Fail if ROM funcs are missing.

Belt-and-braces in case we ever get a different RP2040 ROM with missing functions.

Also use constants for the ROM addresses.

Author: thejpster

src/main.rs

@@ -7,15 +7,23 @@ use core::mem::MaybeUninit;
 
 use self::algo::*;
 
-fn find_func<T>(tag: [u8; 2]) -> T {
+fn find_func<T>(tag: [u8; 2]) -> Option<T> {
     let tag = u16::from_le_bytes(tag) as u32;
     type RomTableLookupFn = unsafe extern "C" fn(table: *const u16, code: u32) -> usize;
+    /// This location in flash holds a 16-bit truncated pointer for the ROM lookup function
+    const ROM_TABLE_LOOKUP_PTR: *const u16 = 0x0000_0018 as _;
+    /// This location in flash holds a 16-bit truncated pointer for the ROM function table
+    /// (there's also a ROM data table which we don't need)
+    const FUNC_TABLE: *const u16 = 0x0000_0014 as _;
     unsafe {
-        let lookup_func = core::ptr::read(0x0000_0018 as *const u16) as usize;
+        let lookup_func = ROM_TABLE_LOOKUP_PTR.read() as usize;
         let lookup_func: RomTableLookupFn = core::mem::transmute(lookup_func);
-        let table = core::ptr::read(0x0000_00014 as *const u16) as usize;
+        let table = FUNC_TABLE.read() as usize;
         let result = lookup_func(table as *const u16, tag);
-        core::mem::transmute_copy(&result)
+        if result == 0 {
+            return None;
+        }
+        Some(core::mem::transmute_copy(&result))
     }
 }
 
@@ -29,15 +37,15 @@ struct ROMFuncs {
 }
 
 impl ROMFuncs {
-    fn load() -> Self {
-        ROMFuncs {
-            connect_internal_flash: find_func(*b"IF"),
-            flash_exit_xip: find_func(*b"EX"),
-            flash_range_erase: find_func(*b"RE"),
-            flash_range_program: find_func(*b"RP"),
-            flash_flush_cache: find_func(*b"FC"),
-            flash_enter_cmd_xip: find_func(*b"CX"),
-        }
+    fn load() -> Option<Self> {
+        Some(ROMFuncs {
+            connect_internal_flash: find_func(*b"IF")?,
+            flash_exit_xip: find_func(*b"EX")?,
+            flash_range_erase: find_func(*b"RE")?,
+            flash_range_program: find_func(*b"RP")?,
+            flash_flush_cache: find_func(*b"FC")?,
+            flash_enter_cmd_xip: find_func(*b"CX")?,
+        })
     }
 }
 
@@ -54,8 +62,9 @@ const FLASH_BASE: u32 = 0x1000_0000;
 
 impl FlashAlgo for RP2040Algo {
     fn new(_address: u32, _clock: u32, _function: u32) -> Result<Self, ErrorCode> {
-        let funcs = ROMFuncs::load();
-
+        let Some(funcs) = ROMFuncs::load() else {
+            return Err(ErrorCode::new(1).unwrap());
+        };
         (funcs.connect_internal_flash)();
         (funcs.flash_exit_xip)();
         Ok(Self { funcs })