1 // SPDX-License-Identifier: GPL-2.0 1
2 #include <linux/kernel.h>
3 #include <linux/init.h>
4 #include <linux/memblock.h>
5
6 #include <asm/setup.h>
7 #include <asm/bios_ebda.h>
8
9 /*
10 * This function reserves all conventional PC
11 * firmware memory areas (some of which are da
12 * are code), that must not be used by the ker
13 * RAM.
14 *
15 * The BIOS places the EBDA/XBDA at the top of
16 * memory, and usually decreases the reported
17 * conventional memory (int 0x12) too.
18 *
19 * This means that as a first approximation on
20 * guess the reserved BIOS area by looking at
21 * value and assume that everything above that
22 * reserved.
23 *
24 * But life in firmware country is not that si
25 *
26 * - This code also contains a quirk for Dell
27 * to reserve the EBDA area in the 'RAM size
28 *
29 * - The same quirk also avoids a problem with
30 * chipset: reserve a page before VGA to pre
31 * into it (errata #56). (Usually the page i
32 * unless you have no PS/2 mouse plugged in.
33 *
34 * - Plus paravirt systems don't have a reliab
35 * 'BIOS RAM size' pointer we can rely on, s
36 * them too.
37 *
38 * Due to those various problems this function
39 * very conservative and tries to err on the s
40 * too much, to not risk reserving too little.
41 *
42 * Losing a small amount of memory in the bott
43 * rarely a problem, as long as we have enough
44 * the SMP bootup trampoline which *must* be i
45 *
46 * Using memory that is in use by the BIOS or
47 * the BIOS didn't shut down *is* a big proble
48 * obviously.
49 */
50
51 #define BIOS_RAM_SIZE_KB_PTR 0x413
52
53 #define BIOS_START_MIN 0x20000U
54 #define BIOS_START_MAX 0x9f000U
55
56 void __init reserve_bios_regions(void)
57 {
58 unsigned int bios_start, ebda_start;
59
60 /*
61 * NOTE: In a paravirtual environment
62 * area is absent. We'll just have to
63 * paravirt case can handle memory set
64 * without our help.
65 */
66 if (!x86_platform.legacy.reserve_bios_
67 return;
68
69 /*
70 * BIOS RAM size is encoded in kilobyt
71 * to bytes to get a first guess at wh
72 * firmware area starts:
73 */
74 bios_start = *(unsigned short *)__va(B
75 bios_start <<= 10;
76
77 /*
78 * If bios_start is less than 128K, as
79 * and bump it up to 640K. Similarly,
80 * don't trust it.
81 */
82 if (bios_start < BIOS_START_MIN || bio
83 bios_start = BIOS_START_MAX;
84
85 /* Get the start address of the EBDA p
86 ebda_start = get_bios_ebda();
87
88 /*
89 * If the EBDA start address is sane a
90 * then also reserve everything from t
91 * the BIOS region.
92 */
93 if (ebda_start >= BIOS_START_MIN && eb
94 bios_start = ebda_start;
95
96 /* Reserve all memory between bios_sta
97 memblock_reserve(bios_start, 0x100000
98 }
99
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