Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/jesse/openvswitch

[GitHub/mt8127/android_kernel_alcatel_ttab.git] / kernel / debug / debug_core.c

/*
 * Kernel Debug Core
 *
 * Maintainer: Jason Wessel <jason.wessel@windriver.com>
 *
 * Copyright (C) 2000-2001 VERITAS Software Corporation.
 * Copyright (C) 2002-2004 Timesys Corporation
 * Copyright (C) 2003-2004 Amit S. Kale <amitkale@linsyssoft.com>
 * Copyright (C) 2004 Pavel Machek <pavel@ucw.cz>
 * Copyright (C) 2004-2006 Tom Rini <trini@kernel.crashing.org>
 * Copyright (C) 2004-2006 LinSysSoft Technologies Pvt. Ltd.
 * Copyright (C) 2005-2009 Wind River Systems, Inc.
 * Copyright (C) 2007 MontaVista Software, Inc.
 * Copyright (C) 2008 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 * Contributors at various stages not listed above:
 *  Jason Wessel ( jason.wessel@windriver.com )
 *  George Anzinger <george@mvista.com>
 *  Anurekh Saxena (anurekh.saxena@timesys.com)
 *  Lake Stevens Instrument Division (Glenn Engel)
 *  Jim Kingdon, Cygnus Support.
 *
 * Original KGDB stub: David Grothe <dave@gcom.com>,
 * Tigran Aivazian <tigran@sco.com>
 *
 * This file is licensed under the terms of the GNU General Public License
 * version 2. This program is licensed "as is" without any warranty of any
 * kind, whether express or implied.
 */
#include <linux/pid_namespace.h>
#include <linux/clocksource.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
#include <linux/console.h>
#include <linux/threads.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/string.h>
#include <linux/delay.h>
#include <linux/sched.h>
#include <linux/sysrq.h>
#include <linux/reboot.h>
#include <linux/init.h>
#include <linux/kgdb.h>
#include <linux/kdb.h>
#include <linux/pid.h>
#include <linux/smp.h>
#include <linux/mm.h>
#include <linux/rcupdate.h>

#include <asm/cacheflush.h>
#include <asm/byteorder.h>
#include <linux/atomic.h>

#include "debug_core.h"

static int kgdb_break_asap;

struct debuggerinfo_struct kgdb_info[NR_CPUS];

/**
 * kgdb_connected - Is a host GDB connected to us?
 */
int                             kgdb_connected;
EXPORT_SYMBOL_GPL(kgdb_connected);

/* All the KGDB handlers are installed */
int                     kgdb_io_module_registered;

/* Guard for recursive entry */
static int                      exception_level;

struct kgdb_io          *dbg_io_ops;
static DEFINE_SPINLOCK(kgdb_registration_lock);

/* Action for the reboot notifiter, a global allow kdb to change it */
static int kgdbreboot;
/* kgdb console driver is loaded */
static int kgdb_con_registered;
/* determine if kgdb console output should be used */
static int kgdb_use_con;
/* Flag for alternate operations for early debugging */
bool dbg_is_early = true;
/* Next cpu to become the master debug core */
int dbg_switch_cpu;

/* Use kdb or gdbserver mode */
int dbg_kdb_mode = 1;

static int __init opt_kgdb_con(char *str)
{
        kgdb_use_con = 1;
        return 0;
}

early_param("kgdbcon", opt_kgdb_con);

module_param(kgdb_use_con, int, 0644);
module_param(kgdbreboot, int, 0644);

/*
 * Holds information about breakpoints in a kernel. These breakpoints are
 * added and removed by gdb.
 */
static struct kgdb_bkpt         kgdb_break[KGDB_MAX_BREAKPOINTS] = {
        [0 ... KGDB_MAX_BREAKPOINTS-1] = { .state = BP_UNDEFINED }
};

/*
 * The CPU# of the active CPU, or -1 if none:
 */
atomic_t                        kgdb_active = ATOMIC_INIT(-1);
EXPORT_SYMBOL_GPL(kgdb_active);
static DEFINE_RAW_SPINLOCK(dbg_master_lock);
static DEFINE_RAW_SPINLOCK(dbg_slave_lock);

