual in a volatile way. This method works as all
//! of the volatile wrapper types are the same size as their contained values.
#![no_std]
use core::ptr;
/// A wrapper type around a volatile variable, which allows for volatile reads and writes
/// to the contained value. The stored type needs to be `Copy`, as volatile reads and writes
/// take and return copies of the value.
///
/// The size of this struct is the same as the size of the contained type.
#[derive(Debug, Clone)]
pub struct Volatile<T: Copy>(T);
impl<T: Copy> Volatile<T> {
/// Construct a new volatile instance wrapping the given value.
///
/// ```rust
/// use volatile::Volatile;
///
/// let value = Volatile::new(0u32);
/// ```
///
/// # Panics
///
/// This method never panics.
#[cfg(feature="const_fn")]
pub const fn new(value: T) -> Volatile<T> {
Volatile(value)
}
/// Construct a new volatile instance wrapping the given value.
///
/// ```rust
/// use volatile::Volatile;
///
/// let value = Volatile::new(0u32);
/// ```
///
/// # Panics
///
/// This method never panics.
#[cfg(not(feature="const_fn"))]
pub fn new(value: T) -> Volatile<T> {
Volatile(value)
}
/// Performs a volatile read of the contained value, returning a copy
/// of the read value. Volatile reads are guaranteed not to be optimized
/// away by the compiler, but by themselves do not have atomic ordering
/// guarantees. To also get atomicity, consider looking at the `Atomic` wrapper type.
///
/// ```rust
/// use volatile::Volatile;
///
/// let value = Volatile::new(42u32);
///
/// assert_eq!(value.read(), 42u32);
/// ```
///
/// # Panics
///
/// This method never panics.
pub fn read(&self) -> T {
// UNSAFE: Safe, as we know that our internal value exists.
unsafe { ptr::read_volatile(&self.0) }
}
/// Performs a volatile write, setting the contained value to the given value `value`. Volatile
/// writes are guaranteed to not be optimized away by the compiler, but by themselves do not
/// have atomic ordering guarantees. To also get atomicity, consider looking at the `Atomic`
/// wrapper type.
///
/// ```rust
/// use volatile::Volatile;
///
/// let mut value = Volatile::new(0u32);
///
/// value.write(42u32);
///
/// assert_eq!(value.read(), 42u32);
/// ```
///
/// # Panics
///
/// This method never panics.
pub fn write(&mut self, value: T) {
// UNSAFE: Safe, as we know that our internal value exists.
unsafe { ptr::write_volatile(&mut self.0, value) };
}
/// Performs a volatile read of the contained value, passes a mutable reference to it to the
/// function `f`, and then performs a volatile write of the (potentially updated) value back to
/// the contained value.
///
/// ```rust
/// use volatile::Volatile;
///
/// let mut value = Volatile::new(21u32);
///
/// value.update(|val_ref| *val_ref *= 2);
///
/// assert_eq!(value.read(), 42u32);
/// ```
///
/// # Panics
///
/// Ths method never panics.
pub fn update<F>(&mut self, f: F)
where F: FnOnce(&mut T)
{
let mut value = self.read();
f(&mut value);
self.write(value);
}
}
/// A volatile wrapper which only allows read operations.
///
/// The size of this struct is the same as the contained type.
#[derive(Debug, Clone)]
pub struct ReadOnly<T: Copy>(Volatile<T>);
impl<T: Copy> ReadOnly<T> {
/// Construct a new read-only volatile wrapper wrapping the given value.
///
/// ```rust
/// use volatile::ReadOnly;
///
/// let value = ReadOnly::new(42u32);
/// ```
///
/// # Panics
///
/// This function never panics.
#[cfg(feature = "const_fn")]
pub const fn new(value: T) -> ReadOnly<T> {
ReadOnly(Volatile::new(value))
}
/// Construct a new read-only volatile wrapper wrapping the given value.
///
/// ```rust
/// use volatile::ReadOnly;
///
/// let value = ReadOnly::new(42u32);
/// ```
///
/// # Panics
///
/// This function never panics.
#[cfg(not(feature = "const_fn"))]
pub fn new(value: T) -> ReadOnly<T> {
ReadOnly(Volatile::new(value))
}
/// Perform a volatile read of the contained value, returning a copy of the read value.
/// Functionally equivalent to `Volatile::read`.
///
/// ```rust
/// use volatile::ReadOnly;
///
/// let value = ReadOnly::new(42u32);
/// assert_eq!(value.read(), 42u32);
/// ```
///
/// # Panics
///
/// This function never panics.
pub fn read(&self) -> T {
self.0.read()
}
}
/// A volatile wrapper which only allows write operations.
///
/// The size of this struct is the same as the contained type.
#[derive(Debug, Clone)]
pub struct WriteOnly<T: Copy>(Volatile<T>);
impl<T: Copy> WriteOnly<T> {
/// Constructs a new write only volatile wrapper around the given value.
///
/// ```rust
/// use volatile::WriteOnly;
///
/// let value = WriteOnly::new(0u32);
/// ```
///
/// # Panics
///
/// This function never panics.
#[cfg(feature = "const_fn")]
pub const fn new(value: T) -> WriteOnly<T> {
WriteOnly(Volatile::new(value))
}
/// Constructs a new write only volatile wrapper around the given value.
///
/// ```rust
/// use volatile::WriteOnly;
///
/// let value = WriteOnly::new(0u32);
/// ```
///
/// # Panics
///
/// This function never panics.
#[cfg(not(feature = "const_fn"))]
pub fn new(value: T) -> WriteOnly<T> {
WriteOnly(Volatile::new(value))
}
/// Performs a volatile write of value `value` into the contained value. Functionally identical
/// to `Volatile::write`.
///
/// ```rust
/// use volatile::WriteOnly;
///
/// let mut value = WriteOnly::new(0u32);
///
/// value.write(42u32);
/// ```
///
/// # Panics
///
/// This method never panics.
pub fn write(&mut self, value: T) {
self.0.write(value)
}
}
/// A volatile wrapper which allows both read and write operations;
/// functionally equivalent to the `Volatile` type, as it is a type
/// alias for it.
///
/// The size of this struct is the same as the contained type.
pub type ReadWrite<T> = Volatile<T>;
#[cfg(test)]
mod tests {
use super::Volatile;
#[test]
fn test_read() {
assert_eq!(Volatile(42).read(), 42);
}
#[test]
fn test_write() {
let mut volatile = Volatile(42);
volatile.write(50);
assert_eq!(volatile.0, 50);
}
#[test]
fn test_update() {
let mut volatile = Volatile(42);
volatile.update(|v| *v += 1);
assert_eq!(volatile.0, 43);
}
#[test]
fn test_pointer_recast() {
let mut target_value = 0u32;
let target_ptr: *mut u32 = &mut target_value;
let volatile_ptr = target_ptr as *mut Volatile<u32>;
// UNSAFE: Safe, as we know the value exists on the stack.
unsafe { (*volatile_ptr).write(42u32); }
assert_eq!(target_value, 42u32);
}
}