v4.19.13 snapshot.
diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
new file mode 100644
index 0000000..64a31a6
--- /dev/null
+++ b/arch/x86/boot/a20.c
@@ -0,0 +1,165 @@
+/* -*- linux-c -*- ------------------------------------------------------- *
+ *
+ * Copyright (C) 1991, 1992 Linus Torvalds
+ * Copyright 2007-2008 rPath, Inc. - All Rights Reserved
+ * Copyright 2009 Intel Corporation; author H. Peter Anvin
+ *
+ * This file is part of the Linux kernel, and is made available under
+ * the terms of the GNU General Public License version 2.
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * 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;
+}