Add an rp2040 specific #[entry] macro. (#300)

* Add an rp2040 specific #[entry] macro.

This macro extends the one from cortex-m-rt by code to unlock
all spinlocks on boot.

* Idiomatic pointer arithmetic

Apply suggestion by @9names, improving address calculations.
(This doesn't change the generated code at opt levels 2 or "z".)

Co-authored-by: 9names <60134748+9names@users.noreply.github.com>

Cargo.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "rp2040-hal",
+    "rp2040-hal-macros",
     "boards/adafruit-feather-rp2040",
     "boards/adafruit-itsy-bitsy-rp2040",
     "boards/adafruit-kb2040",

rp2040-hal-macros/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+description = "Macros used by rp2040-hal"
+license = "MIT OR Apache-2.0"
+name = "rp2040-hal-macros"
+readme = "README.md"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+quote = "1.0"
+proc-macro2 = "1.0"
+cortex-m-rt = "0.7.0"
+
+[dependencies.syn]
+features = ["extra-traits", "full"]
+version = "1.0"
+

rp2040-hal-macros/README.md

@@ -0,0 +1,27 @@
+# `rp2040-hal-macros`
+
+Macros used by rp2040-hal.
+
+## Entry macro
+
+Extension of the `cortex-m-rt` `#[entry]` with rp2040 specific initialization code.
+
+Currently, it just unlocks all spinlocks before calling the entry function.
+
+# License
+
+Licensed under either of
+
+- Apache License, Version 2.0 (`APACHE2.0` or
+  http://www.apache.org/licenses/LICENSE-2.0)
+
+- MIT license (`MIT` or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+

rp2040-hal-macros/src/lib.rs

@@ -0,0 +1,59 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::Span;
+use quote::quote;
+use syn::{parse, parse_macro_input, Item, ItemFn, Stmt};
+
+#[proc_macro_attribute]
+pub fn entry(args: TokenStream, input: TokenStream) -> TokenStream {
+    let mut f = parse_macro_input!(input as ItemFn);
+
+    if !args.is_empty() {
+        return parse::Error::new(Span::call_site(), "This attribute accepts no arguments")
+            .to_compile_error()
+            .into();
+    }
+
+    let clear_locks: TokenStream = quote!(unsafe {
+        const SIO_BASE: u32 = 0xd0000000;
+        const SPINLOCK0_PTR: *mut u32 = (SIO_BASE + 0x100) as *mut u32;
+        const SPINLOCK_COUNT: usize = 32;
+        for i in 0..SPINLOCK_COUNT {
+            SPINLOCK0_PTR.wrapping_add(i).write_volatile(1);
+        }
+    })
+    .into();
+    let clear_locks = parse_macro_input!(clear_locks as Stmt);
+
+    // statics must stay first so cortex_m_rt::entry still finds them
+    let stmts = insert_after_static(f.block.stmts, clear_locks);
+    f.block.stmts = stmts;
+
+    quote!(
+        #[::cortex_m_rt::entry]
+        #f
+    )
+    .into()
+}
+
+/// Insert new statements after initial block of statics
+fn insert_after_static(stmts: impl IntoIterator<Item = Stmt>, insert: Stmt) -> Vec<Stmt> {
+    let mut istmts = stmts.into_iter();
+    let mut stmts = vec![];
+    for stmt in istmts.by_ref() {
+        match stmt {
+            Stmt::Item(Item::Static(var)) => {
+                stmts.push(Stmt::Item(Item::Static(var)));
+            }
+            _ => {
+                stmts.push(insert);
+                stmts.push(stmt);
+                break;
+            }
+        }
+    }
+    stmts.extend(istmts);
+
+    stmts
+}

rp2040-hal/Cargo.toml

@@ -11,6 +11,7 @@ license = "MIT OR Apache-2.0"
 
 [dependencies]
 cortex-m = "0.7.2"
+cortex-m-rt = ">=0.6.15,<0.8"
 embedded-hal = { version = "0.2.5", features = ["unproven"] }
 eh1_0_alpha = { version = "=1.0.0-alpha.7", package="embedded-hal", optional=true }
 embedded-time = "0.12.0"
@@ -19,6 +20,7 @@ nb = "1.0"
 rp2040-pac = "0.3.0"
 paste = "1.0"
 pio = "0.2.0"
+rp2040-hal-macros = { version = "0.1.0", path = "../rp2040-hal-macros" }
 usb-device = "0.2.8"
 vcell = "0.1"
 void = { version = "1.0.2", default-features = false }

rp2040-hal/src/lib.rs

@@ -49,6 +49,15 @@ pub mod xosc;
 pub use adc::Adc;
 pub use clocks::Clock;
 pub use i2c::I2C;
+/// Attribute to declare the entry point of the program
+///
+/// This is based on and can be used like the [entry attribute from
+/// cortex-m-rt](https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.entry.html).
+///
+/// It extends that macro with code to unlock all spinlocks at the beginning
+/// of `main`. As spinlocks are not automatically unlocked on software resets,
+/// this can prevent unexpected deadlocks when running from a debugger.
+pub use rp2040_hal_macros::entry;
 pub use sio::Sio;
 pub use spi::Spi;
 pub use timer::Timer;