[platform][pc] Add basic UART support and connect the console to it.

This is especially useful when using the "nographic" option of qemu like this:

qemu -kernel build-pc-x86/lk.bin -nographic

Define WITH_CGA_CONSOLE=1 to enable the CGA console instead.

platform/pc/debug.c

@@ -25,21 +25,86 @@
 #include <printf.h>
 #include <kernel/thread.h>
 #include <arch/x86.h>
+#include <lib/cbuf.h>
+#include <platform/interrupts.h>
 #include <platform/pc/memmap.h>
 #include <platform/console.h>
 #include <platform/keyboard.h>
 #include <platform/debug.h>
 
+static int uart_baud_rate = 115200;
+static int uart_io_port = 0x3f8;
+
+static cbuf_t uart_rx_buf;
+
+static enum handler_return uart_irq_handler(void *arg)
+{
+	unsigned char c;
+	bool resched = false;
+
+	while (inp(uart_io_port + 5) & (1<<0)) {
+		c = inp(uart_io_port + 0);
+		cbuf_write(&uart_rx_buf, &c, 1, false);
+		resched = true;
+	}
+
+	return resched ? INT_RESCHEDULE : INT_NO_RESCHEDULE;
+}
+
+void platform_init_uart(void)
+{
+	/* configure the uart */
+	int divisor = 115200 / uart_baud_rate;
+
+	/* get basic config done so that tx functions */
+	outp(uart_io_port + 3, 0x80); // set up to load divisor latch
+	outp(uart_io_port + 0, divisor & 0xff); // lsb
+	outp(uart_io_port + 1, divisor >> 8); // msb
+	outp(uart_io_port + 3, 3); // 8N1
+	outp(uart_io_port + 2, 0x07); // enable FIFO, clear, 14-byte threshold
+}
+
+void uart_init(void)
+{
+	/* finish uart init to get rx going */
+	cbuf_initialize(&uart_rx_buf, 16);
+
+	register_int_handler(0x24, uart_irq_handler, NULL);
+	unmask_interrupt(0x24);
+
+	outp(uart_io_port + 1, 0x1); // enable receive data available interrupt
+}
+
+void uart_putc(char c)
+{
+	while ((inp(uart_io_port + 5) & (1<<6)) == 0)
+		;
+	outp(uart_io_port + 0, c);
+}
+
+int uart_getc(char *c, bool wait)
+{
+	return cbuf_read(&uart_rx_buf, c, 1, wait);
+}
+
 void _dputc(char c)
 {
+#if WITH_CGA_CONSOLE
 	cputc(c);
+#else
+	uart_putc(c);
+#endif
 }
 
 int dgetc(char *c, bool wait)
 {
+#if WITH_CGA_CONSOLE
 	int ret =  platform_read_key(c);
 	//if (ret < 0)
 	//	arch_idle();
+#else
+	int ret = uart_getc(c, wait);
+#endif
 	
 	return ret;
 }

platform/pc/platform.c

@@ -30,6 +30,7 @@
 #include <platform/console.h>
 #include <platform/keyboard.h>
 #include <dev/pci.h>
+#include <dev/uart.h>
 
 extern multiboot_info_t *_multiboot_info;
 extern unsigned int _heap_end;
@@ -75,6 +76,8 @@ void platform_init_multiboot_info(void)
 
 void platform_early_init(void)
 {
+	platform_init_uart();
+
 	/* update the heap end so we can take advantage of more ram */
 	platform_init_multiboot_info();
 	
@@ -90,6 +93,8 @@ void platform_early_init(void)
 
 void platform_init(void)
 {
+	uart_init();
+
 	platform_init_keyboard();
 	
 	pci_init();

platform/pc/platform_p.h

@@ -25,6 +25,7 @@
 
 void platform_init_interrupts(void);
 void platform_init_timer(void);
+void platform_init_uart(void);
 
 #endif