1 files changed, 163 insertions, 0 deletions
diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
new file mode 100644
index 000000000..a2b6b4289
--- /dev/null
+++ b/arch/x86/boot/a20.c
@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* -*- linux-c -*- ------------------------------------------------------- *
+ *
+ *   Copyright (C) 1991, 1992 Linus Torvalds
+ *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
+ *   Copyright 2009 Intel Corporation; author H. Peter Anvin
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * Enable A20 gate (return -1 on failure)
+ */
+
+#include "boot.h"
+
+#define MAX_8042_LOOPS	100000
+#define MAX_8042_FF	32
+
+static int empty_8042(void)
+{
+	u8 status;
+	int loops = MAX_8042_LOOPS;
+	int ffs   = MAX_8042_FF;
+
+	while (loops--) {
+		io_delay();
+
+		status = inb(0x64);
+		if (status == 0xff) {
+			/* FF is a plausible, but very unlikely status */
+			if (!--ffs)
+				return -1; /* Assume no KBC present */
+		}
+		if (status & 1) {
+			/* Read and discard input data */
+			io_delay();
+			(void)inb(0x60);
+		} else if (!(status & 2)) {
+			/* Buffers empty, finished! */
+			return 0;
+		}
+	}
+
+	return -1;
+}
+
+/* Returns nonzero if the A20 line is enabled.  The memory address
+   used as a test is the int $0x80 vector, which should be safe. */
+
+#define A20_TEST_ADDR	(4*0x80)
+#define A20_TEST_SHORT  32
+#define A20_TEST_LONG	2097152	/* 2^21 */
+
+static int a20_test(int loops)
+{
+	int ok = 0;
+	int saved, ctr;
+
+	set_fs(0x0000);
+	set_gs(0xffff);
+
+	saved = ctr = rdfs32(A20_TEST_ADDR);
+
+	while (loops--) {
+		wrfs32(++ctr, A20_TEST_ADDR);
+		io_delay();	/* Serialize and make delay constant */
+		ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
+		if (ok)
+			break;
+	}
+
+	wrfs32(saved, A20_TEST_ADDR);
+	return ok;
+}
+
+/* Quick test to see if A20 is already enabled */
+static int a20_test_short(void)
+{
+	return a20_test(A20_TEST_SHORT);
+}
+
+/* Longer test that actually waits for A20 to come on line; this
+   is useful when dealing with the KBC or other slow external circuitry. */
+static int a20_test_long(void)
+{
+	return a20_test(A20_TEST_LONG);
+}
+
+static void enable_a20_bios(void)
+{
+	struct biosregs ireg;
+
+	initregs(&ireg);
+	ireg.ax = 0x2401;
+	intcall(0x15, &ireg, NULL);
+}
+
+static void enable_a20_kbc(void)
+{
+	empty_8042();
+
+	outb(0xd1, 0x64);	/* Command write */
+	empty_8042();
+
+	outb(0xdf, 0x60);	/* A20 on */
+	empty_8042();
+
+	outb(0xff, 0x64);	/* Null command, but UHCI wants it */
+	empty_8042();
+}
+
+static void enable_a20_fast(void)
+{
+	u8 port_a;
+
+	port_a = inb(0x92);	/* Configuration port A */
+	port_a |=  0x02;	/* Enable A20 */
+	port_a &= ~0x01;	/* Do not reset machine */
+	outb(port_a, 0x92);
+}
+
+/*
+ * Actual routine to enable A20; return 0 on ok, -1 on failure
+ */
+
+#define A20_ENABLE_LOOPS 255	/* Number of times to try */
+
+int enable_a20(void)
+{
+       int loops = A20_ENABLE_LOOPS;
+       int kbc_err;
+
+       while (loops--) {
+	       /* First, check to see if A20 is already enabled
+		  (legacy free, etc.) */
+	       if (a20_test_short())
+		       return 0;
+	       
+	       /* Next, try the BIOS (INT 0x15, AX=0x2401) */
+	       enable_a20_bios();
+	       if (a20_test_short())
+		       return 0;
+	       
+	       /* Try enabling A20 through the keyboard controller */
+	       kbc_err = empty_8042();
+
+	       if (a20_test_short())
+		       return 0; /* BIOS worked, but with delayed reaction */
+	
+	       if (!kbc_err) {
+		       enable_a20_kbc();
+		       if (a20_test_long())
+			       return 0;
+	       }
+	       
+	       /* Finally, try enabling the "fast A20 gate" */
+	       enable_a20_fast();
+	       if (a20_test_long())
+		       return 0;
+       }
+       
+       return -1;
+}