drivers/firmware/google/memconsole-coreboot.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * memconsole-coreboot.c
   4  *
   5  * Memory based BIOS console accessed through coreboot table.
   6  *
   7  * Copyright 2017 Google Inc.
   8  */
   9
  10 #include <linux/device.h>
  11 #include <linux/io.h>
  12 #include <linux/kernel.h>
  13 #include <linux/module.h>
  14
  15 #include "memconsole.h"
  16 #include "coreboot_table.h"
  17
  18 #define CB_TAG_CBMEM_CONSOLE    0x17
  19
  20 /* CBMEM firmware console log descriptor. */
  21 struct cbmem_cons {
  22         u32 size_dont_access_after_boot;
  23         u32 cursor;
  24         u8  body[];
  25 } __packed;
  26
  27 #define CURSOR_MASK ((1 << 28) - 1)
  28 #define OVERFLOW (1 << 31)
  29
  30 static struct cbmem_cons *cbmem_console;
  31 static u32 cbmem_console_size;
  32
  33 /*
  34  * The cbmem_console structure is read again on every access because it may
  35  * change at any time if runtime firmware logs new messages. This may rarely
  36  * lead to race conditions where the firmware overwrites the beginning of the
  37  * ring buffer with more lines after we have already read |cursor|. It should be
  38  * rare and harmless enough that we don't spend extra effort working around it.
  39  */
  40 static ssize_t memconsole_coreboot_read(char *buf, loff_t pos, size_t count)
  41 {
  42         u32 cursor = cbmem_console->cursor & CURSOR_MASK;
  43         u32 flags = cbmem_console->cursor & ~CURSOR_MASK;
  44         u32 size = cbmem_console_size;
  45         struct seg {    /* describes ring buffer segments in logical order */
  46                 u32 phys;       /* physical offset from start of mem buffer */
  47                 u32 len;        /* length of segment */
  48         } seg[2] = { {0}, {0} };
  49         size_t done = 0;
  50         int i;
  51
  52         if (flags & OVERFLOW) {
  53                 if (cursor > size)      /* Shouldn't really happen, but... */
  54                         cursor = 0;
  55                 seg[0] = (struct seg){.phys = cursor, .len = size - cursor};
  56                 seg[1] = (struct seg){.phys = 0, .len = cursor};
  57         } else {
  58                 seg[0] = (struct seg){.phys = 0, .len = min(cursor, size)};
  59         }
  60
  61         for (i = 0; i < ARRAY_SIZE(seg) && count > done; i++) {
  62                 done += memory_read_from_buffer(buf + done, count - done, &pos,
  63                         cbmem_console->body + seg[i].phys, seg[i].len);
  64                 pos -= seg[i].len;
  65         }
  66         return done;
  67 }
  68
  69 static int memconsole_probe(struct coreboot_device *dev)
  70 {
  71         struct cbmem_cons *tmp_cbmc;
  72
  73         tmp_cbmc = memremap(dev->cbmem_ref.cbmem_addr,
  74                             sizeof(*tmp_cbmc), MEMREMAP_WB);
  75
  76         if (!tmp_cbmc)
  77                 return -ENOMEM;
  78
  79         /* Read size only once to prevent overrun attack through /dev/mem. */
  80         cbmem_console_size = tmp_cbmc->size_dont_access_after_boot;
  81         cbmem_console = devm_memremap(&dev->dev, dev->cbmem_ref.cbmem_addr,
  82                                  cbmem_console_size + sizeof(*cbmem_console),
  83                                  MEMREMAP_WB);
  84         memunmap(tmp_cbmc);
  85
  86         if (IS_ERR(cbmem_console))
  87                 return PTR_ERR(cbmem_console);
  88
  89         memconsole_setup(memconsole_coreboot_read);
  90
  91         return memconsole_sysfs_init();
  92 }
  93
  94 static int memconsole_remove(struct coreboot_device *dev)
  95 {
  96         memconsole_exit();
  97
  98         return 0;
  99 }
 100
 101 static struct coreboot_driver memconsole_driver = {
 102         .probe = memconsole_probe,
 103         .remove = memconsole_remove,
 104         .drv = {
 105                 .name = "memconsole",
 106         },
 107         .tag = CB_TAG_CBMEM_CONSOLE,
 108 };
 109 module_coreboot_driver(memconsole_driver);
 110
 111 MODULE_AUTHOR("Google, Inc.");
 112 MODULE_LICENSE("GPL");