Add new system for startup.

Now instead of init() and main() being responsible for all
initialization, individual modules can link in their own
initialization routines.

There are 7 levels for these initializiation routines.

So far these are how the levels are defined

level 0 - Here the world is dark. Nothing is initialized.
          This level is responsible for initializing the
          system clock.

level 1 - The system clock has been configured, but nothing else. Not
          even global variables. This level is responsible for loading
          the data sections from flash and clearing the .bss section.

level 2 - USART2 is enabled and set to be the main kernel logging
          vehicle. From this point on klogf(...) can be used.

level 3 - The NVIC is reset to point to the flash. From this point
          on interrupts can be received. I expect this is where
          most core initialization routines will take place

levels 4 to 7 - User initializiation levels.

main - main() is called after all 8 initialization levels have executed,
       so in a sense main() is like a 9th initialization level, except
       that there is can be only one main() routine whereas there can be
       multiple initalization routines per level.

1 files changed, 55 insertions, 0 deletions

diff --git a/02-usart/src/kern/log.c b/02-usart/src/kern/log.c
new file mode 100644
index 0000000..a217183
--- /dev/null
+++ b/02-usart/src/kern/log.c
@@ -0,0 +1,55 @@
+#include "arch/stm32l4xxx/peripherals/usart.h"
+#include "arch/stm32l4xxx/peripherals/clock.h"
+
+#include "kern/log.h"
+#include "kern/init.h"
+#include "kern/gpio/gpio_manager.h"
+
+#include "kern/common.h"
+
+void setup_usart2(uint32_t baud_rate);
+
+/** This module requires an initialization routine. This is a level2 routine,
+ * so anything running at level3 or lower is guaranteed to have access
+ * to the klong. */
+init2()
+{
+  setup_usart2(115200);
+  regset(USART2.c_r1, usart_txeie, 1);
+  regset(USART2.c_r1, usart_rxneie, 1);
+  usart_set_enabled(&USART2, USART_ENABLE_TX | USART_ENABLE_RX);
+
+  klogf("klog() enabled on USART2\n");
+}
+
+void klogf(const char* fmt, ...)
+{
+  va_list l;
+  va_start(l, fmt);
+
+  usart_vprintf(&USART2, fmt, l);
+}
+
+void setup_usart2(uint32_t baud_rate)
+{
+  enable_hsi(&RCC, true);
+
+  int ec = 0;
+  gpio_enable_alternate_function(
+      GPIO_ALTERNATE_FUNCTION_USART2_TX, GPIO_PIN_PA2, &ec);
+
+  gpio_enable_alternate_function(
+      GPIO_ALTERNATE_FUNCTION_USART2_RX, GPIO_PIN_PA15, &ec);
+
+  set_usart2_clock_src(&RCC, USART_CLK_SRC_HSI16);
+  set_usart2_clock_enabled(&RCC, USART_CLK_SRC_HSI16);
+
+  /* De-assert reset of USART2 */
+  regset(RCC.apb1rst1_r, rcc_usart2rst, 0);
+
+  USART2.c_r1 = 0;
+  USART2.c_r2 = 0;
+  USART2.c_r3 = 0;
+
+  usart_set_divisor(&USART2, 16000000 / baud_rate);
+}