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();