[PATCH v4 0/12] KGDB/KDB FIQ (NMI) debugger

Hi Brian,

On Mon, Aug 20, 2012 at 01:51:33PM -0700, Brian Swetland wrote:

quoted

- KGDB/KDB FIQ debugger shell is synchronous. In Google's version you
  could have a dedicated shell always running in the FIQ context,

[...]

The main reason we did this asynchronously was that it's entirely possible
to get the occasional random character on the debug serial port (which is
often multiplexed with the audio path on the headphone jack), and having
the device freeze mysteriously when this happens is problematic.

Since the FIQ debugger is incredibly useful for diagnosing "my device is
stuck" type problems, we tend to leave it enabled on large numbers of
devices during internal testing, so that if somebody runs into a problem
an engineer can plug in a serial debug cable and take a look.  It's
important that the presence of the debug feature doesn't lead to instability,
and thus we don't want a single random character to stop the normal
operation ofthe device.

Yup, and that's why in my approach I implemented a tiny async "shell" on
to of KDB, the shell accepts just one command "$3#33" -- GDB-protocol
escape sequence:

/**
 * kgdb_nmi_poll_knock - Check if it is time to enter the debugger
 *
 * "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 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.
 ...

I.e. the kernel will print this prompt on the NMI debugger console:

        Type $3#33 to enter the debugger>

And this command will be processed asynchronously.

Thanks!

Anton.