From b490e6917df7087ba0064d1973679c2b4ea7fda0 Mon Sep 17 00:00:00 2001
From: R <rqou@berkeley.edu>
Date: Sun, 14 Apr 2024 18:10:59 +0100
Subject: [PATCH] xxx try to fix ram boot bug

ram boot doesn't seem to work if there's no flash application for some reason
---
 bootloader.c |  9 +++++++++
 startup.S    | 13 ++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/bootloader.c b/bootloader.c
index 7d2438e..9ac3a8a 100644
--- a/bootloader.c
+++ b/bootloader.c
@@ -188,6 +188,8 @@ extern uint32_t UF2_GOT_BLOCKS[36];
 
 #define R16_BKP_DATAR10     (*(volatile uint32_t*)0x40006C28)
 
+#define R32_PWR_CTLR        (*(volatile uint32_t*)0x40007000)
+
 #define R32_RCC_CTLR        (*(volatile uint32_t*)0x40021000)
 #define R32_RCC_CFGR0       (*(volatile uint32_t*)0x40021004)
 #define R32_RCC_APB2PCENR   (*(volatile uint32_t*)0x40021018)
@@ -348,6 +350,11 @@ static void make_msc_csw(uint32_t dCSWTag, uint32_t error) {
 }
 
 __attribute__((naked)) int main(void) {
+    // Make sure this stuff is enabled
+    R32_RCC_APB1PCENR |= (1 << 27) | (1 << 28);
+    R32_PWR_CTLR |= 1 << 8;
+    R16_BKP_DATAR10 = 0;
+
     // PLL setup: system clock 96 MHz
     R32_EXTEN_CTR |= (1 << 4);  // sneaky div2
     R32_RCC_CFGR0 = (R32_RCC_CFGR0 & ~0xff0000) | (0b01101000 << 16);
@@ -855,6 +862,8 @@ __attribute__((naked)) int main(void) {
                         R32_EXTEN_CTR &= ~(1 << 1);
                         if (ADDRESS_HI >> 8 == 0x20) {
                             // ram boot
+                            R32_RCC_CFGR0 = (R32_RCC_CFGR0 & ~0b11) | 0b00;
+                            while ((R32_RCC_CFGR0 & 0b1100) != 0b0000) {}
                             ((void (*)())0x20000000)();
                         } else {
                             // wtf why does this work and the other stuff doesn't?
diff --git a/startup.S b/startup.S
index be0bf86..e3b7721 100644
--- a/startup.S
+++ b/startup.S
@@ -19,6 +19,12 @@
 
 .global _start
 _start:
+    // Weird things seem to happen if you fall off the end of flash
+    la a0, _bootloader_limit
+    lw a0, (a0)
+    la a1, 0xe339e339
+    beq a0, a1, _enter_bootloader
+
     // Use Adafruit(?)-inspired boot entry, namely "double-reset in short succession to enter bootloader"
     // 1. If the reset wasn't triggered by the nRST pin, go to application
     // 2. On nRST, if not first reset, go to bootloader
@@ -45,7 +51,7 @@ _start:
     la a3, R16_BKP_DATAR10
     lhu a1, (a3)
     la a2, BOOT_MAGIC_APP_IMMEDIATELY
-    beq a1, a2, _enter_application_code_magic
+    beq a1, a2, _enter_application_code
     la a2, BOOT_MAGIC_BOOTLOADER
     beq a1, a2, _enter_bootloader
 
@@ -68,9 +74,8 @@ _start:
     sw zero, (STK_SR-SYSTICK_BASE)(a0)
     // If the delay expires, boot the application after all
 
-_enter_application_code_magic:
-    sh zero, (a3)
 _enter_application_code:
+    sh zero, (a3)
     // Clear all of these enables that we had set
     la a0, R32_PWR_CTLR
     lw a1, (a0)
@@ -85,8 +90,6 @@ _enter_application_code:
     j _bootloader_limit
 
 _enter_bootloader:
-    sh zero, (a3)
-
     // Set up global pointer
 .option push
 .option norelax