drivers/firmware/google/memconsole-coreboot.c

   1 /*
   2  * memconsole-coreboot.c
   3  *
   4  * Memory based BIOS console accessed through coreboot table.
   5  *
   6  * Copyright 2017 Google Inc.
   7  *
   8  * This program is free software; you can redistribute it and/or modify
   9  * it under the terms of the GNU General Public License v2.0 as published by
  10  * the Free Software Foundation.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  */
  17
  18 #include <linux/device.h>
  19 #include <linux/kernel.h>
  20 #include <linux/module.h>
  21
  22 #include "memconsole.h"
  23 #include "coreboot_table.h"
  24
  25 #define CB_TAG_CBMEM_CONSOLE    0x17
  26
  27 /* CBMEM firmware console log descriptor. */
  28 struct cbmem_cons {
  29         u32 size_dont_access_after_boot;
  30         u32 cursor;
  31         u8  body[0];
  32 } __packed;
  33
  34 #define CURSOR_MASK ((1 << 28) - 1)
  35 #define OVERFLOW (1 << 31)
  36
  37 static struct cbmem_cons __iomem *cbmem_console;
  38 static u32 cbmem_console_size;
  39
  40 /*
  41  * The cbmem_console structure is read again on every access because it may
  42  * change at any time if runtime firmware logs new messages. This may rarely
  43  * lead to race conditions where the firmware overwrites the beginning of the
  44  * ring buffer with more lines after we have already read |cursor|. It should be
  45  * rare and harmless enough that we don't spend extra effort working around it.
  46  */
  47 static ssize_t memconsole_coreboot_read(char *buf, loff_t pos, size_t count)
  48 {
  49         u32 cursor = cbmem_console->cursor & CURSOR_MASK;
  50         u32 flags = cbmem_console->cursor & ~CURSOR_MASK;
  51         u32 size = cbmem_console_size;
  52         struct seg {    /* describes ring buffer segments in logical order */
  53                 u32 phys;       /* physical offset from start of mem buffer */
  54                 u32 len;        /* length of segment */
  55         } seg[2] = { {0}, {0} };
  56         size_t done = 0;
  57         int i;
  58
  59         if (flags & OVERFLOW) {
  60                 if (cursor > size)      /* Shouldn't really happen, but... */
  61                         cursor = 0;
  62                 seg[0] = (struct seg){.phys = cursor, .len = size - cursor};
  63                 seg[1] = (struct seg){.phys = 0, .len = cursor};
  64         } else {
  65                 seg[0] = (struct seg){.phys = 0, .len = min(cursor, size)};
  66         }
  67
  68         for (i = 0; i < ARRAY_SIZE(seg) && count > done; i++) {
  69                 done += memory_read_from_buffer(buf + done, count - done, &pos,
  70                         cbmem_console->body + seg[i].phys, seg[i].len);
  71                 pos -= seg[i].len;
  72         }
  73         return done;
  74 }
  75
  76 static int memconsole_probe(struct coreboot_device *dev)
  77 {
  78         struct cbmem_cons __iomem *tmp_cbmc;
  79
  80         tmp_cbmc = memremap(dev->cbmem_ref.cbmem_addr,
  81                             sizeof(*tmp_cbmc), MEMREMAP_WB);
  82
  83         if (!tmp_cbmc)
  84                 return -ENOMEM;
  85
  86         /* Read size only once to prevent overrun attack through /dev/mem. */
  87         cbmem_console_size = tmp_cbmc->size_dont_access_after_boot;
  88         cbmem_console = memremap(dev->cbmem_ref.cbmem_addr,
  89                                  cbmem_console_size + sizeof(*cbmem_console),
  90                                  MEMREMAP_WB);
  91         memunmap(tmp_cbmc);
  92
  93         if (!cbmem_console)
  94                 return -ENOMEM;
  95
  96         memconsole_setup(memconsole_coreboot_read);
  97
  98         return memconsole_sysfs_init();
  99 }
 100
 101 static int memconsole_remove(struct coreboot_device *dev)
 102 {
 103         memconsole_exit();
 104
 105         if (cbmem_console)
 106                 memunmap(cbmem_console);
 107
 108         return 0;
 109 }
 110
 111 static struct coreboot_driver memconsole_driver = {
 112         .probe = memconsole_probe,
 113         .remove = memconsole_remove,
 114         .drv = {
 115                 .name = "memconsole",
 116         },
 117         .tag = CB_TAG_CBMEM_CONSOLE,
 118 };
 119
 120 static void coreboot_memconsole_exit(void)
 121 {
 122         coreboot_driver_unregister(&memconsole_driver);
 123 }
 124
 125 static int __init coreboot_memconsole_init(void)
 126 {
 127         return coreboot_driver_register(&memconsole_driver);
 128 }
 129
 130 module_exit(coreboot_memconsole_exit);
 131 module_init(coreboot_memconsole_init);
 132
 133 MODULE_AUTHOR("Google, Inc.");
 134 MODULE_LICENSE("GPL");