rtic/stable/book/en/by-example/delay.html
2024-10-24 05:57:30 +00:00

487 lines
25 KiB
HTML

<!DOCTYPE HTML>
<html lang="en" class="light" dir="ltr">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Delay and Timeout using Monotonics - Real-Time Interrupt-driven Concurrency</title>
<!-- Custom HTML head -->
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff">
<link rel="icon" href="../favicon.svg">
<link rel="shortcut icon" href="../favicon.png">
<link rel="stylesheet" href="../css/variables.css">
<link rel="stylesheet" href="../css/general.css">
<link rel="stylesheet" href="../css/chrome.css">
<link rel="stylesheet" href="../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="../highlight.css">
<link rel="stylesheet" href="../tomorrow-night.css">
<link rel="stylesheet" href="../ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body class="sidebar-visible no-js">
<div id="body-container">
<!-- Provide site root to javascript -->
<script>
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script>
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script>
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('light')
html.classList.add(theme);
var body = document.querySelector('body');
body.classList.remove('no-js')
body.classList.add('js');
</script>
<input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
<!-- Hide / unhide sidebar before it is displayed -->
<script>
var body = document.querySelector('body');
var sidebar = null;
var sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
}
sidebar_toggle.checked = sidebar === 'visible';
body.classList.remove('sidebar-visible');
body.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter"><li class="chapter-item expanded affix "><a href="../preface.html">Preface</a></li><li class="chapter-item expanded affix "><li class="spacer"></li><li class="chapter-item expanded "><a href="../starting_a_project.html"><strong aria-hidden="true">1.</strong> Starting a new project</a></li><li class="chapter-item expanded "><a href="../by-example.html"><strong aria-hidden="true">2.</strong> RTIC by example</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../by-example/app.html"><strong aria-hidden="true">2.1.</strong> The app</a></li><li class="chapter-item expanded "><a href="../by-example/hardware_tasks.html"><strong aria-hidden="true">2.2.</strong> Hardware tasks</a></li><li class="chapter-item expanded "><a href="../by-example/software_tasks.html"><strong aria-hidden="true">2.3.</strong> Software tasks & spawn</a></li><li class="chapter-item expanded "><a href="../by-example/resources.html"><strong aria-hidden="true">2.4.</strong> Resources</a></li><li class="chapter-item expanded "><a href="../by-example/app_init.html"><strong aria-hidden="true">2.5.</strong> The init task</a></li><li class="chapter-item expanded "><a href="../by-example/app_idle.html"><strong aria-hidden="true">2.6.</strong> The idle task</a></li><li class="chapter-item expanded "><a href="../by-example/channel.html"><strong aria-hidden="true">2.7.</strong> Channel based communication</a></li><li class="chapter-item expanded "><a href="../by-example/delay.html" class="active"><strong aria-hidden="true">2.8.</strong> Delay and Timeout using Monotonics</a></li><li class="chapter-item expanded "><a href="../by-example/app_minimal.html"><strong aria-hidden="true">2.9.</strong> The minimal app</a></li><li class="chapter-item expanded "><a href="../by-example/tips/index.html"><strong aria-hidden="true">2.10.</strong> Tips & Tricks</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../by-example/tips/destructureing.html"><strong aria-hidden="true">2.10.1.</strong> Resource de-structure-ing</a></li><li class="chapter-item expanded "><a href="../by-example/tips/indirection.html"><strong aria-hidden="true">2.10.2.</strong> Avoid copies when message passing</a></li><li class="chapter-item expanded "><a href="../by-example/tips/static_lifetimes.html"><strong aria-hidden="true">2.10.3.</strong> 'static super-powers</a></li><li class="chapter-item expanded "><a href="../by-example/tips/view_code.html"><strong aria-hidden="true">2.10.4.</strong> Inspecting generated code</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../monotonic_impl.html"><strong aria-hidden="true">3.</strong> Monotonics & the Timer Queue</a></li><li class="chapter-item expanded "><a href="../rtic_vs.html"><strong aria-hidden="true">4.</strong> RTIC vs. the world</a></li><li class="chapter-item expanded "><a href="../rtic_and_embassy.html"><strong aria-hidden="true">5.</strong> RTIC and Embassy</a></li><li class="chapter-item expanded "><a href="../awesome_rtic.html"><strong aria-hidden="true">6.</strong> Awesome RTIC examples</a></li><li class="chapter-item expanded affix "><li class="spacer"></li><li class="chapter-item expanded "><a href="../migration_v1_v2.html"><strong aria-hidden="true">7.