From a113f9dd9886e48914ba48b1f02ecf3ad0fc00d3 Mon Sep 17 00:00:00 2001
From: Warner Losh <imp@FreeBSD.org>
Date: Fri, 20 Oct 2023 16:07:03 -0600
Subject: [PATCH] uart: Support EARLY_PRINTF on x86 for port-mapped COM ports

Support early printf for the ns8250 uart driver. Adding
	options UART_NS8250_EARLY_PORT=0xYYY
	options EARLY_PRINTF
to your kernel config will enable it. The code is rather simple minded,
so caveat emptor. This will enable printf before cninit. cninit
automatically disables this and switches to the real routine. It only
works for port-mapped COM ports, and only if you know the port's address
at compile time. It's intended for be a debugging aide, not a general
purpose thing.

Sponsored by:		Netflix
Reviewed by:		emaste
Differential Revision:	https://reviews.freebsd.org/D42306
---
 sys/conf/options.amd64         |  3 +++
 sys/conf/options.i386          |  3 +++
 sys/dev/uart/uart_dev_ns8250.c | 21 +++++++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/sys/conf/options.amd64 b/sys/conf/options.amd64
index 8402c89640f..df18abfa8e1 100644
--- a/sys/conf/options.amd64
+++ b/sys/conf/options.amd64
@@ -65,3 +65,6 @@ NO_LEGACY_PCIB		opt_cpu.h
 
 # Compatibility with Linux MP table bugs.
 MPTABLE_LINUX_BUG_COMPAT
+
+# x86 specific uart options
+UART_NS8250_EARLY_PORT	opt_uart.h
diff --git a/sys/conf/options.i386 b/sys/conf/options.i386
index c827e0bb7a6..b98591a2a7d 100644
--- a/sys/conf/options.i386
+++ b/sys/conf/options.i386
@@ -109,3 +109,6 @@ NO_LEGACY_PCIB		opt_cpu.h
 
 # Compatibility with Linux MP table bugs.
 MPTABLE_LINUX_BUG_COMPAT
+
+# x86 specific uart options
+UART_NS8250_EARLY_PORT	opt_uart.h
diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
index de9b67c6bef..3787c6ed4c9 100644
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -78,6 +78,27 @@ static int broken_txfifo = 0;
 SYSCTL_INT(_hw, OID_AUTO, broken_txfifo, CTLFLAG_RWTUN,
 	&broken_txfifo, 0, "UART FIFO has QEMU emulation bug");
 
+/*
+ * To use early printf on x86, add the following to your kernel config:
+ *
+ * options UART_NS8250_EARLY_PORT=0x3f8
+ * options EARLY_PRINTF
+*/
+#if defined(EARLY_PRINTF) && (defined(__amd64__) || defined(__i386__))
+static void
+uart_ns8250_early_putc(int c)
+{
+	u_int stat = UART_NS8250_EARLY_PORT + REG_LSR;
+	u_int tx = UART_NS8250_EARLY_PORT + REG_DATA;
+	int limit = 10000; /* 10ms is plenty of time */
+
+	while ((inb(stat) & LSR_THRE) == 0 && --limit > 0)
+		continue;
+	outb(tx, c);
+}
+early_putc_t *early_putc = uart_ns8250_early_putc;
+#endif /* EARLY_PRINTF */
+
 /*
  * Clear pending interrupts. THRE is cleared by reading IIR. Data
  * that may have been received gets lost here.