Re: [PATCH] perf tool: report user-friendly error from timechart

[David Ahern <dsahern@gmail.com>]

On 10/3/13 6:38 AM, Ingo Molnar wrote:
>
> * Ramkumar Ramachandra <artagnon@gmail.com> wrote:
>
>> +	/* Perform a quick sanity check */
>> +	if (!is_valid_tracepoint("power:cpu_frequency")) {
>> +		fprintf(stderr, "Error:\tNo permissions to read $debugfs/tracing/events/power/cpu_frequency\n");
>> +		fprintf(stderr, "Hint:\tChange the permissions of debugfs: /sys/kernel/debug\n");
>> +		fprintf(stderr, "\tThe directory will be present if your kernel was compiled with debugfs support.\n");
>
> Is missing permissions the only way how is_valid_tracepoint() can fail?
>
> What if debugfs has the right permissions but CONFIG_TRACEPOINTS is
> disabled in the kernel?

There are a number of reasons that function can fail. The complete 
solution is to plumb various error numbers and on failure request a 
string for that failure. Take a look at util/target.[ch] as an example.

The comment applies to the perf-trace patch as well, but it gets more 
complicated to handle the error paths from perf_evsel__newtp when they 
dip into the tracepoint code

David