</strong> Migrating from v1.0.x to v2.0.0</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../migration_v1_v2/monotonics.html"><strong aria-hidden="true">7.1.</strong> Migrating to rtic-monotonics</a></li><li class="chapter-item expanded "><a href="../migration_v1_v2/async_tasks.html"><strong aria-hidden="true">7.2.</strong> Software tasks must now be async</a></li><li class="chapter-item expanded "><a href="../migration_v1_v2/rtic-sync.html"><strong aria-hidden="true">7.3.</strong> Using and understanding rtic-sync</a></li><li class="chapter-item expanded "><a href="../migration_v1_v2/complete_example.html"><strong aria-hidden="true">7.4.</strong> A code example on migration</a></li></ol></li><li class="chapter-item expanded "><li class="spacer"></li><li class="chapter-item expanded "><a href="../internals.html"><strong aria-hidden="true">8.</strong> Under the hood</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../internals/targets.html"><strong aria-hidden="true">8.1.</strong> Cortex-M architectures</a></li></ol></li></ol>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle">
<div class="sidebar-resize-indicator"></div>
</div>
</nav>
<!-- Track and set sidebar scroll position -->
<script>
var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
sidebarScrollbox.addEventListener('click', function(e) {
if (e.target.tagName === 'A') {
sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
}
}, { passive: true });
var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
sessionStorage.removeItem('sidebar-scroll');
if (sidebarScrollTop) {
// preserve sidebar scroll position when navigating via links within sidebar
sidebarScrollbox.scrollTop = sidebarScrollTop;
} else {
// scroll sidebar to current active section when navigating via "next/previous chapter" buttons
var activeSection = document.querySelector('#sidebar .active');
if (activeSection) {
activeSection.scrollIntoView({ block: 'center' });
}
}
</script>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky">
<div class="left-buttons">
<label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</label>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Real-Time Interrupt-driven Concurrency</h1>
<div class="right-buttons">
<a href="../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/rtic-rs/rtic" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script>
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1 id="tasks-with-delay"><a class="header" href="#tasks-with-delay">Tasks with delay</a></h1>
<p>A convenient way to express miniminal timing requirements is by delaying progression.</p>
<p>This can be achieved by instantiating a monotonic timer (for implementations, see <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics"><code>rtic-monotonics</code></a>):</p>
<pre><code class="language-rust noplayground">...
#[init]
fn init(cx: init::Context) -&gt; (Shared, Local) {
hprintln!("init");
Mono::start(cx.core.SYST, 12_000_000);
...</code></pre>
<p>A <em>software</em> task can <code>await</code> the delay to expire:</p>
<pre><code class="language-rust noplayground">#[task]
async fn foo(_cx: foo::Context) {
...
Systick::delay(100.millis()).await;
...
}
</code></pre>
<details>
<summary>A complete example</summary>
<pre><code class="language-rust noplayground">//! examples/async-delay.rs
#![no_main]
#![no_std]
#![deny(warnings)]
#![deny(unsafe_code)]
#![deny(missing_docs)]
use panic_semihosting as _;
#[rtic::app(device = lm3s6965, dispatchers = [SSI0, UART0], peripherals = true)]
mod app {
use cortex_m_semihosting::{debug, hprintln};
use rtic_monotonics::systick::prelude::*;
systick_monotonic!(Mono, 100);
#[shared]
struct Shared {}
#[local]
struct Local {}
#[init]
fn init(cx: init::Context) -&gt; (Shared, Local) {
hprintln!("init");
Mono::start(cx.core.SYST, 12_000_000);
foo::spawn().ok();
bar::spawn().ok();
baz::spawn().ok();
(Shared {}, Local {})
}
#[task]
async fn foo(_cx: foo::Context) {
hprintln!("hello from foo");
Mono::delay(100.millis()).await;
hprintln!("bye from foo");
}
#[task]
async fn bar(_cx: bar::Context) {
hprintln!("hello from bar");
Mono::delay(200.millis()).await;
hprintln!("bye from bar");
}
#[task]
async fn baz(_cx: baz::Context) {
hprintln!("hello from baz");
Mono::delay(300.millis()).await;
hprintln!("bye from baz");
debug::exit(debug::EXIT_SUCCESS);
}
}</code></pre>
<pre><code class="language-console">$ cargo xtask qemu --verbose --example async-delay --features test-critical-section
</code></pre>
<pre><code class="language-console">init
hello from bar
hello from baz
hello from foo
bye from foo
bye from bar
bye from baz
</code></pre>
</details>
<blockquote>
<p>Interested in contributing new implementations of <a href="https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html"><code>Monotonic</code></a>, or more information about the inner workings of monotonics?
