From 31e80067387f7ac85221d004deaad9d1bf3460e6 Mon Sep 17 00:00:00 2001
From: Scott Mabin <scott@mabez.dev>
Date: Wed, 8 Jun 2022 14:54:18 +0100
Subject: [PATCH] arch:riscv32

- Add basic riscv32 executor
- Add 16MHZ timer support
---
 embassy/Cargo.toml                   |  1 +
 embassy/src/executor/arch/riscv32.rs | 74 ++++++++++++++++++++++++++++
 embassy/src/executor/mod.rs          |  5 ++
 embassy/src/time/mod.rs              |  3 ++
 4 files changed, 83 insertions(+)
 create mode 100644 embassy/src/executor/arch/riscv32.rs

diff --git a/embassy/Cargo.toml b/embassy/Cargo.toml
index 1d193d6a..d2c96e30 100644
--- a/embassy/Cargo.toml
+++ b/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 = []
 
diff --git a/embassy/src/executor/arch/riscv32.rs b/embassy/src/executor/arch/riscv32.rs
new file mode 100644
index 00000000..7a7d5698
--- /dev/null
+++ b/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
+            }
+        }
+    }
+}
diff --git a/embassy/src/executor/mod.rs b/embassy/src/executor/mod.rs
index 5e5c18c0..ca8488a9 100644
--- a/embassy/src/executor/mod.rs
+++ b/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;
diff --git a/embassy/src/time/mod.rs b/embassy/src/time/mod.rs
index ac84b8f8..018e01c8 100644
--- a/embassy/src/time/mod.rs
+++ b/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