/// Mutex for SMP-safe locking

use crate::cortex_a9::asm;

pub struct Mutex {
    state: u32,
}

const UNLOCKED_MUTEX: u32 = 0;
const LOCKED_MUTEX: u32 = 1;

impl Mutex {
    pub const fn new_unlocked() -> Mutex {
        Mutex { state: UNLOCKED_MUTEX }
    }

    pub const fn new_locked() -> Mutex {
        Mutex { state: LOCKED_MUTEX }
    }

    // inlining causes problems with the labels
    #[inline(never)]
    pub fn acquire(&mut self) {
        unsafe {
            // code adapted from an example by ARM at
            // http://infocenter.arm.com (Home > ARM Synchronization
            // Primitives > Practical uses > Implementing a mutex)
            asm!("
                  mutex_acquire_label1: 
                          ldrex r2, [$0];
                          cmp r2, $1;
                          beq mutex_acquire_label2;
                          strexne r2, $1, [$0];
                          cmpne r2, 1;
                          beq mutex_acquire_label1;
                          dmb;
                          b mutex_acquire_label3;
                  mutex_acquire_label2:
                          wfe;
                          b mutex_acquire_label1;
                  mutex_acquire_label3: ;
                 "
                  ::
                 // inputs
                 "r" (&mut self.state as *mut _ as u32), "r" (LOCKED_MUTEX)
                  :
                 // clobbers
                 "r2"
                  :
                 "volatile"
            );
        }
    }

    pub fn release(&mut self) {
        unsafe {
            asm!("
                  dmb;
                  str $1, [$0];
                  dsb;
                  sev;
                 "
                  ::
                 // inputs
                 "r" (&mut self.state as *mut _ as u32), "r" (UNLOCKED_MUTEX)
                  ::
                 "volatile"
            );
        }
    }
}