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