1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * memconsole-x86-legacy.c
4  *
5  * EBDA specific parts of the memory based BIOS console.
6  *
7  * Copyright 2017 Google Inc.
8  */
9 
10 #include <linux/kernel.h>
11 #include <linux/module.h>
12 #include <linux/dmi.h>
13 #include <linux/mm.h>
14 #include <asm/bios_ebda.h>
15 #include <linux/acpi.h>
16 
17 #include "memconsole.h"
18 
19 #define BIOS_MEMCONSOLE_V1_MAGIC	0xDEADBABE
20 #define BIOS_MEMCONSOLE_V2_MAGIC	(('M')|('C'<<8)|('O'<<16)|('N'<<24))
21 
22 struct biosmemcon_ebda {
23 	u32 signature;
24 	union {
25 		struct {
26 			u8  enabled;
27 			u32 buffer_addr;
28 			u16 start;
29 			u16 end;
30 			u16 num_chars;
31 			u8  wrapped;
32 		} __packed v1;
33 		struct {
34 			u32 buffer_addr;
35 			/* Misdocumented as number of pages! */
36 			u16 num_bytes;
37 			u16 start;
38 			u16 end;
39 		} __packed v2;
40 	};
41 } __packed;
42 
43 static char *memconsole_baseaddr;
44 static size_t memconsole_length;
45 
46 static ssize_t memconsole_read(char *buf, loff_t pos, size_t count)
47 {
48 	return memory_read_from_buffer(buf, count, &pos, memconsole_baseaddr,
49 				       memconsole_length);
50 }
51 
52 static void found_v1_header(struct biosmemcon_ebda *hdr)
53 {
54 	pr_info("memconsole: BIOS console v1 EBDA structure found at %p\n",
55 		hdr);
56 	pr_info("memconsole: BIOS console buffer at 0x%.8x, start = %d, end = %d, num = %d\n",
57 		hdr->v1.buffer_addr, hdr->v1.start,
58 		hdr->v1.end, hdr->v1.num_chars);
59 
60 	memconsole_baseaddr = phys_to_virt(hdr->v1.buffer_addr);
61 	memconsole_length = hdr->v1.num_chars;
62 	memconsole_setup(memconsole_read);
63 }
64 
65 static void found_v2_header(struct biosmemcon_ebda *hdr)
66 {
67 	pr_info("memconsole: BIOS console v2 EBDA structure found at %p\n",
68 		hdr);
69 	pr_info("memconsole: BIOS console buffer at 0x%.8x, start = %d, end = %d, num_bytes = %d\n",
70 		hdr->v2.buffer_addr, hdr->v2.start,
71 		hdr->v2.end, hdr->v2.num_bytes);
72 
73 	memconsole_baseaddr = phys_to_virt(hdr->v2.buffer_addr + hdr->v2.start);
74 	memconsole_length = hdr->v2.end - hdr->v2.start;
75 	memconsole_setup(memconsole_read);
76 }
77 
78 /*
79  * Search through the EBDA for the BIOS Memory Console, and
80  * set the global variables to point to it.  Return true if found.
81  */
82 static bool memconsole_ebda_init(void)
83 {
84 	unsigned int address;
85 	size_t length, cur;
86 
87 	address = get_bios_ebda();
88 	if (!address) {
89 		pr_info("memconsole: BIOS EBDA non-existent.\n");
90 		return false;
91 	}
92 
93 	/* EBDA length is byte 0 of EBDA (in KB) */
94 	length = *(u8 *)phys_to_virt(address);
95 	length <<= 10; /* convert to bytes */
96 
97 	/*
98 	 * Search through EBDA for BIOS memory console structure
99 	 * note: signature is not necessarily dword-aligned
100 	 */
101 	for (cur = 0; cur < length; cur++) {
102 		struct biosmemcon_ebda *hdr = phys_to_virt(address + cur);
103 
104 		/* memconsole v1 */
105 		if (hdr->signature == BIOS_MEMCONSOLE_V1_MAGIC) {
106 			found_v1_header(hdr);
107 			return true;
108 		}
109 
110 		/* memconsole v2 */
111 		if (hdr->signature == BIOS_MEMCONSOLE_V2_MAGIC) {
112 			found_v2_header(hdr);
113 			return true;
114 		}
115 	}
116 
117 	pr_info("memconsole: BIOS console EBDA structure not found!\n");
118 	return false;
119 }
120 
121 static const struct dmi_system_id memconsole_dmi_table[] __initconst = {
122 	{
123 		.ident = "Google Board",
124 		.matches = {
125 			DMI_MATCH(DMI_BOARD_VENDOR, "Google, Inc."),
126 		},
127 	},
128 	{}
129 };
130 MODULE_DEVICE_TABLE(dmi, memconsole_dmi_table);
131 
132 static bool __init memconsole_find(void)
133 {
134 	if (!dmi_check_system(memconsole_dmi_table))
135 		return false;
136 
137 	return memconsole_ebda_init();
138 }
139 
140 static int __init memconsole_x86_init(void)
141 {
142 	if (!memconsole_find())
143 		return -ENODEV;
144 
145 	return memconsole_sysfs_init();
146 }
147 
148 static void __exit memconsole_x86_exit(void)
149 {
150 	memconsole_exit();
151 }
152 
153 module_init(memconsole_x86_init);
154 module_exit(memconsole_x86_exit);
155 
156 MODULE_AUTHOR("Google, Inc.");
157 MODULE_LICENSE("GPL");
158