/*
 * We use NR_CPUs not PERCPU, in case kgdb is used to debug early
 * bootup code (which might not have percpu set up yet):
 */
static atomic_t                 masters_in_kgdb;
static atomic_t                 slaves_in_kgdb;
static atomic_t                 kgdb_break_tasklet_var;
atomic_t                        kgdb_setting_breakpoint;

struct task_struct              *kgdb_usethread;
struct task_struct              *kgdb_contthread;

int                             kgdb_single_step;
static pid_t                    kgdb_sstep_pid;

/* to keep track of the CPU which is doing the single stepping*/
atomic_t                        kgdb_cpu_doing_single_step = ATOMIC_INIT(-1);

/*
 * If you are debugging a problem where roundup (the collection of
 * all other CPUs) is a problem [this should be extremely rare],
 * then use the nokgdbroundup option to avoid roundup. In that case
 * the other CPUs might interfere with your debugging context, so
 * use this with care:
 */
static int kgdb_do_roundup = 1;

static int __init opt_nokgdbroundup(char *str)
{
        kgdb_do_roundup = 0;

        return 0;
}

early_param("nokgdbroundup", opt_nokgdbroundup);

/*
 * Finally, some KGDB code :-)
 */

/*
 * Weak aliases for breakpoint management,
 * can be overriden by architectures when needed:
 */
int __weak kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
{
        int err;

        err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
                                BREAK_INSTR_SIZE);
        if (err)
                return err;
        err = probe_kernel_write((char *)bpt->bpt_addr,
                                 arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
        return err;
}

int __weak kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
{
        return probe_kernel_write((char *)bpt->bpt_addr,
                                  (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
}

int __weak kgdb_validate_break_address(unsigned long addr)
{
        struct kgdb_bkpt tmp;
        int err;
        /* Validate setting the breakpoint and then removing it.  If the
         * remove fails, the kernel needs to emit a bad message because we
         * are deep trouble not being able to put things back the way we
         * found them.
         */
        tmp.bpt_addr = addr;
        err = kgdb_arch_set_breakpoint(&tmp);
        if (err)
                return err;
        err = kgdb_arch_remove_breakpoint(&tmp);
        if (err)
                printk(KERN_ERR "KGDB: Critical breakpoint error, kernel "
                   "memory destroyed at: %lx", addr);
        return err;
}

unsigned long __weak kgdb_arch_pc(int exception, struct pt_regs *regs)
{
        return instruction_pointer(regs);
}

int __weak kgdb_arch_init(void)
{
        return 0;
}

int __weak kgdb_skipexception(int exception, struct pt_regs *regs)
{
        return 0;
}

/*
 * Some architectures need cache flushes when we set/clear a
 * breakpoint:
 */
static void kgdb_flush_swbreak_addr(unsigned long addr)
{
        if (!CACHE_FLUSH_IS_SAFE)
                return;

        if (current->mm && current->mm->mmap_cache) {
                flush_cache_range(current->mm->mmap_cache,
                                  addr, addr + BREAK_INSTR_SIZE);
        }
        /* Force flush instruction cache if it was outside the mm */
        flush_icache_range(addr, addr + BREAK_INSTR_SIZE);
}

/*
 * SW breakpoint management:
 */
int dbg_activate_sw_breakpoints(void)
{
        int error;
        int ret = 0;
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_SET)
                        continue;

                error = kgdb_arch_set_breakpoint(&kgdb_break[i]);
                if (error) {
                        ret = error;
                        printk(KERN_INFO "KGDB: BP install failed: %lx",
                               kgdb_break[i].bpt_addr);
                        continue;
                }

                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                kgdb_break[i].state = BP_ACTIVE;
        }
        return ret;
}

int dbg_set_sw_break(unsigned long addr)
{
        int err = kgdb_validate_break_address(addr);
        int breakno = -1;
        int i;

        if (err)
                return err;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_SET) &&
                                        (kgdb_break[i].bpt_addr == addr))
                        return -EEXIST;
        }
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state == BP_REMOVED &&
                                        kgdb_break[i].bpt_addr == addr) {
                        breakno = i;
                        break;
                }
        }

        if (breakno == -1) {
                for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                        if (kgdb_break[i].state == BP_UNDEFINED) {
                                breakno = i;
                                break;
                        }
                }
        }

        if (breakno == -1)
                return -E2BIG;

        kgdb_break[breakno].state = BP_SET;
        kgdb_break[breakno].type = BP_BREAKPOINT;
        kgdb_break[breakno].bpt_addr = addr;

        return 0;
}

int dbg_deactivate_sw_breakpoints(void)
{
        int error;
        int ret = 0;
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_ACTIVE)
                        continue;
                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                if (error) {
                        printk(KERN_INFO "KGDB: BP remove failed: %lx\n",
                               kgdb_break[i].bpt_addr);
                        ret = error;
                }

                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                kgdb_break[i].state = BP_SET;
        }
        return ret;
}

int dbg_remove_sw_break(unsigned long addr)
{
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_SET) &&
                                (kgdb_break[i].bpt_addr == addr)) {
                        kgdb_break[i].state = BP_REMOVED;
                        return 0;
                }
        }
        return -ENOENT;
}

int kgdb_isremovedbreak(unsigned long addr)
{
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_REMOVED) &&
                                        (kgdb_break[i].bpt_addr == addr))
                        return 1;
        }
        return 0;
}

int dbg_remove_all_break(void)
{
        int error;
        int i;

        /* Clear memory breakpoints. */
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_ACTIVE)
                        goto setundefined;
                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                if (error)
                        printk(KERN_ERR "KGDB: breakpoint remove failed: %lx\n",
                               kgdb_break[i].bpt_addr);
setundefined:
                kgdb_break[i].state = BP_UNDEFINED;
        }

        /* Clear hardware breakpoints. */
        if (arch_kgdb_ops.remove_all_hw_break)
                arch_kgdb_ops.remove_all_hw_break();

        return 0;
}

/*
 * Return true if there is a valid kgdb I/O module.  Also if no
 * debugger is attached a message can be printed to the console about
 * waiting for the debugger to attach.
 *
 * The print_wait argument is only to be true when called from inside
 * the core kgdb_handle_exception, because it will wait for the
 * debugger to attach.
 */
static int kgdb_io_ready(int print_wait)
{
        if (!dbg_io_ops)
                return 0;
        if (kgdb_connected)
                return 1;
        if (atomic_read(&kgdb_setting_breakpoint))
                return 1;
        if (print_wait) {
#ifdef CONFIG_KGDB_KDB
                if (!dbg_kdb_mode)
                        printk(KERN_CRIT "KGDB: waiting... or $3#33 for KDB\n");
#else
                printk(KERN_CRIT "KGDB: Waiting for remote debugger\n");
#endif
        }
        return 1;
}

static int kgdb_reenter_check(struct kgdb_state *ks)
{
        unsigned long addr;

        if (atomic_read(&kgdb_active) != raw_smp_processor_id())
                return 0;

        /* Panic on recursive debugger calls: */
        exception_level++;
        addr = kgdb_arch_pc(ks->ex_vector, ks->linux_regs);
        dbg_deactivate_sw_breakpoints();

        /*
         * If the break point removed ok at the place exception
         * occurred, try to recover and print a warning to the end
         * user because the user planted a breakpoint in a place that
         * KGDB needs in order to function.
         */
        if (dbg_remove_sw_break(addr) == 0) {
                exception_level = 0;
                kgdb_skipexception(ks->ex_vector, ks->linux_regs);
                dbg_activate_sw_breakpoints();
                printk(KERN_CRIT "KGDB: re-enter error: breakpoint removed %lx\n",
                        addr);
                WARN_ON_ONCE(1);

                return 1;
        }
        dbg_remove_all_break();
        kgdb_skipexception(ks->ex_vector, ks->linux_regs);

        if (exception_level > 1) {
                dump_stack();
                panic("Recursive entry to debugger");
        }

        printk(KERN_CRIT "KGDB: re-enter exception: ALL breakpoints killed\n");
#ifdef CONFIG_KGDB_KDB
        /* Allow kdb to debug itself one level */
        return 0;
#endif
        dump_stack();
        panic("Recursive entry to debugger");

        return 1;
}

static void dbg_touch_watchdogs(void)
{
        touch_softlockup_watchdog_sync();
        clocksource_touch_watchdog();
        rcu_cpu_stall_reset();
}

