Merge #804

804: Preliminary RISCV support r=Dirbaio a=MabezDev

- ~~Moves the default Interrupt implementation into a cortex_m specific module~~
- Adds a RISCV32 executor based on [osobiehl](https://github.com/osobiehl)'s work in [esp32c3 mess work](https://github.com/osobiehl/riscv-embassy-mess-work)

(FYI esp implementation of embassy traits etc, is being developed [here](https://github.com/esp-rs/esp-hal/tree/feature/embassy))

[bonus ascii cinema](https://asciinema.org/a/500857 )

Co-authored-by: Scott Mabin <scott@mabez.dev>

embassy/Cargo.toml

@@ -47,6 +47,7 @@ time = []
 time-tick-32768hz = ["time"]
 time-tick-1000hz = ["time"]
 time-tick-1mhz = ["time"]
+time-tick-16mhz = ["time"]
 
 executor-agnostic = []
 

embassy/src/executor/arch/riscv32.rs

@@ -0,0 +1,74 @@
+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 RISCV
+///
+static SIGNAL_WORK_THREAD_MODE: AtomicBool = AtomicBool::new(false);
+
+/// RISCV32 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.
+                critical_section::with(|_| {
+                    // if there is work to do, loop back to polling
+                    // TODO can we relax this?
+                    if SIGNAL_WORK_THREAD_MODE.load(Ordering::SeqCst) {
+                        SIGNAL_WORK_THREAD_MODE.store(false, Ordering::SeqCst);
+                    }
+                    // if not, wait for interrupt
+                    else {
+                        core::arch::asm!("wfi");
+                    }
+                });
+                // if an interrupt occurred while waiting, it will be serviced here
+            }
+        }
+    }
+}

embassy/src/executor/mod.rs

@@ -18,6 +18,11 @@ cfg_if::cfg_if! {
         mod arch;
         pub use arch::*;
     }
+    else if #[cfg(target_arch="riscv32")] {
+        #[path="arch/riscv32.rs"]
+        mod arch;
+        pub use arch::*;
+    }
     else if #[cfg(feature="wasm")] {
         #[path="arch/wasm.rs"]
         mod arch;

embassy/src/time/mod.rs

@@ -68,6 +68,9 @@ const TPS: u64 = 32_768;
 #[cfg(feature = "time-tick-1mhz")]
 const TPS: u64 = 1_000_000;
 
+#[cfg(feature = "time-tick-16mhz")]
+const TPS: u64 = 16_000_000;
+
 /// Ticks per second of the global timebase.
 ///
 /// This value is specified by the `time-tick-*` Cargo features, which