trace.c

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/*
 * drivers/base/power/trace.c
 *
 * Copyright (C) 2006 Linus Torvalds
 *
 * Trace facility for suspend/resume problems, when none of the
 * devices may be working.
 */

#include <linux/resume-trace.h>
#include <linux/export.h>
#include <linux/rtc.h>

#include <asm/rtc.h>

#include "power.h"

/*
 * Horrid, horrid, horrid.
 *
 * It turns out that the _only_ piece of hardware that actually
 * keeps its value across a hard boot (and, more importantly, the
 * POST init sequence) is literally the realtime clock.
 *
 * Never mind that an RTC chip has 114 bytes (and often a whole
 * other bank of an additional 128 bytes) of nice SRAM that is
 * _designed_ to keep data - the POST will clear it. So we literally
 * can just use the few bytes of actual time data, which means that
 * we're really limited.
 *
 * It means, for example, that we can't use the seconds at all
 * (since the time between the hang and the boot might be more
 * than a minute), and we'd better not depend on the low bits of
 * the minutes either.
 *
 * There are the wday fields etc, but I wouldn't guarantee those
 * are dependable either. And if the date isn't valid, either the
 * hw or POST will do strange things.
 *
 * So we're left with:
 *  - year: 0-99
 *  - month: 0-11
 *  - day-of-month: 1-28
 *  - hour: 0-23
 *  - min: (0-30)*2
 *
 * Giving us a total range of 0-16128000 (0xf61800), ie less
 * than 24 bits of actual data we can save across reboots.
 *
 * And if your box can't boot in less than three minutes,
 * you're screwed.
 *
 * Now, almost 24 bits of data is pitifully small, so we need
 * to be pretty dense if we want to use it for anything nice.
 * What we do is that instead of saving off nice readable info,
 * we save off _hashes_ of information that we can hopefully
 * regenerate after the reboot.
 *
 * In particular, this means that we might be unlucky, and hit
 * a case where we have a hash collision, and we end up not
 * being able to tell for certain exactly which case happened.
 * But that's hopefully unlikely.
 *
 * What we do is to take the bits we can fit, and split them
 * into three parts (16*997*1009 = 16095568), and use the values
 * for:
 *  - 0-15: user-settable
 *  - 0-996: file + line number
 *  - 0-1008: device
 */
#define USERHASH (16)
#define FILEHASH (997)
#define DEVHASH (1009)

#define DEVSEED (7919)

static unsigned int dev_hash_value;

static int set_magic_time(unsigned int user, unsigned int file, unsigned int device)
{
    unsigned int n = user + USERHASH*(file + FILEHASH*device);

    // June 7th, 2006
    static struct rtc_time time = {
        .tm_sec = 0,
        .tm_min = 0,
        .tm_hour = 0,
        .tm_mday = 7,
        .tm_mon = 5,    // June - counting from zero
        .tm_year = 106,
        .tm_wday = 3,
        .tm_yday = 160,
        .tm_isdst = 1
    };

    time.tm_year = (n % 100);
    n /= 100;
    time.tm_mon = (n % 12);
    n /= 12;
    time.tm_mday = (n % 28) + 1;
    n /= 28;
    time.tm_hour = (n % 24);
    n /= 24;
    time.tm_min = (n % 20) * 3;
    n /= 20;
    set_rtc_time(&time);
    return n ? -1 : 0;
}

static unsigned int read_magic_time(void)
{
    struct rtc_time time;
    unsigned int val;

    get_rtc_time(&time);
    pr_info("RTC time: %2d:%02d:%02d, date: %02d/%02d/%02d\n",
        time.tm_hour, time.tm_min, time.tm_sec,
        time.tm_mon + 1, time.tm_mday, time.tm_year % 100);
    val = time.tm_year;             /* 100 years */
    if (val > 100)
        val -= 100;
    val += time.tm_mon * 100;           /* 12 months */
    val += (time.tm_mday-1) * 100 * 12;     /* 28 month-days */
    val += time.tm_hour * 100 * 12 * 28;        /* 24 hours */
    val += (time.tm_min / 3) * 100 * 12 * 28 * 24;  /* 20 3-minute intervals */
    return val;
}