static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
                int exception_state)
{
        unsigned long flags;
        int sstep_tries = 100;
        int error;
        int cpu;
        int trace_on = 0;
        int online_cpus = num_online_cpus();

        kgdb_info[ks->cpu].enter_kgdb++;
        kgdb_info[ks->cpu].exception_state |= exception_state;

        if (exception_state == DCPU_WANT_MASTER)
                atomic_inc(&masters_in_kgdb);
        else
                atomic_inc(&slaves_in_kgdb);

        if (arch_kgdb_ops.disable_hw_break)
                arch_kgdb_ops.disable_hw_break(regs);

acquirelock:
        /*
         * Interrupts will be restored by the 'trap return' code, except when
         * single stepping.
         */
        local_irq_save(flags);

        cpu = ks->cpu;
        kgdb_info[cpu].debuggerinfo = regs;
        kgdb_info[cpu].task = current;
        kgdb_info[cpu].ret_state = 0;
        kgdb_info[cpu].irq_depth = hardirq_count() >> HARDIRQ_SHIFT;

        /* Make sure the above info reaches the primary CPU */
        smp_mb();

        if (exception_level == 1) {
                if (raw_spin_trylock(&dbg_master_lock))
                        atomic_xchg(&kgdb_active, cpu);
                goto cpu_master_loop;
        }

        /*
         * CPU will loop if it is a slave or request to become a kgdb
         * master cpu and acquire the kgdb_active lock:
         */
        while (1) {
cpu_loop:
                if (kgdb_info[cpu].exception_state & DCPU_NEXT_MASTER) {
                        kgdb_info[cpu].exception_state &= ~DCPU_NEXT_MASTER;
                        goto cpu_master_loop;
                } else if (kgdb_info[cpu].exception_state & DCPU_WANT_MASTER) {
                        if (raw_spin_trylock(&dbg_master_lock)) {
                                atomic_xchg(&kgdb_active, cpu);
                                break;
                        }
                } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
                        if (!raw_spin_is_locked(&dbg_slave_lock))
                                goto return_normal;
                } else {
return_normal:
                        /* Return to normal operation by executing any
                         * hw breakpoint fixup.
                         */
                        if (arch_kgdb_ops.correct_hw_break)
                                arch_kgdb_ops.correct_hw_break();
                        if (trace_on)
                                tracing_on();
                        kgdb_info[cpu].exception_state &=
                                ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
                        kgdb_info[cpu].enter_kgdb--;
                        smp_mb__before_atomic_dec();
                        atomic_dec(&slaves_in_kgdb);
                        dbg_touch_watchdogs();
                        local_irq_restore(flags);
                        return 0;
                }
                cpu_relax();
        }

        /*
         * For single stepping, try to only enter on the processor
         * that was single stepping.  To guard against a deadlock, the
         * kernel will only try for the value of sstep_tries before
         * giving up and continuing on.
         */
        if (atomic_read(&kgdb_cpu_doing_single_step) != -1 &&
            (kgdb_info[cpu].task &&
             kgdb_info[cpu].task->pid != kgdb_sstep_pid) && --sstep_tries) {
                atomic_set(&kgdb_active, -1);
                raw_spin_unlock(&dbg_master_lock);
                dbg_touch_watchdogs();
                local_irq_restore(flags);

                goto acquirelock;
        }

        if (!kgdb_io_ready(1)) {
                kgdb_info[cpu].ret_state = 1;
                goto kgdb_restore; /* No I/O connection, resume the system */
        }

        /*
         * Don't enter if we have hit a removed breakpoint.
         */
        if (kgdb_skipexception(ks->ex_vector, ks->linux_regs))
                goto kgdb_restore;

        /* Call the I/O driver's pre_exception routine */
        if (dbg_io_ops->pre_exception)
                dbg_io_ops->pre_exception();

        /*
         * Get the passive CPU lock which will hold all the non-primary
         * CPU in a spin state while the debugger is active
         */
        if (!kgdb_single_step)
                raw_spin_lock(&dbg_slave_lock);

#ifdef CONFIG_SMP
        /* Signal the other CPUs to enter kgdb_wait() */
        if ((!kgdb_single_step) && kgdb_do_roundup)
                kgdb_roundup_cpus(flags);
#endif

