Expose __syscall and add new bootstrap method

Author: adamgreig

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Added
+
+- New assembly methods `asm::sh_syscall`, `asm::bootstrap`, and
+  `asm::bootload`.
+
 ## [v0.7.0] - 2020-11-09
 
 ### Added

asm/inline.rs

@@ -177,11 +177,25 @@ pub unsafe fn __wfi() {
 
 /// Semihosting syscall.
 #[inline(always)]
-pub unsafe fn __syscall(mut nr: u32, arg: u32) -> u32 {
+pub unsafe fn __sh_syscall(mut nr: u32, arg: u32) -> u32 {
     asm!("bkpt #0xab", inout("r0") nr, in("r1") arg);
     nr
 }
 
+/// Bootstrap: ensure we are using the main stack, then write `msp` to MSP and jump to `rv`.
+#[inline(always)]
+pub unsafe fn __bootstrap(msp: u32, rv: u32) {
+    asm!(
+        "msr CONTROL, {}",
+        "isb",
+        "msr MSP, {}",
+        "bx {}",
+        in(reg) 0,
+        in(reg) msp,
+        in(reg) rv,
+    );
+}
+
 // v7m *AND* v8m.main, but *NOT* v8m.base
 #[cfg(any(armv7m, armv8m_main))]
 pub use self::v7m::*;

asm/lib.rs

@@ -72,7 +72,8 @@ shims! {
     fn __udf();
     fn __wfe();
     fn __wfi();
-    fn __syscall(nr: u32, arg: u32) -> u32;
+    fn __sh_syscall(nr: u32, arg: u32) -> u32;
+    fn __bootstrap(msp: u32, rv: u32);
 }
 
 // v7m *AND* v8m.main, but *NOT* v8m.base

src/asm.rs

@@ -164,3 +164,44 @@ pub fn ttat(addr: *mut u32) -> u32 {
 pub unsafe fn bx_ns(addr: u32) {
     call_asm!(__bxns(addr: u32));
 }
+
+/// Semihosing syscall.
+///
+/// This method is used by cortex-m-semihosting to provide semihosting syscalls.
+#[inline]
+pub unsafe fn sh_syscall(nr: u32, arg: u32) -> u32 {
+    call_asm!(__sh_syscall(nr: u32, arg: u32) -> u32)
+}
+
+/// Bootstrap.
+///
+/// Sets the active stack to the main stack, updates the main stack pointer to `msp`,
+/// then jumps execution to the address in `rv`.
+/// Writes `msp` to the MSP special register, then jumps to the address in `rv`.
+#[inline]
+pub unsafe fn bootstrap(msp: *const u32, rv: *const u32) -> ! {
+    let msp = msp as u32;
+    let rv = rv as u32;
+    call_asm!(__bootstrap(msp: u32, rv: u32));
+    core::hint::unreachable_unchecked();
+}
+
+/// Bootload.
+///
+/// Reads the initial stack pointer value and reset vector from
+/// the provided vector table address, sets the active stack to
+/// the main stack, sets the main stack pointer to the new initial
+/// stack pointer view, then jumps to the reset vector.
+///
+/// # Safety
+///
+/// The provided `vector_table` must point to a valid vector
+/// table, with a valid stack pointer as the first word and
+/// a valid reset vector (in thumb mode, with the least significant
+/// bit cleared) as the second word.
+#[inline]
+pub unsafe fn bootload(vector_table: *const u32) -> ! {
+    let msp = core::ptr::read_volatile(vector_table);
+    let rv = core::ptr::read_volatile(vector_table.offset(1));
+    bootstrap(msp as *const u32, rv as *const u32);
+}