Add pre-default-start-trap feature

Author: bjoernQ

riscv-rt/Cargo.toml

@@ -46,3 +46,4 @@ no-xie-xip = []
 device = []
 memory = []
 defmt = ["dep:defmt"]
+pre-default-start-trap = ["riscv-rt-macros/pre-default-start-trap"]

riscv-rt/macros/Cargo.toml

@@ -25,3 +25,4 @@ syn = { version = "2.0", features = ["extra-traits", "full"] }
 s-mode = []
 v-trap = []
 u-boot = []
+pre-default-start-trap = []

riscv-rt/macros/src/lib.rs

@@ -606,13 +606,24 @@ pub fn default_start_trap(_input: TokenStream) -> TokenStream {
     #[cfg(not(feature = "s-mode"))]
     let ret = "mret";
 
+    let pre_default_start_trap = if cfg!(feature = "pre-default-start-trap") {
+        r#"
+        j _pre_default_start_trap
+.global _pre_default_start_trap_ret
+_pre_default_start_trap_ret:
+        "#
+    } else {
+        ""
+    };
+
     format!(
         r#"
 core::arch::global_asm!(
 ".section .trap.start, \"ax\"
 .balign 4 /* Alignment required for xtvec */
 .global _default_start_trap
 _default_start_trap:
+    {pre_default_start_trap}
     addi sp, sp, - {trap_size} * {width}
     {store}
     add a0, sp, zero

riscv-rt/src/lib.rs

@@ -598,6 +598,15 @@
 //! because when booting from elf, U-boot passes `argc` and `argv`. This feature also implies `single-hart`.
 //! The only way to get boot-hart is through fdt, so other harts initialization is up to you.
 //!
+//! ## `pre-default-start-trap`
+//!
+//! This provides a mechanism to execute custom code prior to `default_start_trap`.
+//!
+//! To use it, the user must define a symbol named `_pre_default_start_trap`, which the system will jump to.
+//! After executing the custom code, control should return by jumping to `_pre_default_start_trap_ret`.
+//!
+//! It is expected that the custom code does not clobber any registers.
+//!
 //! [attr-entry]: attr.entry.html
 //! [attr-exception]: attr.exception.html
 //! [attr-external-interrupt]: attr.external_interrupt.html