tracing: Add 'last error' error facility for hist triggers
With the addition of variables and actions, it's become necessary to provide more detailed error information to users about syntax errors. Add a 'last error' facility accessible via the erroring event's 'hist' file. Reading the hist file after an error will display more detailed information about what went wrong, if information is available. This extended error information will be available until the next hist trigger command for that event. # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger echo: write error: Invalid argument # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist ERROR: Couldn't yyy: zzz Last command: xxx Also add specific error messages for variable and action errors. Link: http://lkml.kernel.org/r/64e9c422fc8aeafcc2f7a3b4328c0cffe7969129.1516069914.git.tom.zanussi@linux.intel.com Signed-off-by:Tom Zanussi <tom.zanussi@linux.intel.com> Signed-off-by: