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    Subject[PATCH 08/11] ARM: kgdb_fiq: Implement knocking into KDB via escape command
    Date
    As Colin Cross noticed, serial ports could be noisy, so occasional
    characters once in a while are possible. So, considering the noise
    possibility, entering the debugger on any received byte is unacceptable
    for production devices.

    This changes KGDB FIQ behaviour in a such way so that we have to type the
    GDB-protocol "$3#33" command to actually enter the debugger, the kernel
    will print the following prompt:

    Type $3#33 to enter the debugger>

    This is the exactly the same command we use to escape from KGDB to KDB,
    so it should be all pretty familiar.

    For convenience, there is a kgdb_fiq.knock kernel command line option,
    when set to 0, this turns the special command to just a return key
    press, so the kernel will be printing this:

    Hit <return> to enter the debugger>

    And for the cases when NMI connected to a dedicated button, the knocking
    can be disabled altogether by setting kgdb_fiq.knock to -1.

    Suggested-by: Colin Cross <ccross@android.com>
    Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
    ---
    arch/arm/kernel/kgdb_fiq.c | 58 ++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 58 insertions(+)

    diff --git a/arch/arm/kernel/kgdb_fiq.c b/arch/arm/kernel/kgdb_fiq.c
    index 72a62c7..2bf4467 100644
    --- a/arch/arm/kernel/kgdb_fiq.c
    +++ b/arch/arm/kernel/kgdb_fiq.c
    @@ -18,6 +18,7 @@
    #include <linux/slab.h>
    #include <linux/errno.h>
    #include <linux/hardirq.h>
    +#include <linux/kdb.h>
    #include <linux/kgdb.h>
    #include <asm/fiq.h>
    #include <asm/exception.h>
    @@ -26,6 +27,60 @@ static int kgdb_fiq_enabled;
    module_param_named(enable, kgdb_fiq_enabled, int, 0600);
    MODULE_PARM_DESC(enable, "set to 1 to enable FIQ KGDB");

    +static int kgdb_fiq_knock = 1;
    +module_param_named(knock, kgdb_fiq_knock, int, 0600);
    +MODULE_PARM_DESC(knock, "if set to 1 (default), the special '$3#33' command "
    + "must be used to enter the debugger; when set to 0, "
    + "hitting return key is enough to enter the debugger; "
    + "when set to -1, the debugger is entered immediately "
    + "upon NMI");
    +
    +/*
    + * "Serial ports are often noisy, especially when muxed over another port (we
    + * often use serial over the headset connector). Noise on the async command
    + * line just causes characters that are ignored, on a command line that blocked
    + * execution noise would be catastrophic." -- Colin Cross
    + *
    + * So, this small function implements KGDB/KDB knocking on the serial line: we
    + * won't enter the debugger until we receive a known magic phrase (which is
    + * actually "$3#33", known as "escape to KDB" command. If knocking is disabled,
    + * just pressing the return key is enough to enter the debugger.
    + */
    +static bool kgdb_fiq_poll_knock(void)
    +{
    + static int n;
    + int c = -1;
    + get_char_func *getc;
    + char magic[] = "$3#33";
    + size_t m = strlen(magic);
    +
    + if (kgdb_fiq_knock < 0)
    + return 1;
    +
    + for (getc = &kdb_poll_funcs[0]; *getc; ++getc) {
    + c = (*getc)();
    + if (c >= 0)
    + break;
    + }
    +
    + if (!kgdb_fiq_knock && (c == '\r' || c == '\n')) {
    + return 1;
    + } else if (c == magic[n]) {
    + kdb_printf("%c", c);
    + n = (n + 1) % m;
    + if (!n)
    + return 1;
    + } else {
    + n = 0;
    + kdb_printf("\r%s %s to enter the debugger> %*s",
    + kgdb_fiq_knock ? "Type" : "Hit",
    + kgdb_fiq_knock ? magic : "<return>", m, "");
    + memset(magic, '\b', m);
    + kdb_printf("%s", magic);
    + }
    + return 0;
    +}
    +
    static unsigned int kgdb_fiq;
    static void (*kgdb_enable_fiq)(unsigned int irq, bool on);
    static bool (*is_kgdb_fiq)(unsigned int irq);
    @@ -36,6 +91,9 @@ asmlinkage void __exception_irq_entry kgdb_fiq_do_handle(struct pt_regs *regs)
    return;
    dbg_io_ops->clear_irqs();

    + if (!kgdb_fiq_poll_knock())
    + return;
    +
    nmi_enter();
    kgdb_handle_exception(1, 0, 0, regs);
    nmi_exit();
    --
    1.7.10.4


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