Add skip-data-init feature to disable .data section initialization

cortex-m-rt/CHANGELOG.md

@@ -10,6 +10,7 @@ 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-init` feature to optionally skip copying the `.data` section.
 
 ## [v0.7.5]
 

cortex-m-rt/Cargo.toml

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

cortex-m-rt/examples/skip-data-init.rs

@@ -0,0 +1,38 @@
+//! Example demonstrating the skip-data-init feature
+//!
+//! This feature is useful when using bootloaders (like RP2040's boot2) that:
+//! 1. Copy all data from Flash to RAM
+//! 2. Unmap the Flash from memory space
+//! 3. Jump to the Reset handler
+//!
+//! In such scenarios, the default cortex-m-rt data initialization would fail
+//! because it tries to copy from Flash which is no longer accessible.
+//!
+//! To use this feature, enable it in your Cargo.toml:
+//! ```toml
+//! [dependencies]
+//! cortex-m-rt = { version = "0.7", features = ["skip-data-init"] }
+//! ```
+//!
+//! And ensure your bootloader or linker script properly initializes .data before
+//! jumping to Reset.
+
+#![no_main]
+#![no_std]
+
+use panic_halt as _;
+
+use cortex_m_rt::entry;
+
+static mut COUNTER: u32 = 42;
+
+#[entry]
+fn main() -> ! {
+    unsafe {
+        COUNTER += 1;
+    }
+
+    loop {
+        cortex_m::asm::nop();
+    }
+}

cortex-m-rt/src/lib.rs

@@ -208,6 +208,30 @@
 //! 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-init`
+//!
+//! If this feature is enabled, the `.data` section initialization is skipped during startup.
+//! By default, cortex-m-rt copies the `.data` section from its load address (LMA) in Flash
+//! to its virtual address (VMA) in RAM. However, in some scenarios this copy is unnecessary
+//! or undesirable:
+//!
+//! - When using bootloaders like RP2040's boot2 that copy all data from Flash to RAM and then
+//!   unmap the Flash, the cortex-m-rt data initialization would fail because Flash is no longer
+//!   accessible.
+//! - When the `.data` section is already placed in RAM at the correct address (LMA equals VMA).
+//!
+//! When this feature is enabled, it is the user's responsibility to ensure that the `.data`
+//! section is properly initialized before the program's main function is called. This can be
+//! done by:
+//!
+//! - Using a bootloader that copies the data before jumping to the Reset handler
+//! - Setting `__sidata = ADDR(.data)` in the linker script to make LMA equal to VMA (though this
+//!   wastes Flash space)
+//! - Other custom initialization mechanisms
+//!
+//! **WARNING:** Using this feature without ensuring proper `.data` initialization will result
+//! in undefined behavior if your program uses initialized static variables.
+//!
 //! # Inspection
 //!
 //! This section covers how to inspect a binary that builds on top of `cortex-m-rt`.
@@ -489,6 +513,9 @@
 #![deny(missing_docs)]
 #![no_std]
 
+#[cfg(all(feature = "skip-data-init", feature = "zero-init-ram"))]
+compile_error!("features `skip-data-init` and `zero-init-ram` cannot be enabled at the same time");
+
 extern crate cortex_m_rt_macros as macros;
 
 /// The 32-bit value the stack is painted with before the program runs.
@@ -603,6 +630,7 @@ cfg_global_asm! {
      1:",
 
     // Initialise .data memory. `__sdata`, `__sidata`, and `__edata` come from the linker script.
+    #[cfg(not(feature = "skip-data-init"))]
     "ldr r0, =__sdata
      ldr r1, =__edata
      ldr r2, =__sidata