Check out the <a href="../monotonic_impl.html">Implementing a <code>Monotonic</code></a> chapter!</p>
</blockquote>
<h2 id="timeout"><a class="header" href="#timeout">Timeout</a></h2>
<p>Rust <a href="https://doc.rust-lang.org/std/future/trait.Future.html"><code>Future</code></a>s (underlying Rust <code>async</code>/<code>await</code>) are composable. This makes it possible to <code>select</code> in between <code>Futures</code> that have completed.</p>
<p>A common use case is transactions with an associated timeout. In the examples shown below, we introduce a fake HAL device that performs some transaction. We have modelled the time it takes based on the input parameter (<code>n</code>) as <code>350ms + n * 100ms</code>.</p>
<p>Using the <code>select_biased</code> macro from the <code>futures</code> crate it may look like this:</p>
<pre><code class="language-rust noplayground noplayground"> // Call hal with short relative timeout using `select_biased`
select_biased! {
v = hal_get(1).fuse() =&gt; hprintln!("hal returned {}", v),
_ = Mono::delay(200.millis()).fuse() =&gt; hprintln!("timeout", ), // this will finish first
}
// Call hal with long relative timeout using `select_biased`
select_biased! {
v = hal_get(1).fuse() =&gt; hprintln!("hal returned {}", v), // hal finish first
_ = Mono::delay(1000.millis()).fuse() =&gt; hprintln!("timeout", ),
}</code></pre>
<p>Assuming the <code>hal_get</code> will take 450ms to finish, a short timeout of 200ms will expire before <code>hal_get</code> can complete.</p>
<p>Extending the timeout to 1000ms would cause <code>hal_get</code> will to complete first.</p>
<p>Using <code>select_biased</code> any number of futures can be combined, so its very powerful. However, as the timeout pattern is frequently used, more ergonomic support is baked into RTIC, provided by the <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics"><code>rtic-monotonics</code></a> and <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-time"><code>rtic-time</code></a> crates.</p>
<p>Rewriting the second example from above using <code>timeout_after</code> gives:</p>
<pre><code class="language-rust noplayground"> // get the current time instance
let mut instant = Mono::now();
// do this 3 times
for n in 0..3 {
// absolute point in time without drift
instant += 1000.millis();
Mono::delay_until(instant).await;
// absolute point in time for timeout
let timeout = instant + 500.millis();
hprintln!("now is {:?}, timeout at {:?}", Mono::now(), timeout);
match Mono::timeout_at(timeout, hal_get(n)).await {
Ok(v) =&gt; hprintln!("hal returned {} at time {:?}", v, Mono::now()),
_ =&gt; hprintln!("timeout"),
}
}</code></pre>
<p>In cases where you want exact control over time without drift we can use exact points in time using <code>Instant</code>, and spans of time using <code>Duration</code>. Operations on the <code>Instant</code> and <code>Duration</code> types come from the [<code>fugit</code>] crate.</p>
<p><code>let mut instant = Systick::now()</code> sets the starting time of execution.</p>
<p>We want to call <code>hal_get</code> after 1000ms relative to this starting time. This can be accomplished by using <code>Systick::delay_until(instant).await</code>.</p>
<p>Then, we define a point in time called <code>timeout</code>, and call <code>Systick::timeout_at(timeout, hal_get(n)).await</code>.</p>
<p>For the first iteration of the loop, with <code>n == 0</code>, the <code>hal_get</code> will take 350ms (and finishes before the timeout).</p>
<p>For the second iteration, with <code>n == 1</code>, the <code>hal_get</code> will take 450ms (and again succeeds to finish before the timeout).</p>
<p>For the third iteration, with <code>n == 2</code>, <code>hal_get</code> will take 550ms to finish, in which case we will run into a timeout.</p>
<details>
<summary>A complete example</summary>
<pre><code class="language-rust noplayground">//! examples/async-timeout.rs
#![no_main]
