f4discovery: Use Magic constant in RAM for Bootloader entry.

src/platforms/f4discovery/Makefile.inc

@@ -7,7 +7,7 @@ CFLAGS += -Istm32/include -mcpu=cortex-m4 -mthumb \
 	-DSTM32F4 -DF4DISCOVERY -I../libopencm3/include \
 	-Iplatforms/stm32
 
-LDFLAGS = -lopencm3_stm32f4 -Wl,--defsym,_stack=0x20006000 \
+LDFLAGS = -lopencm3_stm32f4 -Wl,--defsym,_stack=0x20010000 \
 	-Wl,-T,platforms/stm32/f4discovery.ld -nostartfiles -lc -lnosys \
 	-Wl,-Map=mapfile -mthumb -mcpu=cortex-m4 -Wl,-gc-sections \
 	-mfloat-abi=hard -mfpu=fpv4-sp-d16 \

src/platforms/f4discovery/platform.c

@@ -38,13 +38,27 @@
 #include <libopencm3/cm3/cortex.h>
 
 jmp_buf fatal_error_jmpbuf;
+extern uint32_t _ebss;
 
 void platform_init(void)
 {
+	volatile uint32_t *magic = (uint32_t *) &_ebss;
 	/* Check the USER button*/
 	rcc_periph_clock_enable(RCC_GPIOA);
-	if(gpio_get(GPIOA, GPIO0)) {
-		platform_request_boot();
+	if (gpio_get(GPIOA, GPIO0) ||
+	   ((magic[0] == BOOTMAGIC0) && (magic[1] == BOOTMAGIC1))) {
+		magic[0] = 0;
+		magic[1] = 0;
+		/* Assert blue LED as indicator we are in the bootloader */
+		rcc_periph_clock_enable(RCC_GPIOD);
+		gpio_mode_setup(LED_PORT, GPIO_MODE_OUTPUT,
+						GPIO_PUPD_NONE, LED_BOOTLOADER);
+		gpio_set(LED_PORT, LED_BOOTLOADER);
+		/* Jump to the built in bootloader by mapping System flash.
+		   As we just come out of reset, no other deinit is needed!*/
+		rcc_periph_clock_enable(RCC_SYSCFG);
+		SYSCFG_MEMRM &= ~3;
+		SYSCFG_MEMRM |=  1;
 		scb_reset_core();
 	}
 
@@ -89,36 +103,8 @@ const char *platform_target_voltage(void)
 
 void platform_request_boot(void)
 {
-	/* Disconnect USB cable */
-	if (RCC_AHB2ENR & RCC_AHB2ENR_OTGFSEN) {
-		usbd_disconnect(usbdev, 1);
-		nvic_disable_irq(USB_IRQ);
-		rcc_periph_clock_disable(RCC_OTGFS);
-	}
-
-	/* Assert blue LED as indicator we are in the bootloader */
-	gpio_mode_setup(LED_PORT, GPIO_MODE_OUTPUT,
-		GPIO_PUPD_NONE, LED_BOOTLOADER);
-	gpio_set(LED_PORT, LED_BOOTLOADER);
-	/* Disable used ports beside PORTD.*/
-	rcc_periph_clock_disable(RCC_GPIOA);
-	rcc_periph_clock_disable(RCC_GPIOC);
-	rcc_periph_clock_disable(USBUSART_CLK);
-	/* Reset Systick.*/
-	systick_interrupt_disable();
-	STK_CSR = 0;
-	STK_RVR = 0;
-	STK_CVR = 0;
-	/*Disable Interrupts.*/
-	cm_disable_interrupts();
-	/* Switch back to HSI.*/
-	while (!(RCC_CR & RCC_CR_HSIRDY))
-		RCC_CR |= RCC_CR_HSION;
-	while (RCC_CFGR & (RCC_CFGR_SWS_HSE | RCC_CFGR_SWS_PLL))
-		RCC_CFGR &= ~(RCC_CFGR_SWS_HSE | RCC_CFGR_SWS_PLL);
-
-	/* Map System flash at 0.*/
-	rcc_periph_clock_enable(RCC_SYSCFG);
-	SYSCFG_MEMRM &= ~3;
-	SYSCFG_MEMRM |=  1;
+	uint32_t *magic = (uint32_t *) &_ebss;
+	magic[0] = BOOTMAGIC0;
+	magic[1] = BOOTMAGIC1;
+	scb_reset_system();
 }

src/platforms/f4discovery/platform.h

@@ -80,6 +80,8 @@
 #define LED_IDLE_RUN	GPIO13
 #define LED_ERROR	GPIO14
 #define LED_BOOTLOADER	GPIO15
+#define BOOTMAGIC0 0xb007da7a
+#define BOOTMAGIC1 0xbaadfeed
 
 #define TMS_SET_MODE() \
 	gpio_mode_setup(TMS_PORT, GPIO_MODE_OUTPUT, \