Nadir serial COM1 polling driver

include/serial.h

@@ -0,0 +1,35 @@
+/* Nadir serial log: COM1 (16550 UART) polling driver.
+ *
+ * Runs in 64-bit long mode at Ring 0 (CPL 0); uses `inb`/`outb` which fault
+ * with #GP if executed above CPL 0, so this driver is kernel-only.
+ * Ports: COM1 0x3F8-0x3FF (data, int-enable, divisor latch, FIFO, line
+ * control, modem control, line status). QEMU/SeaBIOS provides COM1; the
+ * VGA console stays primary and serial is the secondary log.
+ * Fault behavior: port I/O cannot fault at CPL 0. The TX poll spins on
+ * LSR bit 5 and hangs only if the UART hardware is absent or wedged —
+ * call serial_ok() first to check LSR sanity.
+ *
+ * Preconditions: call serial_init() once before any put. serial_puts()
+ * requires a valid NUL-terminated string (NULL is ignored, not crashed
+ * on). Failure modes: on absent hardware, init writes go nowhere and
+ * serial_ok() returns 0; TX polls may then spin forever.
+ */
+
+#ifndef NADIR_SERIAL_H
+#define NADIR_SERIAL_H
+
+/* Program COM1 for 38400 8N1 polling (interrupts off, FIFO on).
+ * Ring 0 only. Safe to call again to re-program the port. */
+void serial_init(void);
+
+/* Block until the THR is empty (LSR bit 5), then emit one byte. */
+void serial_putchar(char c);
+
+/* Emit a NUL-terminated string via serial_putchar(). NULL is a no-op. */
+void serial_puts(const char *s);
+
+/* Nonzero if a UART looks present (LSR reads back sane, not 0xFF
+ * as an open bus returns). Zero means "do not trust TX polls". */
+int serial_ok(void);
+
+#endif /* NADIR_SERIAL_H */