stlink: Add option to turn SWIM/RST into UART RX/TX

Building for `stlink` with `SWIM_AS_UART=1` now enables the use of the
SWIM and RST ports as RX and TX on cheap ST-Link V2 clones.

This is done by using USART1 on the alternative port instead of USART2.

src/platforms/stlink/Makefile.inc

@@ -25,6 +25,10 @@ else
 LDFLAGS += --specs=nosys.specs
 endif
 
+ifeq ($(SWIM_AS_UART), 1)
+CFLAGS += -DSWIM_AS_UART=1
+endif
+
 VPATH += platforms/stm32
 
 SRC += 	cdcacm.c	\
@@ -47,4 +51,3 @@ blackmagic_dfu.elf: usbdfu.o dfucore.o dfu_f1.o stlink_common.o serialno.o
 
 host_clean:
 	-$(Q)$(RM) *.bin *elf *hex
-

src/platforms/stlink/platform.c

@@ -81,6 +81,12 @@ void platform_init(void)
 	if (rev > 1) /* Reconnect USB */
 		gpio_set(GPIOA, GPIO15);
 	cdcacm_init();
+
+#ifdef SWIM_AS_UART
+	gpio_primary_remap(AFIO_MAPR_SWJ_CFG_FULL_SWJ,
+			   AFIO_MAPR_USART1_REMAP);
+#endif
+
 	/* Don't enable UART if we're being debugged. */
 	if (!(SCS_DEMCR & SCS_DEMCR_TRCENA))
 		usbuart_init();

src/platforms/stlink/platform.h

@@ -105,15 +105,24 @@ int usbuart_debug_write(const char *buf, size_t len);
 #define IRQ_PRI_USB_VBUS	(14 << 4)
 #define IRQ_PRI_SWO_DMA			(0 << 4)
 
+#ifdef SWIM_AS_UART
+#define USBUSART USART1
+#define USBUSART_CR1 USART1_CR1
+#define USBUSART_DR USART1_DR
+#define USBUSART_IRQ NVIC_USART1_IRQ
+#define USBUSART_CLK RCC_USART1
+#define USBUSART_ISR(x) usart1_isr(x)
+#else
 #define USBUSART USART2
 #define USBUSART_CR1 USART2_CR1
 #define USBUSART_DR USART2_DR
 #define USBUSART_IRQ NVIC_USART2_IRQ
 #define USBUSART_CLK RCC_USART2
+#define USBUSART_ISR(x) usart2_isr(x)
+#endif
 #define USBUSART_PORT GPIOA
 #define USBUSART_TX_PIN GPIO2
 #define USBUSART_RX_PIN GPIO3
-#define USBUSART_ISR(x) usart2_isr(x)
 
 #define USBUSART_DMA_BUS DMA1
 #define USBUSART_DMA_CLK RCC_DMA1
@@ -179,5 +188,4 @@ extern uint32_t detect_rev(void);
 #define snprintf sniprintf
 #endif
 
-
 #endif