Merge pull request #291 from jannic/avoid-64bit-division

Avoid 64bit division
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9names 2022-02-27 10:42:56 +11:00 committed by GitHub
commit 111654fc28
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4 changed files with 85 additions and 25 deletions

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@ -48,3 +48,7 @@ jobs:
with:
command: test
args: --doc --target x86_64-unknown-linux-gnu --features chrono
- uses: actions-rs/cargo@v1
with:
command: test
args: --tests --target x86_64-unknown-linux-gnu

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@ -13,7 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
### Changed
- Updated embedded-hal alpha support to version 1.0.0-alpha.7
- Update embedded-hal alpha support to version 1.0.0-alpha.7
- Avoid 64-bit division in clock calculations
## [0.3.0] - 2021-12-19

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@ -180,19 +180,13 @@ macro_rules! clock {
#[doc = "Configure `"$name"`"]
fn configure_clock<S: ValidSrc<$name>>(&mut self, src: &S, freq: Hertz) -> Result<(), ClockError>{
let src_freq: Hertz<u64> = src.get_freq().into();
let src_freq: Hertz<u32> = src.get_freq().into();
if freq.gt(&src_freq){
return Err(ClockError::CantIncreaseFreq);
}
// Div register is 24.8) int.frac divider so multiply by 2^8 (left shift by 8)
let shifted_src_freq = src_freq * (1 << 8);
let div = if freq.eq(&src_freq) {
1 << 8
} else {
(shifted_src_freq / freq.integer() as u64).integer() as u32
};
let div = fractional_div(src_freq.integer(), freq.integer()).ok_or(ClockError::FrequencyTooLow)?;
// If increasing divisor, set divisor before source. Otherwise set source
// before divisor. This avoids a momentary overspeed when e.g. switching
@ -223,8 +217,7 @@ macro_rules! clock {
self.set_div(div);
// Store the configured frequency
// div contains both the integer part and the fractional part so we need to shift the src_freq equally
self.frequency = (shifted_src_freq / div as u64).try_into().map_err(|_| ClockError::FrequencyToHigh)?;
self.frequency = fractional_div(src_freq.integer(), div).ok_or(ClockError::FrequencyTooHigh)?.Hz();
Ok(())
}
@ -337,19 +330,13 @@ macro_rules! stoppable_clock {
#[doc = "Configure `"$name"`"]
fn configure_clock<S: ValidSrc<$name>>(&mut self, src: &S, freq: Hertz) -> Result<(), ClockError>{
let src_freq: Hertz<u64> = src.get_freq().into();
let src_freq: Hertz<u32> = src.get_freq().into();
if freq.gt(&src_freq){
return Err(ClockError::CantIncreaseFreq);
}
// Div register is 24.8) int.frac divider so multiply by 2^8 (left shift by 8)
let shifted_src_freq = src_freq * (1 << 8);
let div = if freq.eq(&src_freq) {
1 << 8
} else {
(shifted_src_freq / freq.integer() as u64).integer() as u32
};
let div = fractional_div(src_freq.integer(), freq.integer()).ok_or(ClockError::FrequencyTooLow)?;
// If increasing divisor, set divisor before source. Otherwise set source
// before divisor. This avoids a momentary overspeed when e.g. switching
@ -386,7 +373,7 @@ macro_rules! stoppable_clock {
self.set_div(div);
// Store the configured frequency
self.frequency = (shifted_src_freq / div as u64).try_into().map_err(|_| ClockError::FrequencyToHigh)?;
self.frequency = fractional_div(src_freq.integer(), div).ok_or(ClockError::FrequencyTooHigh)?.Hz();
Ok(())
}

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@ -70,10 +70,7 @@ use crate::{
watchdog::Watchdog,
xosc::{setup_xosc_blocking, CrystalOscillator, Error as XoscError, Stable},
};
use core::{
convert::{Infallible, TryInto},
marker::PhantomData,
};
use core::{convert::Infallible, marker::PhantomData};
use embedded_time::rate::*;
use pac::{CLOCKS, PLL_SYS, PLL_USB, RESETS, XOSC};
@ -102,11 +99,15 @@ impl ShareableClocks {
}
/// Something when wrong setting up the clock
#[non_exhaustive]
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub enum ClockError {
/// The frequency desired is higher than the source frequency
CantIncreaseFreq,
/// The desired frequency is to high (would overflow an u32)
FrequencyToHigh,
FrequencyTooHigh,
/// The desired frequency is too low (divider can't reach the desired value)
FrequencyTooLow,
}
/// For clocks
@ -354,3 +355,70 @@ pub fn init_clocks_and_plls(
.map_err(InitError::ClockError)?;
Ok(clocks)
}
// Calculates (numerator<<8)/denominator, avoiding 64bit division
// Returns None if the result would not fit in 32 bit.
fn fractional_div(numerator: u32, denominator: u32) -> Option<u32> {
if denominator.eq(&numerator) {
return Some(1 << 8);
}
let div_int = numerator / denominator;
if div_int >= 1 << 24 {
return None;
}
let div_rem = numerator - (div_int * denominator);
let div_frac = if div_rem < 1 << 24 {
// div_rem is small enough to shift it by 8 bits without overflow
(div_rem << 8) / denominator
} else {
// div_rem is too large. Shift denominator right, instead.
// As 1<<24 < div_rem < denominator, relative error caused by the
// lost lower 8 bits of denominator is smaller than 2^-16
(div_rem) / (denominator >> 8)
};
Some((div_int << 8) + div_frac)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_fractional_div() {
// easy values
assert_eq!(fractional_div(1, 1), Some(1 << 8));
// typical values
assert_eq!(fractional_div(125_000_000, 48_000_000), Some(666));
assert_eq!(fractional_div(48_000_000, 46875), Some(1024 << 8));
// resulting frequencies
assert_eq!(
fractional_div(
125_000_000,
fractional_div(125_000_000, 48_000_000).unwrap()
),
Some(48_048_048)
);
assert_eq!(
fractional_div(48_000_000, fractional_div(48_000_000, 46875).unwrap()),
Some(46875)
);
// not allowed in src/clocks/mod.rs, but should still deliver correct results
assert_eq!(fractional_div(1, 2), Some(128));
assert_eq!(fractional_div(1, 256), Some(1));
assert_eq!(fractional_div(1, 257), Some(0));
// borderline cases
assert_eq!(fractional_div((1 << 24) - 1, 1), Some(((1 << 24) - 1) << 8));
assert_eq!(fractional_div(1 << 24, 1), None);
assert_eq!(fractional_div(1 << 24, 2), Some(1 << (23 + 8)));
assert_eq!(fractional_div(1 << 24, (1 << 24) + 1), Some(1 << 8));
assert_eq!(fractional_div(u32::MAX, u32::MAX), Some(1 << 8));
}
}