From c861f2df836599a4bf2886a0cfbabd1ace9f5b4d Mon Sep 17 00:00:00 2001
From: topquark12 <aw@m-labs.hk>
Date: Tue, 12 Jan 2021 15:27:30 +0800
Subject: [PATCH] dfu: documentation

---
 src/dfu.rs | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/dfu.rs b/src/dfu.rs
index 89b473b..0668a88 100644
--- a/src/dfu.rs
+++ b/src/dfu.rs
@@ -1,23 +1,30 @@
 use cortex_m_rt::{pre_init};
 
+/// RAM location used to store DFU trigger message
 const DFU_MSG_ADDR: usize = 0x2001BC00;
+
+/// DFU trigger message 
 const DFU_TRIG_MSG: usize = 0xDECAFBAD;
 
+/// Set DFU trigger message
 pub unsafe fn trig_dfu() {
     let dfu_msg_addr = DFU_MSG_ADDR as *mut usize;
     *dfu_msg_addr = DFU_TRIG_MSG;
 }
 
+/// Called by reset handler in lib.rs immediately after reset, checks if booting into dfu is needed
 #[pre_init]
-#[no_mangle]
 unsafe fn __pre_init() {
 
     let dfu_msg_addr = DFU_MSG_ADDR as *mut usize;
     
+    // Check DFU trigger message
     if *dfu_msg_addr == DFU_TRIG_MSG{
 
+        // Reset message
         *dfu_msg_addr = 0x00000000;
 
+        // Enable system config controller clock
         const RCC_APB2ENR: *mut u32 = 0xE000_ED88 as *mut u32;
         const RCC_APB2ENR_ENABLE_SYSCFG_CLOCK: u32 = 0x00004000;
 
@@ -26,6 +33,7 @@ unsafe fn __pre_init() {
             *RCC_APB2ENR | RCC_APB2ENR_ENABLE_SYSCFG_CLOCK,
         );
 
+        // Bypass BOOT pins and remap bootloader to 0x00000000
         const SYSCFG_MEMRMP: *mut u32 = 0x40013800 as *mut u32;
         const SYSCFG_MEMRMP_MAP_ROM: u32 = 0x00000001;
 
@@ -34,8 +42,11 @@ unsafe fn __pre_init() {
             *SYSCFG_MEMRMP | SYSCFG_MEMRMP_MAP_ROM,
         );
 
+        // Set stack pointer to bootloader location
         asm!("LDR R0, =0x1FFF0000");
         asm!("LDR SP,[R0, #0]");
+
+        // Jump to bootloader
         asm!("LDR R0,[R0, #4]");
         asm!("BX R0");
     }