        /*
         * Wait for the other CPUs to be notified and be waiting for us:
         */
        while (kgdb_do_roundup && (atomic_read(&masters_in_kgdb) +
                                atomic_read(&slaves_in_kgdb)) != online_cpus)
                cpu_relax();

        /*
         * At this point the primary processor is completely
         * in the debugger and all secondary CPUs are quiescent
         */
        dbg_deactivate_sw_breakpoints();
        kgdb_single_step = 0;
        kgdb_contthread = current;
        exception_level = 0;
        trace_on = tracing_is_on();
        if (trace_on)
                tracing_off();

        while (1) {
cpu_master_loop:
                if (dbg_kdb_mode) {
                        kgdb_connected = 1;
                        error = kdb_stub(ks);
                        if (error == -1)
                                continue;
                        kgdb_connected = 0;
                } else {
                        error = gdb_serial_stub(ks);
                }

                if (error == DBG_PASS_EVENT) {
                        dbg_kdb_mode = !dbg_kdb_mode;
                } else if (error == DBG_SWITCH_CPU_EVENT) {
                        kgdb_info[dbg_switch_cpu].exception_state |=
                                DCPU_NEXT_MASTER;
                        goto cpu_loop;
                } else {
                        kgdb_info[cpu].ret_state = error;
                        break;
                }
        }

        /* Call the I/O driver's post_exception routine */
        if (dbg_io_ops->post_exception)
                dbg_io_ops->post_exception();

        if (!kgdb_single_step) {
                raw_spin_unlock(&dbg_slave_lock);
                /* Wait till all the CPUs have quit from the debugger. */
                while (kgdb_do_roundup && atomic_read(&slaves_in_kgdb))
                        cpu_relax();
        }

kgdb_restore:
        if (atomic_read(&kgdb_cpu_doing_single_step) != -1) {
                int sstep_cpu = atomic_read(&kgdb_cpu_doing_single_step);
                if (kgdb_info[sstep_cpu].task)
                        kgdb_sstep_pid = kgdb_info[sstep_cpu].task->pid;
                else
                        kgdb_sstep_pid = 0;
        }
        if (arch_kgdb_ops.correct_hw_break)
                arch_kgdb_ops.correct_hw_break();
        if (trace_on)
                tracing_on();

        kgdb_info[cpu].exception_state &=
                ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
        kgdb_info[cpu].enter_kgdb--;
        smp_mb__before_atomic_dec();
        atomic_dec(&masters_in_kgdb);
        /* Free kgdb_active */
        atomic_set(&kgdb_active, -1);
        raw_spin_unlock(&dbg_master_lock);
        dbg_touch_watchdogs();
        local_irq_restore(flags);

        return kgdb_info[cpu].ret_state;
}

/*
 * kgdb_handle_exception() - main entry point from a kernel exception
 *
 * Locking hierarchy:
 *      interface locks, if any (begin_session)
 *      kgdb lock (kgdb_active)
 */
int
kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs)
{
        struct kgdb_state kgdb_var;
        struct kgdb_state *ks = &kgdb_var;

        ks->cpu                 = raw_smp_processor_id();
        ks->ex_vector           = evector;
        ks->signo               = signo;
        ks->err_code            = ecode;
        ks->kgdb_usethreadid    = 0;
        ks->linux_regs          = regs;

        if (kgdb_reenter_check(ks))
                return 0; /* Ouch, double exception ! */
        if (kgdb_info[ks->cpu].enter_kgdb != 0)
                return 0;

        return kgdb_cpu_enter(ks, regs, DCPU_WANT_MASTER);
}

int kgdb_nmicallback(int cpu, void *regs)
{
#ifdef CONFIG_SMP
        struct kgdb_state kgdb_var;
        struct kgdb_state *ks = &kgdb_var;

        memset(ks, 0, sizeof(struct kgdb_state));
        ks->cpu                 = cpu;
        ks->linux_regs          = regs;

        if (kgdb_info[ks->cpu].enter_kgdb == 0 &&
                        raw_spin_is_locked(&dbg_master_lock)) {
                kgdb_cpu_enter(ks, regs, DCPU_IS_SLAVE);
                return 0;
        }
#endif
        return 1;
}

static void kgdb_console_write(struct console *co, const char *s,
   unsigned count)
{
        unsigned long flags;

