From mboxrd@z Thu Jan 1 00:00:00 1970 From: anton.vorontsov@linaro.org (Anton Vorontsov) Date: Mon, 20 Aug 2012 14:24:51 -0700 Subject: [PATCH v4 0/12] KGDB/KDB FIQ (NMI) debugger In-Reply-To: References: <20120820144449.GA32560@lizard> Message-ID: <20120820212451.GA15823@lizard> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Brian, On Mon, Aug 20, 2012 at 01:51:33PM -0700, Brian Swetland wrote: > > - 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.