memconsole.c

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/*
 * memconsole.c
 *
 * Infrastructure for importing the BIOS memory based console
 * into the kernel log ringbuffer.
 *
 * Copyright 2010 Google Inc. All rights reserved.
 */

#include <linux/ctype.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <linux/kobject.h>
#include <linux/module.h>
#include <linux/dmi.h>
#include <asm/bios_ebda.h>

#define BIOS_MEMCONSOLE_V1_MAGIC    0xDEADBABE
#define BIOS_MEMCONSOLE_V2_MAGIC    (('M')|('C'<<8)|('O'<<16)|('N'<<24))

struct biosmemcon_ebda {
    u32 signature;
    union {
        struct {
            u8  enabled;
            u32 buffer_addr;
            u16 start;
            u16 end;
            u16 num_chars;
            u8  wrapped;
        } __packed v1;
        struct {
            u32 buffer_addr;
            /* Misdocumented as number of pages! */
            u16 num_bytes;
            u16 start;
            u16 end;
        } __packed v2;
    };
} __packed;

static char *memconsole_baseaddr;
static size_t memconsole_length;

static ssize_t memconsole_read(struct file *filp, struct kobject *kobp,
                   struct bin_attribute *bin_attr, char *buf,
                   loff_t pos, size_t count)
{
    return memory_read_from_buffer(buf, count, &pos, memconsole_baseaddr,
                       memconsole_length);
}

static struct bin_attribute memconsole_bin_attr = {
    .attr = {.name = "log", .mode = 0444},
    .read = memconsole_read,
};


static void found_v1_header(struct biosmemcon_ebda *hdr)
{
    printk(KERN_INFO "BIOS console v1 EBDA structure found at %p\n", hdr);
    printk(KERN_INFO "BIOS console buffer at 0x%.8x, "
           "start = %d, end = %d, num = %d\n",
           hdr->v1.buffer_addr, hdr->v1.start,
           hdr->v1.end, hdr->v1.num_chars);

    memconsole_length = hdr->v1.num_chars;
    memconsole_baseaddr = phys_to_virt(hdr->v1.buffer_addr);
}

static void found_v2_header(struct biosmemcon_ebda *hdr)
{
    printk(KERN_INFO "BIOS console v2 EBDA structure found at %p\n", hdr);
    printk(KERN_INFO "BIOS console buffer at 0x%.8x, "
           "start = %d, end = %d, num_bytes = %d\n",
           hdr->v2.buffer_addr, hdr->v2.start,
           hdr->v2.end, hdr->v2.num_bytes);

    memconsole_length = hdr->v2.end - hdr->v2.start;
    memconsole_baseaddr = phys_to_virt(hdr->v2.buffer_addr
                       + hdr->v2.start);
}

/*
 * Search through the EBDA for the BIOS Memory Console, and
 * set the global variables to point to it.  Return true if found.
 */
static bool found_memconsole(void)
{
    unsigned int address;
    size_t length, cur;

    address = get_bios_ebda();
    if (!address) {
        printk(KERN_INFO "BIOS EBDA non-existent.\n");
        return false;
    }

    /* EBDA length is byte 0 of EBDA (in KB) */
    length = *(u8 *)phys_to_virt(address);
    length <<= 10; /* convert to bytes */

    /*
     * Search through EBDA for BIOS memory console structure
     * note: signature is not necessarily dword-aligned
     */
    for (cur = 0; cur < length; cur++) {
        struct biosmemcon_ebda *hdr = phys_to_virt(address + cur);

        /* memconsole v1 */
        if (hdr->signature == BIOS_MEMCONSOLE_V1_MAGIC) {
            found_v1_header(hdr);
            return true;
        }

        /* memconsole v2 */
        if (hdr->signature == BIOS_MEMCONSOLE_V2_MAGIC) {
            found_v2_header(hdr);
            return true;
        }
    }

    printk(KERN_INFO "BIOS console EBDA structure not found!\n");
    return false;
}

static struct dmi_system_id memconsole_dmi_table[] __initdata = {
    {
        .ident = "Google Board",
        .matches = {
            DMI_MATCH(DMI_BOARD_VENDOR, "Google, Inc."),
        },
    },
    {}
};
MODULE_DEVICE_TABLE(dmi, memconsole_dmi_table);

static int __init memconsole_init(void)
{
    int ret;

    if (!dmi_check_system(memconsole_dmi_table))
        return -ENODEV;

    if (!found_memconsole())
        return -ENODEV;

    memconsole_bin_attr.size = memconsole_length;

    ret = sysfs_create_bin_file(firmware_kobj, &memconsole_bin_attr);

    return ret;
}

static void __exit memconsole_exit(void)
{
    sysfs_remove_bin_file(firmware_kobj, &memconsole_bin_attr);
}

module_init(memconsole_init);
module_exit(memconsole_exit);

MODULE_AUTHOR("Google, Inc.");
MODULE_LICENSE("GPL");