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<h1 class="menu-title">Real-Time Interrupt-driven Concurrency</h1>
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<main>
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<h1 id="using-indirection-for-faster-message-passing"><a class="header" href="#using-indirection-for-faster-message-passing">Using indirection for faster message passing</a></h1>
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<p>Message passing always involves copying the payload from the sender into a static variable and then from the static variable into the receiver. Thus sending a large buffer, like a <code>[u8; 128]</code>, as a message involves two expensive
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<code>memcpy</code>s.</p>
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<p>Indirection can minimize message passing overhead: instead of sending the buffer by value, one can send an owning pointer into the buffer.</p>
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<p>One can use a global memory allocator to achieve indirection (<code>alloc::Box</code>, <code>alloc::Rc</code>, etc.), which requires using the nightly channel as of Rust v1.37.0, or one can use a statically allocated memory pool like <a href="https://docs.rs/heapless/latest/heapless/pool/index.html"><code>heapless::Pool</code></a>.</p>
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<p>As this example of approach goes completely outside of RTIC resource model with shared and local the program would rely on the correctness of the memory allocator, in this case <code>heapless::pool</code>.</p>
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<p>Here's an example where <code>heapless::Pool</code> is used to "box" buffers of 128 bytes.</p>
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<pre><code class="language-rust noplayground">//! examples/pool.rs
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#![no_main]
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#![no_std]
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#![deny(warnings)]
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use panic_semihosting as _;
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use rtic::app;
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// thumbv6-none-eabi does not support pool
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// This might be better worked around in the build system,
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// but for proof of concept, let's try having one example
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// being different for different backends
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// https://docs.rs/heapless/0.8.0/heapless/pool/index.html#target-support
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cfg_if::cfg_if! {
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if #[cfg(feature = "thumbv6-backend")] {
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// Copy of the smallest.rs example
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#[app(device = lm3s6965)]
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mod app {
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use cortex_m_semihosting::debug;
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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#[init]
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fn init(_: init::Context) -> (Shared, Local) {
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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(Shared {}, Local {})
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}
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}
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} else {
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// Run actual pool example
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use heapless::{
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box_pool,
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pool::boxed::{Box, BoxBlock},
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};
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// Declare a pool containing 8-byte memory blocks
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box_pool!(P: u8);
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const POOL_CAPACITY: usize = 512;
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#[app(device = lm3s6965, dispatchers = [SSI0, QEI0])]
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mod app {
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use crate::{Box, BoxBlock, POOL_CAPACITY};
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use cortex_m_semihosting::debug;
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use lm3s6965::Interrupt;
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// Import the memory pool into scope
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use crate::P;
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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const BLOCK: BoxBlock<u8> = BoxBlock::new();
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#[init(local = [memory: [BoxBlock<u8>; POOL_CAPACITY] = [BLOCK; POOL_CAPACITY]])]
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fn init(cx: init::Context) -> (Shared, Local) {
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for block in cx.local.memory {
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// Give the 'static memory to the pool
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P.manage(block);
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}
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rtic::pend(Interrupt::I2C0);
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(Shared {}, Local {})
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}
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#[task(binds = I2C0, priority = 2)]
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fn i2c0(_: i2c0::Context) {
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// Claim 128 u8 blocks
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let x = P.alloc(128).unwrap();
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// .. send it to the `foo` task
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foo::spawn(x).ok().unwrap();
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// send another 128 u8 blocks to the task `bar`
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bar::spawn(P.alloc(128).unwrap()).ok().unwrap();
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}
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#[task]
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async fn foo(_: foo::Context, _x: Box<P>) {
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// explicitly return the block to the pool
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drop(_x);
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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}
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#[task(priority = 2)]
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async fn bar(_: bar::Context, _x: Box<P>) {
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// this is done automatically so we can omit the call to `drop`
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// drop(_x);
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}
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}
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example pool
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</code></pre>
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<pre><code class="language-console">
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</code></pre>
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