executor/spin: introduce an architecture agnostic executor

Spin polls the raw executor and never sleeps. It is useful for disabling
any power features associated with wfi/wfe-like instructions.

When implementing support for the CH32V30x MCU, the wfi instruction
had issues interacting with the USB OTG peripheral and appeared to be
non-spec-compliant.

1. When sending a USB Data-in packet, the USB peripheral appears to be
unable to read the system main memory while in WFI. This manifests in
the USB peripheral sending all or partially zeroed DATA packets.
Disabling WFI works around this issue.

2. The WFI instruction does not wake up the processor when MIE is
disabled. The MCU provides a WFITOWFE bit to emulate the WFE instruction
on arm, which, when enabled, ignores the MIE and allows the processor to
wake up. This works around the non-compliant WFI implementation.

Co-authored-by: Codetector <[email protected]>
Co-authored-by: Dummyc0m <[email protected]>

1 files changed, 28 insertions, 1 deletions

diff --git a/embassy-executor-macros/src/macros/main.rs b/embassy-executor-macros/src/macros/main.rs
index 26dfa2397..66a3965d0 100644
--- a/embassy-executor-macros/src/macros/main.rs
+++ b/embassy-executor-macros/src/macros/main.rs
@@ -1,5 +1,5 @@
 use darling::export::NestedMeta;
-use darling::FromMeta;
+use darling::{Error, FromMeta};
 use proc_macro2::TokenStream;
 use quote::quote;
 use syn::{Expr, ReturnType, Type};
@@ -50,6 +50,33 @@ pub fn riscv(args: &[NestedMeta]) -> TokenStream {
     }
 }
 
+pub fn spin(args: &[NestedMeta]) -> TokenStream {
+    let maybe_entry = match Args::from_list(args) {
+        Ok(args) => args.entry,
+        Err(e) => return e.write_errors(),
+    };
+
+    let entry = match maybe_entry {
+        Some(str) => str,
+        None => return Error::missing_field("entry").write_errors(),
+    };
+    let entry = match Expr::from_string(&entry) {
+        Ok(expr) => expr,
+        Err(e) => return e.write_errors(),
+    };
+
+    quote! {
+        #[#entry]
+        fn main() -> ! {
+            let mut executor = ::embassy_executor::Executor::new();
+            let executor = unsafe { __make_static(&mut executor) };
+            executor.run(|spawner| {
+                spawner.must_spawn(__embassy_main(spawner));
+            })
+        }
+    }
+}
+
 pub fn cortex_m() -> TokenStream {
     quote! {
         #[cortex_m_rt::entry]