1#include <linux/kernel.h>
 2#include <linux/init.h>
 3#include <linux/memblock.h>
 4
 5#include <asm/setup.h>
 6#include <asm/bios_ebda.h>
 7
 8/*
 9 * This function reserves all conventional PC system BIOS related
10 * firmware memory areas (some of which are data, some of which
11 * are code), that must not be used by the kernel as available
12 * RAM.
13 *
14 * The BIOS places the EBDA/XBDA at the top of conventional
15 * memory, and usually decreases the reported amount of
16 * conventional memory (int 0x12) too.
17 *
18 * This means that as a first approximation on most systems we can
19 * guess the reserved BIOS area by looking at the low BIOS RAM size
20 * value and assume that everything above that value (up to 1MB) is
21 * reserved.
22 *
23 * But life in firmware country is not that simple:
24 *
25 * - This code also contains a quirk for Dell systems that neglect
26 *   to reserve the EBDA area in the 'RAM size' value ...
27 *
28 * - The same quirk also avoids a problem with the AMD768MPX
29 *   chipset: reserve a page before VGA to prevent PCI prefetch
30 *   into it (errata #56). (Usually the page is reserved anyways,
31 *   unless you have no PS/2 mouse plugged in.)
32 *
33 * - Plus paravirt systems don't have a reliable value in the
34 *   'BIOS RAM size' pointer we can rely on, so we must quirk
35 *   them too.
36 *
37 * Due to those various problems this function is deliberately
38 * very conservative and tries to err on the side of reserving
39 * too much, to not risk reserving too little.
40 *
41 * Losing a small amount of memory in the bottom megabyte is
42 * rarely a problem, as long as we have enough memory to install
43 * the SMP bootup trampoline which *must* be in this area.
44 *
45 * Using memory that is in use by the BIOS or by some DMA device
46 * the BIOS didn't shut down *is* a big problem to the kernel,
47 * obviously.
48 */
49
50#define BIOS_RAM_SIZE_KB_PTR	0x413
51
52#define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
53#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
54
55void __init reserve_bios_regions(void)
56{
57	unsigned int bios_start, ebda_start;
58
59	/*
60	 * NOTE: In a paravirtual environment the BIOS reserved
61	 * area is absent. We'll just have to assume that the
62	 * paravirt case can handle memory setup correctly,
63	 * without our help.
64	 */
65	if (!x86_platform.legacy.reserve_bios_regions)
66		return;
67
68	/*
69	 * BIOS RAM size is encoded in kilobytes, convert it
70	 * to bytes to get a first guess at where the BIOS
71	 * firmware area starts:
72	 */
73	bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
74	bios_start <<= 10;
75
76	/*
77	 * If bios_start is less than 128K, assume it is bogus
78	 * and bump it up to 640K.  Similarly, if bios_start is above 640K,
79	 * don't trust it.
80	 */
81	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
82		bios_start = BIOS_START_MAX;
83
84	/* Get the start address of the EBDA page: */
85	ebda_start = get_bios_ebda();
86
87	/*
88	 * If the EBDA start address is sane and is below the BIOS region,
89	 * then also reserve everything from the EBDA start address up to
90	 * the BIOS region.
91	 */
92	if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
93		bios_start = ebda_start;
94
95	/* Reserve all memory between bios_start and the 1MB mark: */
96	memblock_reserve(bios_start, 0x100000 - bios_start);
97}