        /* If we're debugging, or KGDB has not connected, don't try
         * and print. */
        if (!kgdb_connected || atomic_read(&kgdb_active) != -1 || dbg_kdb_mode)
                return;

        local_irq_save(flags);
        gdbstub_msg_write(s, count);
        local_irq_restore(flags);
}

static struct console kgdbcons = {
        .name           = "kgdb",
        .write          = kgdb_console_write,
        .flags          = CON_PRINTBUFFER | CON_ENABLED,
        .index          = -1,
};

#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_dbg(int key)
{
        if (!dbg_io_ops) {
                printk(KERN_CRIT "ERROR: No KGDB I/O module available\n");
                return;
        }
        if (!kgdb_connected) {
#ifdef CONFIG_KGDB_KDB
                if (!dbg_kdb_mode)
                        printk(KERN_CRIT "KGDB or $3#33 for KDB\n");
#else
                printk(KERN_CRIT "Entering KGDB\n");
#endif
        }

        kgdb_breakpoint();
}

static struct sysrq_key_op sysrq_dbg_op = {
        .handler        = sysrq_handle_dbg,
        .help_msg       = "debug(G)",
        .action_msg     = "DEBUG",
};
#endif

static int kgdb_panic_event(struct notifier_block *self,
                            unsigned long val,
                            void *data)
{
        if (dbg_kdb_mode)
                kdb_printf("PANIC: %s\n", (char *)data);
        kgdb_breakpoint();
        return NOTIFY_DONE;
}

static struct notifier_block kgdb_panic_event_nb = {
       .notifier_call   = kgdb_panic_event,
       .priority        = INT_MAX,
};

void __weak kgdb_arch_late(void)
{
}

void __init dbg_late_init(void)
{
        dbg_is_early = false;
        if (kgdb_io_module_registered)
                kgdb_arch_late();
        kdb_init(KDB_INIT_FULL);
}

static int
dbg_notify_reboot(struct notifier_block *this, unsigned long code, void *x)
{
        /*
         * Take the following action on reboot notify depending on value:
         *    1 == Enter debugger
         *    0 == [the default] detatch debug client
         *   -1 == Do nothing... and use this until the board resets
         */
        switch (kgdbreboot) {
        case 1:
                kgdb_breakpoint();
        case -1:
                goto done;
        }
        if (!dbg_kdb_mode)
                gdbstub_exit(code);
done:
        return NOTIFY_DONE;
}

static struct notifier_block dbg_reboot_notifier = {
        .notifier_call          = dbg_notify_reboot,
        .next                   = NULL,
        .priority               = INT_MAX,
};

static void kgdb_register_callbacks(void)
{
        if (!kgdb_io_module_registered) {
                kgdb_io_module_registered = 1;
                kgdb_arch_init();
                if (!dbg_is_early)
                        kgdb_arch_late();
                register_reboot_notifier(&dbg_reboot_notifier);
                atomic_notifier_chain_register(&panic_notifier_list,
                                               &kgdb_panic_event_nb);
#ifdef CONFIG_MAGIC_SYSRQ
                register_sysrq_key('g', &sysrq_dbg_op);
#endif
                if (kgdb_use_con && !kgdb_con_registered) {
                        register_console(&kgdbcons);
                        kgdb_con_registered = 1;
                }
        }
}

static void kgdb_unregister_callbacks(void)
{
        /*
         * When this routine is called KGDB should unregister from the
         * panic handler and clean up, making sure it is not handling any
         * break exceptions at the time.
         */
        if (kgdb_io_module_registered) {
                kgdb_io_module_registered = 0;
                unregister_reboot_notifier(&dbg_reboot_notifier);
                atomic_notifier_chain_unregister(&panic_notifier_list,
                                               &kgdb_panic_event_nb);
                kgdb_arch_exit();
#ifdef CONFIG_MAGIC_SYSRQ
                unregister_sysrq_key('g', &sysrq_dbg_op);
#endif
                if (kgdb_con_registered) {
                        unregister_console(&kgdbcons);
                        kgdb_con_registered = 0;
                }
        }
}

