[embedded] Add comment about UART emulator usage, drop unneccessary attribute((used))

Author: kubamracek

utils/embedded-test-support/arm-qemu-stm32f1/support.c

@@ -54,6 +54,9 @@ void qemu_exit() {
 }
 
 int putchar(int c) {
+  // This is only valid in an emulator (QEMU), and it's skipping a proper configuration of the UART device
+  // and waiting for a "ready to transit" state.
+
   // STM32F1 specific location of USART1 and its DR register
   *(volatile uint32_t *)(0x40013800 + 0x04) = c;
   return c;

utils/embedded-test-support/arm-qemu-stm32f4/support.c

@@ -54,6 +54,9 @@ void qemu_exit() {
 }
 
 int putchar(int c) {
+  // This is only valid in an emulator (QEMU), and it's skipping a proper configuration of the UART device
+  // and waiting for a "ready to transit" state.
+
   // STM32F4 specific location of USART1 and its DR register
   *(volatile uint32_t *)(0x40011000 + 0x04) = c;
   return c;

utils/embedded-test-support/avr-qemu-atmega2560/support.c

@@ -32,8 +32,7 @@ void usart_init() {
   *((volatile char *)0xc1) = 1 << 3; // enable TX on UART0
 }
 
-__attribute__((used))
-static inline void *memcpy_flash_to_sram(void *restrict dst, const void __attribute__((__address_space__(1))) *restrict src, size_t n) {
+void *memcpy_flash_to_sram(void *restrict dst, const void __attribute__((__address_space__(1))) *restrict src, size_t n) {
   for (int i = 0; i < n; i++) {
     ((char *)dst)[i] = ((char __attribute__((__address_space__(1))) *)src)[i];
   }
@@ -46,6 +45,9 @@ void copy_data_from_flash_to_sram() {
 }
 
 int putchar(int c) {
+  // This is only valid in an emulator (QEMU), and it's skipping a proper configuration of the UART device
+  // and waiting for a "ready to transit" state.
+
   // AVR's UART0 DR register
   *((volatile char *)0xc6) = c;
   return c;