pbdrv/uart_debug_first_port: Log on panic.

It's helpful when you crash to see what happens right before you
crash. For example, if you add a log message "about to dereference
sketchy pointer" and then you get a segfault, you'd love to know
whether that message occurred. This change makes it so that at least
on EV3, we will.

This also adds "pb_log" as an alias for pbdrv_uart_debug_printf.
Usually, platforms make it easy to log debug messages, and we should
be no different. C.f. `ESP_LOGI`, Zephyr's `LOG_ERR`, glog's
`LOG(WARNING)` etc.

Tested by adding the following code to pbdrv_init.

```
    pb_log("test\n");

    lwrb_t* rb = (lwrb_t*)0x1281441490;
    pb_log("%p\n", lwrb_get_free(rb));
```

Before this, the "test" would not appear before the panic.
After, it does. If a panic occurs while some messages are pending
in the uart send state, some bytes may be duplicated on the log.

Author: jaguilar

lib/pbio/drv/uart/uart_debug_first_port.c

@@ -22,6 +22,13 @@ static lwrb_t ring_buffer;
 
 static pbdrv_uart_dev_t *debug_uart = NULL;
 
+lwrb_t* pbdrv_uart_debug_get_ring_buffer(void) {
+    if (!lwrb_is_ready(&ring_buffer)) {
+        return NULL;
+    }
+    return &ring_buffer;
+}
+
 /**
  * Formats and stores a string in the UART debug ring buffer.
  *

lib/pbio/drv/uart/uart_debug_first_port.h

@@ -8,6 +8,8 @@
 #include <stdbool.h>
 #include <stdarg.h>
 
+#include <lwrb/lwrb.h>
+
 #include <pbdrv/config.h>
 
 #if PBDRV_CONFIG_UART_DEBUG_FIRST_PORT
@@ -19,6 +21,8 @@ bool pbdrv_uart_debug_is_done(void);
 
 void pbdrv_uart_debug_init(void);
 
+lwrb_t* pbdrv_uart_debug_get_ring_buffer(void);
+
 #else // PBDRV_CONFIG_UART_DEBUG_FIRST_PORT
 
 #define pbdrv_uart_debug_printf(...)
@@ -28,6 +32,13 @@ void pbdrv_uart_debug_init(void);
 
 #define pbdrv_uart_debug_init()
 
+static inline lwrb_t* pbdrv_uart_debug_get_ring_buffer(void) {
+    return NULL;
+}
+
 #endif // PBDRV_CONFIG_UART_DEBUG_FIRST_PORT
 
+// Convenient shorthand for pbdrv_uart_debug_printf.
+#define pb_log(...) pbdrv_uart_debug_printf(__VA_ARGS__)
+
 #endif // _INTERNAL_PBDRV_UART_DEBUG_FIRST_PORT_H_

lib/pbio/platform/ev3/platform.c

@@ -81,6 +81,7 @@
 #include "../../drv/pwm/pwm_ev3.h"
 #include "../../drv/reset/reset_ev3.h"
 #include "../../drv/uart/uart_ev3.h"
+#include "../../drv/uart/uart_debug_first_port.h"
 
 enum {
     LED_DEV_0_STATUS,
@@ -443,6 +444,15 @@ void ev3_panic_handler(int except_type, ev3_panic_ctx *except_data) {
     UARTConfigSetExpClk(SOC_UART_1_REGS, SOC_UART_1_MODULE_FREQ, 115200, UART_WORDL_8BITS, UART_OVER_SAMP_RATE_13);
     UARTFIFOEnable(SOC_UART_1_REGS);
 
+    // Drain whatever is still left in the UART debug FIFO.
+    lwrb_t* rb = pbdrv_uart_debug_get_ring_buffer();
+    if (rb != NULL) {
+        uint8_t byte;
+        while (lwrb_read(rb, &byte, 1) == 1) {
+            UARTCharPut(SOC_UART_1_REGS, byte);
+        }
+    }
+
     panic_puts("********************************************************************************\r\n");
     panic_puts("*                            Pybricks on EV3 Panic                             *\r\n");
     panic_puts("********************************************************************************\r\n");