asm.rs

//! Miscellaneous assembly instructions

#[cfg(cortex_m)]
use core::arch::asm;
use core::sync::atomic::{compiler_fence, Ordering};

/// Puts the processor in Debug state. Debuggers can pick this up as a "breakpoint".
///
/// **NOTE** calling `bkpt` when the processor is not connected to a debugger will cause an
/// exception.
#[cfg(cortex_m)]
#[inline(always)]
pub fn bkpt() {
    unsafe { asm!("bkpt", options(nomem, nostack, preserves_flags)) };
}

/// Blocks the program for *at least* `cycles` CPU cycles.
///
/// This is implemented in assembly so its execution time is independent of the optimization
/// level, however it is dependent on the specific architecture and core configuration.
///
/// NOTE that the delay can take much longer if interrupts are serviced during its execution
/// and the execution time may vary with other factors. This delay is mainly useful for simple
/// timer-less initialization of peripherals if and only if accurate timing is not essential. In
/// any other case please use a more accurate method to produce a delay.
#[cfg(cortex_m)]
#[inline]
pub fn delay(cycles: u32) {
    // The loop will normally take 3 to 4 CPU cycles per iteration, but superscalar cores
    // (eg. Cortex-M7) can potentially do it in 2, so we use that as the lower bound, since delaying
    // for more cycles is okay.
    // Add 1 to prevent an integer underflow which would cause a long freeze
    let real_cycles = 1 + cycles / 2;
    unsafe {
        asm!(
            // Use local labels to avoid R_ARM_THM_JUMP8 relocations which fail on thumbv6m.
            "1:",
            "subs {}, #1",
            "bne 1b",
            inout(reg) real_cycles => _,
            options(nomem, nostack),
        )
    };
}

/// A no-operation. Useful to prevent delay loops from being optimized away.
#[inline(always)]
pub fn nop() {
    // NOTE: This is a `pure` asm block, but applying that option allows the compiler to eliminate
    // the nop entirely (or to collapse multiple subsequent ones). Since the user probably wants N
    // nops when they call `nop` N times, let's not add that option.
    #[cfg(cortex_m)]
    unsafe {
        asm!("nop", options(nomem, nostack, preserves_flags))
    };
}

/// Generate an Undefined Instruction exception.
///
/// Can be used as a stable alternative to `core::intrinsics::abort`.
#[cfg(cortex_m)]
#[inline(always)]
pub fn udf() -> ! {
    unsafe { asm!("udf #0", options(noreturn, nomem, nostack, preserves_flags)) };
}

/// Wait For Event
#[cfg(cortex_m)]
#[inline(always)]
pub fn wfe() {
    unsafe { asm!("wfe", options(nomem, nostack, preserves_flags)) };
}

/// Wait For Interrupt
#[cfg(cortex_m)]
#[inline(always)]
pub fn wfi() {
    unsafe { asm!("wfi", options(nomem, nostack, preserves_flags)) };
}

/// Send Event
#[cfg(cortex_m)]
#[inline(always)]
pub fn sev() {
    unsafe { asm!("sev", options(nomem, nostack, preserves_flags)) };
}

/// Instruction Synchronization Barrier
///
/// Flushes the pipeline in the processor, so that all instructions following the `ISB` are fetched
/// from cache or memory, after the instruction has been completed.
#[inline(always)]
pub fn isb() {
    compiler_fence(Ordering::SeqCst);
    #[cfg(cortex_m)]
    unsafe {
        asm!("isb", options(nomem, nostack, preserves_flags))
    };
    compiler_fence(Ordering::SeqCst);
}

/// Data Synchronization Barrier
///
/// Acts as a special kind of memory barrier. No instruction in program order after this instruction
/// can execute until this instruction completes. This instruction completes only when both:
///
///  * any explicit memory access made before this instruction is complete
///  * all cache and branch predictor maintenance operations before this instruction complete
#[inline(always)]
pub fn dsb() {
    compiler_fence(Ordering::SeqCst);
    #[cfg(cortex_m)]
    unsafe {
        asm!("dsb", options(nomem, nostack, preserves_flags))
    };
    compiler_fence(Ordering::SeqCst);
}

/// Data Memory Barrier
///
/// Ensures that all explicit memory accesses that appear in program order before the `DMB`
/// instruction are observed before any explicit memory accesses that appear in program order
/// after the `DMB` instruction.
#[inline(always)]
pub fn dmb() {
    compiler_fence(Ordering::SeqCst);
    #[cfg(cortex_m)]
    unsafe {
        asm!("dmb", options(nomem, nostack, preserves_flags))
    };
    compiler_fence(Ordering::SeqCst);
}

