Add skip-data-copy feature for when the .data LMA is uninitialized after boot

Author: BenChung

cortex-m-rt/CHANGELOG.md

@@ -10,6 +10,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Mark `pre_init` as deprecated
 - Add `set_msplim` feature to conditionally set the MSPLIM register at device
   reset ([#580]).
+- Add `skip-data-copy` feature to prevent copying the `.data` section from the 
+  load address to the virtual address. Use when the load address is inaccessible 
+  by the time user code runs, such as when copying from flash or when booting
+  from an external device. Note that this relies on the bootloader to have already
+  copied `.data` to the VMA before relinquishing control.
 
 ## [v0.7.5]
 

cortex-m-rt/Cargo.toml

@@ -48,6 +48,7 @@ set-vtor = []
 set-msplim = []
 zero-init-ram = []
 paint-stack = []
+skip-data-copy = []
 
 [package.metadata.docs.rs]
 features = ["device"]

cortex-m-rt/src/lib.rs

@@ -220,6 +220,14 @@
 //! You can then inspect memory during debugging to determine how much of the stack has been used -
 //! where the stack has been used the 'paint' will have been 'scrubbed off' and the memory will
 //! have a value other than `STACK_PAINT_VALUE`.
+//! 
+//! ## `skip-data-copy`
+//! 
+//! Skips copying the .data section (containing the initial values for static variables) when a bootloader
+//! (or other mechanism) is responsible for initializing the section. Use when the code is loaded from a location
+//! that's invalid after the bootloader runs (e.g. copy-from-XIP or copy-from-network boot). For example,
+//! rp2040-boot2 with `BOOT_LOADER_RAM_MEMCPY` (not the default of boot2!) set, which copies the code out
+//! of the XIP flash memory and then disables the XIP peripheral afterwards.
 //!
 //! # Inspection
 //!
@@ -616,6 +624,7 @@ cfg_global_asm! {
      1:",
 
     // Initialise .data memory. `__sdata`, `__sidata`, and `__edata` come from the linker script.
+    #[cfg(not(feature = "skip-data-copy"))]
     "ldr r0, =__sdata
      ldr r1, =__edata
      ldr r2, =__sidata