Use assembly sequences to enable caches.

See #232, which this partially fixes -- there's still the question of
taking an interrupt in the midst of these sequences.

Author: cbiffle

asm-v7.s

@@ -1,3 +1,4 @@
+  .syntax unified
   .cfi_sections .debug_frame
 
   .section .text.__basepri_max
@@ -39,3 +40,39 @@ __faultmask:
   bx lr
   .cfi_endproc
   .size __faultmask, . - __faultmask
+
+  .section .text.__enable_icache
+  .global __enable_icache
+  .thumb_func
+  .cfi_startproc
+__enable_icache:
+  ldr r0, =0xE000ED14       @ CCR
+  mrs r2, PRIMASK           @ save critical nesting info
+  cpsid i                   @ mask interrupts
+  ldr r1, [r0]              @ read CCR
+  orr.w r1, r1, #(1 << 17)  @ Set bit 17, IC
+  str r1, [r0]              @ write it back
+  dsb                       @ ensure store completes
+  isb                       @ synchronize pipeline
+  msr PRIMASK, r2           @ unnest critical section
+  bx lr
+  .cfi_endproc
+  .size __enable_icache, . - __enable_icache
+
+  .section .text.__enable_dcache
+  .global __enable_dcache
+  .thumb_func
+  .cfi_startproc
+__enable_dcache:
+  ldr r0, =0xE000ED14       @ CCR
+  mrs r2, PRIMASK           @ save critical nesting info
+  cpsid i                   @ mask interrupts
+  ldr r1, [r0]              @ read CCR
+  orr.w r1, r1, #(1 << 16)  @ Set bit 16, DC
+  str r1, [r0]              @ write it back
+  dsb                       @ ensure store completes
+  isb                       @ synchronize pipeline
+  msr PRIMASK, r2           @ unnest critical section
+  bx lr
+  .cfi_endproc
+  .size __enable_dcache, . - __enable_dcache

bin/thumbv6m-none-eabi.a

@@ -330,11 +330,16 @@ impl SCB {
         // Invalidate I-Cache
         cbp.iciallu();
 
-        // Enable I-Cache
-        unsafe { self.ccr.modify(|r| r | SCB_CCR_IC_MASK) };
+        // Enable I-cache
+        extern "C" {
+            // see asm-v7m.s
+            fn __enable_icache();
+        }
 
-        crate::asm::dsb();
-        crate::asm::isb();
+        // NOTE(unsafe): The asm routine manages exclusive access to the SCB
+        // registers and applies the proper barriers; it is technically safe on
+        // its own, and is only `unsafe` here because it's `extern "C"`.
+        unsafe { __enable_icache(); }
     }
 
     /// Disables I-Cache if currently enabled
@@ -392,11 +397,16 @@ impl SCB {
         // Invalidate anything currently in the DCache
         self.invalidate_dcache(cpuid);
 
-        // Now turn on the DCache
-        unsafe { self.ccr.modify(|r| r | SCB_CCR_DC_MASK) };
+        // Now turn on the D-cache
+        extern "C" {
+            // see asm-v7m.s
+            fn __enable_dcache();
+        }
 
-        crate::asm::dsb();
-        crate::asm::isb();
+        // NOTE(unsafe): The asm routine manages exclusive access to the SCB
+        // registers and applies the proper barriers; it is technically safe on
+        // its own, and is only `unsafe` here because it's `extern "C"`.
+        unsafe { __enable_dcache(); }
     }
 
     /// Disables D-cache if currently enabled