#![no_std]
#![deny(warnings)]
#![deny(unsafe_code)]
#![deny(missing_docs)]
use cortex_m_semihosting::{debug, hprintln};
use panic_semihosting as _;
use rtic_monotonics::systick::prelude::*;
systick_monotonic!(Mono, 100);
#[rtic::app(device = lm3s6965, dispatchers = [SSI0, UART0], peripherals = true)]
mod app {
use super::*;
use futures::{future::FutureExt, select_biased};
#[shared]
struct Shared {}
#[local]
struct Local {}
// ANCHOR: init
#[init]
fn init(cx: init::Context) -&gt; (Shared, Local) {
hprintln!("init");
Mono::start(cx.core.SYST, 12_000_000);
// ANCHOR_END: init
foo::spawn().ok();
(Shared {}, Local {})
}
#[task]
async fn foo(_cx: foo::Context) {
// ANCHOR: select_biased
// Call hal with short relative timeout using `select_biased`
select_biased! {
v = hal_get(1).fuse() =&gt; hprintln!("hal returned {}", v),
_ = Mono::delay(200.millis()).fuse() =&gt; hprintln!("timeout", ), // this will finish first
}
// Call hal with long relative timeout using `select_biased`
select_biased! {
v = hal_get(1).fuse() =&gt; hprintln!("hal returned {}", v), // hal finish first
_ = Mono::delay(1000.millis()).fuse() =&gt; hprintln!("timeout", ),
}
// ANCHOR_END: select_biased
// ANCHOR: timeout_after_basic
// Call hal with long relative timeout using monotonic `timeout_after`
match Mono::timeout_after(1000.millis(), hal_get(1)).await {
Ok(v) =&gt; hprintln!("hal returned {}", v),
_ =&gt; hprintln!("timeout"),
}
// ANCHOR_END: timeout_after_basic
// ANCHOR: timeout_at_basic
// get the current time instance
let mut instant = Mono::now();
// do this 3 times
for n in 0..3 {
// absolute point in time without drift
instant += 1000.millis();
Mono::delay_until(instant).await;
// absolute point in time for timeout
let timeout = instant + 500.millis();
hprintln!("now is {:?}, timeout at {:?}", Mono::now(), timeout);
match Mono::timeout_at(timeout, hal_get(n)).await {
Ok(v) =&gt; hprintln!("hal returned {} at time {:?}", v, Mono::now()),
_ =&gt; hprintln!("timeout"),
}
}
// ANCHOR_END: timeout_at_basic
debug::exit(debug::EXIT_SUCCESS);
}
}
// Emulate some hal
async fn hal_get(n: u32) -&gt; u32 {
// emulate some delay time dependent on n
let d = 350.millis() + n * 100.millis();
hprintln!("the hal takes a duration of {:?}", d);
Mono::delay(d).await;
// emulate some return value
5
}</code></pre>
<pre><code class="language-console">$ cargo xtask qemu --verbose --example async-timeout --features test-critical-section
</code></pre>
<pre><code class="language-console">init
the hal takes a duration of Duration { ticks: 45 }
timeout
the hal takes a duration of Duration { ticks: 45 }
hal returned 5
the hal takes a duration of Duration { ticks: 45 }
hal returned 5
now is Instant { ticks: 213 }, timeout at Instant { ticks: 263 }
the hal takes a duration of Duration { ticks: 35 }
hal returned 5 at time Instant { ticks: 249 }
now is Instant { ticks: 313 }, timeout at Instant { ticks: 363 }
the hal takes a duration of Duration { ticks: 45 }
hal returned 5 at time Instant { ticks: 359 }
now is Instant { ticks: 413 }, timeout at Instant { ticks: 463 }
the hal takes a duration of Duration { ticks: 55 }
timeout
</code></pre>
</details>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../by-example/channel.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../by-example/app_minimal.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../by-example/channel.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../by-example/app_minimal.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script>
window.playground_copyable = true;
</script>
<script src="../elasticlunr.min.js"></script>
<script src="../mark.min.js"></script>
<script src="../searcher.js"></script>
<script src="../clipboard.min.js"></script>
<script src="../highlight.js"></script>
<script src="../book.js"></script>
<!-- Custom JS scripts -->
<script src="../mermaid.min.js"></script>
<script src="../mermaid-init.js"></script>
</div>
</body>
</html>