On Tue, Apr 21, 2020 at 02:14:41PM -0700, Douglas Anderson wrote: > If you build CONFIG_KGDB_SERIAL_CONSOLE into the kernel then you > should be able to have KGDB init itself at bootup by specifying the > "kgdboc=..." kernel command line parameter. This has worked OK for me > for many years, but on a new device I switched to it stopped working. > > The problem is that on this new device the serial driver gets its > probe deferred. Now when kgdb initializes it can't find the tty > driver and when it gives up it never tries again. > > We could try to find ways to move up the initialization of the serial > driver and such a thing might be worthwhile, but it's nice to be > robust against serial drivers that load late. We could move kgdb to > init itself later but that penalizes our ability to debug early boot > code on systems where the driver inits early. We could roll our own > system of detecting when new tty drivers get loaded and then use that > to figure out when kgdb can init, but that's ugly. > > Instead, let's jump on the -EPROBE_DEFER bandwagon. We'll create a > singleton instance of a "kgdboc" platform device. If we can't find > our tty device when the singleton "kgdboc" probes we'll return > -EPROBE_DEFER which means that the system will call us back later to > try again when the tty device might be there. > > We won't fully transition all of the kgdboc to a platform device > because early kgdb initialization (via the "ekgdboc" kernel command > line parameter) still runs before the platform device has been > created. The kgdb platform device is merely used as a convenient way > to hook into the system's normal probe deferral mechanisms. > > As part of this, we'll ever-so-slightly change how the "kgdboc=..." > kernel command line parameter works. Previously if you booted up and > kgdb couldn't find the tty driver then later reading > '/sys/module/kgdboc/parameters/kgdboc' would return a blank string. > Now kgdb will keep track of the string that came as part of the > command line and give it back to you. It's expected that this should > be an OK change. > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> Reviewed-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