/// Test Target
///
/// Queries the Security state and access permissions of a memory location.
/// Returns a Test Target Response Payload (cf section D1.2.215 of
/// Armv8-M Architecture Reference Manual).
#[inline(always)]
#[cfg(armv8m)]
// The __tt function does not dereference the pointer received.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn tt(addr: *mut u32) -> u32 {
    let mut target = addr as u32;
    unsafe {
        asm!(
            "tt {target}, {target}",
            target = inout(reg) target,
            options(nomem, nostack, preserves_flags),
        )
    };
    target
}

/// Test Target Unprivileged
///
/// Queries the Security state and access permissions of a memory location for an unprivileged
/// access to that location.
/// Returns a Test Target Response Payload (cf section D1.2.215 of
/// Armv8-M Architecture Reference Manual).
#[inline(always)]
#[cfg(armv8m)]
// The __ttt function does not dereference the pointer received.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn ttt(addr: *mut u32) -> u32 {
    let mut target = addr as u32;
    unsafe {
        asm!(
            "ttt {target}, {target}",
            target = inout(reg) target,
            options(nomem, nostack, preserves_flags),
        )
    };
    target
}

/// Test Target Alternate Domain
///
/// Queries the Security state and access permissions of a memory location for a Non-Secure access
/// to that location. This instruction is only valid when executing in Secure state and is
/// undefined if used from Non-Secure state.
/// Returns a Test Target Response Payload (cf section D1.2.215 of
/// Armv8-M Architecture Reference Manual).
#[inline(always)]
#[cfg(armv8m)]
// The __tta function does not dereference the pointer received.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn tta(addr: *mut u32) -> u32 {
    let mut target = addr as u32;
    unsafe {
        asm!(
            "tta {target}, {target}",
            target = inout(reg) target,
            options(nomem, nostack, preserves_flags),
        )
    };
    target
}

/// Test Target Alternate Domain Unprivileged
///
/// Queries the Security state and access permissions of a memory location for a Non-Secure and
/// unprivileged access to that location. This instruction is only valid when executing in Secure
/// state and is undefined if used from Non-Secure state.
/// Returns a Test Target Response Payload (cf section D1.2.215 of
/// Armv8-M Architecture Reference Manual).
#[inline(always)]
#[cfg(armv8m)]
// The __ttat function does not dereference the pointer received.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn ttat(addr: *mut u32) -> u32 {
    let mut target = addr as u32;
    unsafe {
        asm!(
            "ttat {target}, {target}",
            target = inout(reg) target,
            options(nomem, nostack, preserves_flags),
        )
    };
    target
}

/// Branch and Exchange Non-secure
///
/// See section C2.4.26 of Armv8-M Architecture Reference Manual for details.
/// Undefined if executed in Non-Secure state.
#[inline(always)]
#[cfg(armv8m)]
pub unsafe fn bx_ns(addr: u32) {
    asm!("bxns {}", in(reg) addr, options(nomem, nostack, preserves_flags));
}

/// Semihosting syscall.
///
/// This method is used by cortex-m-semihosting to provide semihosting syscalls.
#[cfg(cortex_m)]
#[inline(always)]
pub unsafe fn semihosting_syscall(mut nr: u32, arg: u32) -> u32 {
    asm!("bkpt #0xab", inout("r0") nr, in("r1") arg, options(nostack, preserves_flags));
    nr
}

/// Bootstrap.
///
/// Clears CONTROL.SPSEL (setting the main stack to be the active stack),
/// updates the main stack pointer to the address in `msp`, then jumps
/// to the address in `rv`.
///
/// # Safety
///
/// `msp` and `rv` must point to valid stack memory and executable code,
/// respectively.
#[cfg(cortex_m)]
#[inline]
pub unsafe fn bootstrap(msp: *const u32, rv: *const u32) -> ! {
    // Ensure thumb mode is set.
    let rv = (rv as u32) | 1;
    let msp = msp as u32;
    asm!(
        "mrs {tmp}, CONTROL",
        "bics {tmp}, {spsel}",
        "msr CONTROL, {tmp}",
        "isb",
        "msr MSP, {msp}",
        "bx {rv}",
        // `out(reg) _` is not permitted in a `noreturn` asm! call,
        // so instead use `in(reg) 0` and don't restore it afterwards.
        tmp = in(reg) 0,
        spsel = in(reg) 2,
        msp = in(reg) msp,
        rv = in(reg) rv,
        options(noreturn, nomem, nostack),
    );
}

/// 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, 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 as the second word.
#[cfg(cortex_m)]
#[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);
}