/*
 * This is just the sdbm hash function with a user-supplied
 * seed and final size parameter.
 */
static unsigned int hash_string(unsigned int seed, const char *data, unsigned int mod)
{
    unsigned char c;
    while ((c = *data++) != 0) {
        seed = (seed << 16) + (seed << 6) - seed + c;
    }
    return seed % mod;
}

void set_trace_device(struct device *dev)
{
    dev_hash_value = hash_string(DEVSEED, dev_name(dev), DEVHASH);
}
EXPORT_SYMBOL(set_trace_device);

/*
 * We could just take the "tracedata" index into the .tracedata
 * section instead. Generating a hash of the data gives us a
 * chance to work across kernel versions, and perhaps more
 * importantly it also gives us valid/invalid check (ie we will
 * likely not give totally bogus reports - if the hash matches,
 * it's not any guarantee, but it's a high _likelihood_ that
 * the match is valid).
 */
void generate_resume_trace(const void *tracedata, unsigned int user)
{
    unsigned short lineno = *(unsigned short *)tracedata;
    const char *file = *(const char **)(tracedata + 2);
    unsigned int user_hash_value, file_hash_value;

    user_hash_value = user % USERHASH;
    file_hash_value = hash_string(lineno, file, FILEHASH);
    set_magic_time(user_hash_value, file_hash_value, dev_hash_value);
}
EXPORT_SYMBOL(generate_resume_trace);

extern char __tracedata_start, __tracedata_end;
static int show_file_hash(unsigned int value)
{
    int match;
    char *tracedata;

    match = 0;
    for (tracedata = &__tracedata_start ; tracedata < &__tracedata_end ;
            tracedata += 2 + sizeof(unsigned long)) {
        unsigned short lineno = *(unsigned short *)tracedata;
        const char *file = *(const char **)(tracedata + 2);
        unsigned int hash = hash_string(lineno, file, FILEHASH);
        if (hash != value)
            continue;
        pr_info("  hash matches %s:%u\n", file, lineno);
        match++;
    }
    return match;
}

static int show_dev_hash(unsigned int value)
{
    int match = 0;
    struct list_head *entry;

    device_pm_lock();
    entry = dpm_list.prev;
    while (entry != &dpm_list) {
        struct device * dev = to_device(entry);
        unsigned int hash = hash_string(DEVSEED, dev_name(dev), DEVHASH);
        if (hash == value) {
            dev_info(dev, "hash matches\n");
            match++;
        }
        entry = entry->prev;
    }
    device_pm_unlock();
    return match;
}

static unsigned int hash_value_early_read;

int show_trace_dev_match(char *buf, size_t size)
{
    unsigned int value = hash_value_early_read / (USERHASH * FILEHASH);
    int ret = 0;
    struct list_head *entry;

    /*
     * It's possible that multiple devices will match the hash and we can't
     * tell which is the culprit, so it's best to output them all.
     */
    device_pm_lock();
    entry = dpm_list.prev;
    while (size && entry != &dpm_list) {
        struct device *dev = to_device(entry);
        unsigned int hash = hash_string(DEVSEED, dev_name(dev),
                        DEVHASH);
        if (hash == value) {
            int len = snprintf(buf, size, "%s\n",
                        dev_driver_string(dev));
            if (len > size)
                len = size;
            buf += len;
            ret += len;
            size -= len;
        }
        entry = entry->prev;
    }
    device_pm_unlock();
    return ret;
}

static int early_resume_init(void)
{
    hash_value_early_read = read_magic_time();
    return 0;
}

static int late_resume_init(void)
{
    unsigned int val = hash_value_early_read;
    unsigned int user, file, dev;

    user = val % USERHASH;
    val = val / USERHASH;
    file = val % FILEHASH;
    val = val / FILEHASH;
    dev = val /* % DEVHASH */;

    pr_info("  Magic number: %d:%d:%d\n", user, file, dev);
    show_file_hash(file);
    show_dev_hash(dev);
    return 0;
}

core_initcall(early_resume_init);
late_initcall(late_resume_init);