linux/drivers/base/power/trace.c

// SPDX-License-Identifier: GPL-2.0
/*
 * drivers/base/power/trace.c
 *
 * Copyright (C) 2006 Linus Torvalds
 *
 * Trace facility for suspend/resume problems, when none of the
 * devices may be working.
 */
#define pr_fmt(fmt)

#include <linux/pm-trace.h>
#include <linux/export.h>
#include <linux/rtc.h>
#include <linux/suspend.h>
#include <linux/init.h>

#include <linux/mc146818rtc.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
#define FILEHASH
#define DEVHASH

#define DEVSEED

bool pm_trace_rtc_abused __read_mostly;
EXPORT_SYMBOL_GPL();

static unsigned int dev_hash_value;

static int set_magic_time(unsigned int user, unsigned int file, unsigned int device)
{}

static unsigned int read_magic_time(void)
{}

/*
 * 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)
{}

void set_trace_device(struct device *dev)
{}
EXPORT_SYMBOL();

/*
 * 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_pm_trace(const void *tracedata, unsigned int user)
{}
EXPORT_SYMBOL();

extern char __tracedata_start[], __tracedata_end[];
static int show_file_hash(unsigned int value)
{}

static int show_dev_hash(unsigned int value)
{}

static unsigned int hash_value_early_read;

int show_trace_dev_match(char *buf, size_t size)
{}

static int
pm_trace_notify(struct notifier_block *nb, unsigned long mode, void *_unused)
{}

static struct notifier_block pm_trace_nb =;

static int __init early_resume_init(void)
{}

static int __init late_resume_init(void)
{}

core_initcall(early_resume_init);
late_initcall(late_resume_init);