/*
 * There are times a tasklet needs to be used vs a compiled in
 * break point so as to cause an exception outside a kgdb I/O module,
 * such as is the case with kgdboe, where calling a breakpoint in the
 * I/O driver itself would be fatal.
 */
static void kgdb_tasklet_bpt(unsigned long ing)
{
        kgdb_breakpoint();
        atomic_set(&kgdb_break_tasklet_var, 0);
}

static DECLARE_TASKLET(kgdb_tasklet_breakpoint, kgdb_tasklet_bpt, 0);

void kgdb_schedule_breakpoint(void)
{
        if (atomic_read(&kgdb_break_tasklet_var) ||
                atomic_read(&kgdb_active) != -1 ||
                atomic_read(&kgdb_setting_breakpoint))
                return;
        atomic_inc(&kgdb_break_tasklet_var);
        tasklet_schedule(&kgdb_tasklet_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint);

static void kgdb_initial_breakpoint(void)
{
        kgdb_break_asap = 0;

        printk(KERN_CRIT "kgdb: Waiting for connection from remote gdb...\n");
        kgdb_breakpoint();
}

/**
 *      kgdb_register_io_module - register KGDB IO module
 *      @new_dbg_io_ops: the io ops vector
 *
 *      Register it with the KGDB core.
 */
int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops)
{
        int err;

        spin_lock(&kgdb_registration_lock);

        if (dbg_io_ops) {
                spin_unlock(&kgdb_registration_lock);

                printk(KERN_ERR "kgdb: Another I/O driver is already "
                                "registered with KGDB.\n");
                return -EBUSY;
        }

        if (new_dbg_io_ops->init) {
                err = new_dbg_io_ops->init();
                if (err) {
                        spin_unlock(&kgdb_registration_lock);
                        return err;
                }
        }

        dbg_io_ops = new_dbg_io_ops;

        spin_unlock(&kgdb_registration_lock);

        printk(KERN_INFO "kgdb: Registered I/O driver %s.\n",
               new_dbg_io_ops->name);

        /* Arm KGDB now. */
        kgdb_register_callbacks();

        if (kgdb_break_asap)
                kgdb_initial_breakpoint();

        return 0;
}
EXPORT_SYMBOL_GPL(kgdb_register_io_module);

/**
 *      kkgdb_unregister_io_module - unregister KGDB IO module
 *      @old_dbg_io_ops: the io ops vector
 *
 *      Unregister it with the KGDB core.
 */
void kgdb_unregister_io_module(struct kgdb_io *old_dbg_io_ops)
{
        BUG_ON(kgdb_connected);

        /*
         * KGDB is no longer able to communicate out, so
         * unregister our callbacks and reset state.
         */
        kgdb_unregister_callbacks();

        spin_lock(&kgdb_registration_lock);

        WARN_ON_ONCE(dbg_io_ops != old_dbg_io_ops);
        dbg_io_ops = NULL;

        spin_unlock(&kgdb_registration_lock);

        printk(KERN_INFO
                "kgdb: Unregistered I/O driver %s, debugger disabled.\n",
                old_dbg_io_ops->name);
}
EXPORT_SYMBOL_GPL(kgdb_unregister_io_module);

int dbg_io_get_char(void)
{
        int ret = dbg_io_ops->read_char();
        if (ret == NO_POLL_CHAR)
                return -1;
        if (!dbg_kdb_mode)
                return ret;
        if (ret == 127)
                return 8;
        return ret;
}

/**
 * kgdb_breakpoint - generate breakpoint exception
 *
 * This function will generate a breakpoint exception.  It is used at the
 * beginning of a program to sync up with a debugger and can be used
 * otherwise as a quick means to stop program execution and "break" into
 * the debugger.
 */
void kgdb_breakpoint(void)
{
        atomic_inc(&kgdb_setting_breakpoint);
        wmb(); /* Sync point before breakpoint */
        arch_kgdb_breakpoint();
        wmb(); /* Sync point after breakpoint */
        atomic_dec(&kgdb_setting_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_breakpoint);

static int __init opt_kgdb_wait(char *str)
{
        kgdb_break_asap = 1;

        kdb_init(KDB_INIT_EARLY);
        if (kgdb_io_module_registered)
                kgdb_initial_breakpoint();

        return 0;
}

early_param("kgdbwait", opt_kgdb_wait);