Nadir serial COM1 polling driver

include/serial.h

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+/* 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 */

kernel/arch/x86_64/serial.c

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+/* COM1 16550 UART polling log (see serial.h). */
+
+#include <stdint.h>
+
+#include "io.h"
+#include "serial.h"
+
+/* Register offsets from the COM1 base. */
+#define COM1_BASE 0x3F8U
+#define COM_DATA 0x3F8U /* THR/RBR (DLAB=0) / divisor low (DLAB=1) */
+#define COM_IER 0x3F9U /* interrupt enable / divisor high (DLAB=1) */
+#define COM_FIFO 0x3FAU /* FIFO control */
+#define COM_LCTL 0x3FBU /* line control + DLAB bit 7 */
+#define COM_MCTL 0x3FCU /* modem control */
+#define COM_LSR 0x3FDU /* line status: bit 5 = THR empty, bit 0 = RX ready */
+
+#define LSR_TX_EMPTY 0x20U /* THR holds no byte: safe to write */
+#define LSR_RX_READY 0x01U /* RBR holds a byte (reserved for future RX) */
+
+/* Port I/O owned by include/io.h; no local inb/outb duplicates. */
+
+void serial_init(void)
+{
+    /* No UART interrupts: pure polling, so the 8259/IDT stays untouched. */
+    outb(COM_IER, 0x00);
+    /* DLAB on to program the divisor. */
+    outb(COM_LCTL, 0x80);
+    /* Divisor 3 = 115200/3 = 38400 baud. */
+    outb(COM_DATA, 0x03);
+    outb(COM_IER, 0x00);
+    /* 8N1: 8 bits, no parity, one stop bit; DLAB back off. */
+    outb(COM_LCTL, 0x03);
+    /* FIFO on, clear both queues, 14-byte threshold. */
+    outb(COM_FIFO, 0xC7);
+    /* DTR+RTS on, OUT2 on so the UART drives interrupts (unused). */
+    outb(COM_MCTL, 0x0B);
+}
+
+void serial_putchar(char c)
+{
+    /* Spin until THR is empty; hangs only if hardware is absent/wedged,
+     * which is why callers gate on serial_ok() when unsure. */
+    while ((inb(COM_LSR) & LSR_TX_EMPTY) == 0) {
+        /* Poll. */
+    }
+    outb(COM_DATA, (uint8_t)c);
+}
+
+void serial_puts(const char *s)
+{
+    if (s == 0) {
+        return;
+    }
+    while (*s != '\0') {
+        serial_putchar(*s);
+        s++;
+    }
+}
+
+int serial_ok(void)
+{
+    /* An absent port reads back 0xFF (open bus); a real 16550 never
+     * reports all status bits set at once, so that means no UART. */
+    return inb(COM_LSR) != 0xFFU;
+}