arch: Xtensa

- Adds a executor for the Xtensa arch
- Light sleep implemented with assembly, so we don't pull in the
  xtensa_lx crates (yet)
- lock behind a nightly feature due to Xtensa asm support not upstream

embassy/src/executor/arch/xtensa.rs

@@ -0,0 +1,72 @@
+use core::marker::PhantomData;
+use core::ptr;
+
+use atomic_polyfill::{AtomicBool, Ordering};
+
+use super::{raw, Spawner};
+
+/// global atomic used to keep track of whether there is work to do since sev() is not available on Xtensa
+///
+static SIGNAL_WORK_THREAD_MODE: AtomicBool = AtomicBool::new(false);
+
+/// Xtensa Executor
+pub struct Executor {
+    inner: raw::Executor,
+    not_send: PhantomData<*mut ()>,
+}
+
+impl Executor {
+    /// Create a new Executor.
+    pub fn new() -> Self {
+        Self {
+            // use Signal_Work_Thread_Mode as substitute for local interrupt register
+            inner: raw::Executor::new(
+                |_| {
+                    SIGNAL_WORK_THREAD_MODE.store(true, Ordering::SeqCst);
+                },
+                ptr::null_mut(),
+            ),
+            not_send: PhantomData,
+        }
+    }
+
+    /// Run the executor.
+    ///
+    /// The `init` closure is called with a [`Spawner`] that spawns tasks on
+    /// this executor. Use it to spawn the initial task(s). After `init` returns,
+    /// the executor starts running the tasks.
+    ///
+    /// To spawn more tasks later, you may keep copies of the [`Spawner`] (it is `Copy`),
+    /// for example by passing it as an argument to the initial tasks.
+    ///
+    /// This function requires `&'static mut self`. This means you have to store the
+    /// Executor instance in a place where it'll live forever and grants you mutable
+    /// access. There's a few ways to do this:
+    ///
+    /// - a [Forever](crate::util::Forever) (safe)
+    /// - a `static mut` (unsafe)
+    /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
+    ///
+    /// This function never returns.
+    pub fn run(&'static mut self, init: impl FnOnce(Spawner)) -> ! {
+        init(self.inner.spawner());
+
+        loop {
+            unsafe {
+                self.inner.poll();
+                // we do not care about race conditions between the load and store operations, interrupts
+                // will only set this value to true.
+                // if there is work to do, loop back to polling
+                // TODO can we relax this?
+                critical_section::with(|_| {
+                    if SIGNAL_WORK_THREAD_MODE.load(Ordering::SeqCst) {
+                        SIGNAL_WORK_THREAD_MODE.store(false, Ordering::SeqCst);
+                    } else {
+                        // waiti sets the PS.INTLEVEL when slipping into sleep
+                        core::arch::asm!("waiti 0"); /* wait for any prio interrupt */
+                    }
+                });
+            }
+        }
+    }
+}

embassy/src/executor/mod.rs

@@ -23,6 +23,11 @@ cfg_if::cfg_if! {
         mod arch;
         pub use arch::*;
     }
+    else if #[cfg(all(target_arch="xtensa", feature = "nightly"))] {
+        #[path="arch/xtensa.rs"]
+        mod arch;
+        pub use arch::*;
+    }
     else if #[cfg(feature="wasm")] {
         #[path="arch/wasm.rs"]
         mod arch;

embassy/src/lib.rs

@@ -1,5 +1,6 @@
 #![cfg_attr(not(any(feature = "std", feature = "wasm")), no_std)]
 #![cfg_attr(feature = "nightly", feature(generic_associated_types, type_alias_impl_trait))]
+#![cfg_attr(all(feature = "nightly", target_arch = "xtensa"), feature(asm_experimental_arch))]
 #![allow(clippy::new_without_default)]
 #![doc = include_str!("../../README.md")]