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b2441318 | 1 | // SPDX-License-Identifier: GPL-2.0 |
0c51a965 HY |
2 | #include <linux/kernel.h> |
3 | #include <linux/init.h> | |
72d7c3b3 | 4 | #include <linux/memblock.h> |
0c51a965 HY |
5 | |
6 | #include <asm/setup.h> | |
7 | #include <asm/bios_ebda.h> | |
8 | ||
0c51a965 | 9 | /* |
edce2121 IM |
10 | * This function reserves all conventional PC system BIOS related |
11 | * firmware memory areas (some of which are data, some of which | |
12 | * are code), that must not be used by the kernel as available | |
13 | * RAM. | |
14 | * | |
0c51a965 HY |
15 | * The BIOS places the EBDA/XBDA at the top of conventional |
16 | * memory, and usually decreases the reported amount of | |
edce2121 IM |
17 | * conventional memory (int 0x12) too. |
18 | * | |
19 | * This means that as a first approximation on most systems we can | |
20 | * guess the reserved BIOS area by looking at the low BIOS RAM size | |
21 | * value and assume that everything above that value (up to 1MB) is | |
22 | * reserved. | |
23 | * | |
24 | * But life in firmware country is not that simple: | |
25 | * | |
26 | * - This code also contains a quirk for Dell systems that neglect | |
27 | * to reserve the EBDA area in the 'RAM size' value ... | |
28 | * | |
29 | * - The same quirk also avoids a problem with the AMD768MPX | |
30 | * chipset: reserve a page before VGA to prevent PCI prefetch | |
31 | * into it (errata #56). (Usually the page is reserved anyways, | |
32 | * unless you have no PS/2 mouse plugged in.) | |
33 | * | |
34 | * - Plus paravirt systems don't have a reliable value in the | |
35 | * 'BIOS RAM size' pointer we can rely on, so we must quirk | |
36 | * them too. | |
37 | * | |
38 | * Due to those various problems this function is deliberately | |
39 | * very conservative and tries to err on the side of reserving | |
40 | * too much, to not risk reserving too little. | |
41 | * | |
42 | * Losing a small amount of memory in the bottom megabyte is | |
43 | * rarely a problem, as long as we have enough memory to install | |
44 | * the SMP bootup trampoline which *must* be in this area. | |
7c100936 | 45 | * |
edce2121 IM |
46 | * Using memory that is in use by the BIOS or by some DMA device |
47 | * the BIOS didn't shut down *is* a big problem to the kernel, | |
48 | * obviously. | |
0c51a965 | 49 | */ |
7c100936 | 50 | |
edce2121 | 51 | #define BIOS_RAM_SIZE_KB_PTR 0x413 |
7c100936 | 52 | |
edce2121 IM |
53 | #define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */ |
54 | #define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */ | |
55 | ||
56 | void __init reserve_bios_regions(void) | |
0c51a965 | 57 | { |
edce2121 | 58 | unsigned int bios_start, ebda_start; |
0c51a965 | 59 | |
7c100936 | 60 | /* |
edce2121 IM |
61 | * NOTE: In a paravirtual environment the BIOS reserved |
62 | * area is absent. We'll just have to assume that the | |
63 | * paravirt case can handle memory setup correctly, | |
64 | * without our help. | |
7c100936 | 65 | */ |
edce2121 | 66 | if (!x86_platform.legacy.reserve_bios_regions) |
0c51a965 HY |
67 | return; |
68 | ||
edce2121 IM |
69 | /* |
70 | * BIOS RAM size is encoded in kilobytes, convert it | |
71 | * to bytes to get a first guess at where the BIOS | |
72 | * firmware area starts: | |
73 | */ | |
74 | bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR); | |
75 | bios_start <<= 10; | |
0c51a965 | 76 | |
edce2121 IM |
77 | /* |
78 | * If bios_start is less than 128K, assume it is bogus | |
6a79296c AL |
79 | * and bump it up to 640K. Similarly, if bios_start is above 640K, |
80 | * don't trust it. | |
edce2121 | 81 | */ |
6a79296c | 82 | if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX) |
edce2121 | 83 | bios_start = BIOS_START_MAX; |
0c51a965 | 84 | |
6a79296c AL |
85 | /* Get the start address of the EBDA page: */ |
86 | ebda_start = get_bios_ebda(); | |
87 | ||
edce2121 | 88 | /* |
6a79296c AL |
89 | * If the EBDA start address is sane and is below the BIOS region, |
90 | * then also reserve everything from the EBDA start address up to | |
91 | * the BIOS region. | |
edce2121 | 92 | */ |
6a79296c AL |
93 | if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start) |
94 | bios_start = ebda_start; | |
0c51a965 | 95 | |
edce2121 IM |
96 | /* Reserve all memory between bios_start and the 1MB mark: */ |
97 | memblock_reserve(bios_start, 0x100000 - bios_start); | |
0c51a965 | 98 | } |