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document the limitations of cyccnt::{Instant,Duration}

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This commit is contained in:
Jorge Aparicio 2019-10-15 19:11:35 -05:00
parent eef4e7bf79
commit 6207008884
1 changed files with 19 additions and 5 deletions
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24
src/cyccnt.rs
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@ -1,4 +1,4 @@
//! Data Watchpoint Trace (DWT) unit's CYCle CouNTer //! Data Watchpoint Trace (DWT) unit's CYCle CouNTer (CYCCNT)
use core::{ use core::{
cmp::Ordering, cmp::Ordering,
@ -16,9 +16,15 @@ use crate::Fraction;
/// ///
/// This data type is only available on ARMv7-M /// This data type is only available on ARMv7-M
/// ///
/// Note that this value is tied to the CYCCNT of one core and that sending it a different core /// # Correctness
/// makes it lose its meaning -- each Cortex-M core has its own CYCCNT counter and these are usually ///
/// unsynchronized and they may even be running at different frequencies. /// Adding or subtracting a `Duration` of more than `(1 << 31)` cycles to an `Instant` effectively
/// makes it "wrap around" and creates an incorrect value. This is also true if the operation is
/// done in steps, e.g. `(instant + dur) + dur` where `dur` is `(1 << 30)` ticks.
///
/// In multi-core contexts: this value is tied to the CYCCNT of *one* core so sending it a different
/// core makes it lose its meaning -- each Cortex-M core has its own CYCCNT counter and these are
/// usually unsynchronized and may even be running at different frequencies.
#[derive(Clone, Copy, Eq, PartialEq)] #[derive(Clone, Copy, Eq, PartialEq)]
pub struct Instant { pub struct Instant {
inner: i32, inner: i32,
@ -61,6 +67,8 @@ impl fmt::Debug for Instant {
impl ops::AddAssign<Duration> for Instant { impl ops::AddAssign<Duration> for Instant {
fn add_assign(&mut self, dur: Duration) { fn add_assign(&mut self, dur: Duration) {
// NOTE this is a debug assertion because there's no foolproof way to detect a wrap around;
// the user may write `(instant + dur) + dur` where `dur` is `(1<<31)-1` ticks.
debug_assert!(dur.inner < (1 << 31)); debug_assert!(dur.inner < (1 << 31));
self.inner = self.inner.wrapping_add(dur.inner as i32); self.inner = self.inner.wrapping_add(dur.inner as i32);
} }
@ -77,7 +85,7 @@ impl ops::Add<Duration> for Instant {
impl ops::SubAssign<Duration> for Instant { impl ops::SubAssign<Duration> for Instant {
fn sub_assign(&mut self, dur: Duration) { fn sub_assign(&mut self, dur: Duration) {
// XXX should this be a non-debug assertion? // NOTE see the NOTE in `<Instant as AddAssign<Duration>>::add_assign`
debug_assert!(dur.inner < (1 << 31)); debug_assert!(dur.inner < (1 << 31));
self.inner = self.inner.wrapping_sub(dur.inner as i32); self.inner = self.inner.wrapping_sub(dur.inner as i32);
} }
@ -115,6 +123,12 @@ impl PartialOrd for Instant {
/// A `Duration` type to represent a span of time. /// A `Duration` type to represent a span of time.
/// ///
/// This data type is only available on ARMv7-M /// This data type is only available on ARMv7-M
///
/// # Correctness
///
/// This type is *not* appropriate for representing time spans in the order of, or larger than,
/// seconds because it can hold a maximum of `(1 << 31)` "ticks" where each tick is the inverse of
/// the CPU frequency, which usually is dozens of MHz.
#[derive(Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)] #[derive(Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)]
pub struct Duration { pub struct Duration {
inner: u32, inner: u32,