flash_store: get addresses from linker

Reviewed-on: https://git.m-labs.hk/M-Labs/thermostat/pulls/49
Co-Authored-By: topquark12 <aw@m-labs.hk>
Co-Committed-By: topquark12 <aw@m-labs.hk>

memory.x

@@ -11,5 +11,7 @@ MEMORY
   CCMRAM (rw)     : ORIGIN = 0x10000000, LENGTH = 64K
 }
 
-_stack_start = ORIGIN(CCMRAM) + LENGTH(CCMRAM);
+_flash_start = ORIGIN(FLASH);
+_config_start = ORIGIN(CONFIG);
 _dfu_msg = ORIGIN(DFU_MSG);
+_stack_start = ORIGIN(CCMRAM) + LENGTH(CCMRAM);

src/flash_store.rs

@@ -8,18 +8,29 @@ use sfkv::{Store, StoreBackend};
 /// 16 KiB
 pub const FLASH_SECTOR_SIZE: usize = 0x4000;
 pub const FLASH_SECTOR: u8 = 12;
-pub const FLASH_SECTOR_OFFSET: usize = 0x10_0000;
 static mut BACKUP_SPACE: [u8; FLASH_SECTOR_SIZE] = [0; FLASH_SECTOR_SIZE];
 
+extern "C" {
+    // These are from memory.x
+    static _config_start: usize;
+    static _flash_start: usize;
+}
+
 pub struct FlashBackend {
     flash: FLASH,
 }
 
+fn get_offset() -> usize {
+    unsafe {
+        (&_config_start as *const usize as usize) - (&_flash_start as *const usize as usize)
+    }    
+}
+
 impl StoreBackend for FlashBackend {
     type Data = [u8];
 
     fn data(&self) -> &Self::Data {
-        &self.flash.read()[FLASH_SECTOR_OFFSET..(FLASH_SECTOR_OFFSET + FLASH_SECTOR_SIZE)]
+        &self.flash.read()[get_offset()..(get_offset() + FLASH_SECTOR_SIZE)]
     }
 
     type Error = Error;
@@ -30,10 +41,9 @@ impl StoreBackend for FlashBackend {
 
     fn program(&mut self, offset: usize, payload: &[u8]) -> Result<(), Self::Error> {
         self.flash.unlocked()
-            .program(FLASH_SECTOR_OFFSET + offset, payload.iter().cloned())
+            .program(get_offset() + offset, payload.iter().cloned())
     }
 
-
     fn backup_space(&self) -> &'static mut [u8] {
         unsafe { &mut BACKUP_SPACE }
     }