perf config: Add support for getting config key-value pairs

Add a functionality getting specific config key-value pairs.
For the syntax examples,

    perf config [<file-option>] [section.name ...]

e.g. To query config items 'report.queue-size' and 'report.children', do

    # perf config report.queue-size report.children

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Nambong Ha <over3025@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Wookje Kwon <aweee0@gmail.com>
Link: http://lkml.kernel.org/r/1478241862-31230-2-git-send-email-treeze.taeung@gmail.com
[ Combined patch with docs update with this one ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -8,6 +8,8 @@ perf-config - Get and set variables in a configuration file.
 SYNOPSIS
 --------
 [verse]
+'perf config' [<file-option>] [section.name ...]
+or
 'perf config' [<file-option>] -l | --list
 
 DESCRIPTION
@@ -118,6 +120,22 @@ Given a $HOME/.perfconfig like this:
 		children = true
 		group = true
 
+To query the record mode of call graph, do
+
+	% perf config call-graph.record-mode
+
+If you want to know multiple config key/value pairs, you can do like
+
+	% perf config report.queue-size call-graph.order report.children
+
+To query the config value of sort order of call graph in user config file (i.e. `~/.perfconfig`), do
+
+	% perf config --user call-graph.sort-order
+
+To query the config value of buildid directory in system config file (i.e. `$(sysconf)/perfconfig`), do
+
+	% perf config --system buildid.dir
+
 Variables
 ~~~~~~~~~
 

tools/perf/builtin-config.c

@@ -17,7 +17,7 @@
 static bool use_system_config, use_user_config;
 
 static const char * const config_usage[] = {
-	"perf config [<file-option>] [options]",
+	"perf config [<file-option>] [options] [section.name ...]",
 	NULL
 };
 
@@ -33,6 +33,36 @@ static struct option config_options[] = {
 	OPT_END()
 };
 
+static int show_spec_config(struct perf_config_set *set, const char *var)
+{
+	struct perf_config_section *section;
+	struct perf_config_item *item;
+
+	if (set == NULL)
+		return -1;
+
+	perf_config_items__for_each_entry(&set->sections, section) {
+		if (prefixcmp(var, section->name) != 0)
+			continue;
+
+		perf_config_items__for_each_entry(&section->items, item) {
+			const char *name = var + strlen(section->name) + 1;
+
+			if (strcmp(name, item->name) == 0) {
+				char *value = item->value;
+
+				if (value) {
+					printf("%s=%s\n", var, value);
+					return 0;
+				}
+			}
+
+		}
+	}
+
+	return 0;
+}
+
 static int show_config(struct perf_config_set *set)
 {
 	struct perf_config_section *section;
@@ -54,7 +84,7 @@ static int show_config(struct perf_config_set *set)
 
 int cmd_config(int argc, const char **argv, const char *prefix __maybe_unused)
 {
-	int ret = 0;
+	int i, ret = 0;
 	struct perf_config_set *set;
 	char *user_config = mkpath("%s/.perfconfig", getenv("HOME"));
 
@@ -100,7 +130,11 @@ int cmd_config(int argc, const char **argv, const char *prefix __maybe_unused)
 		}
 		break;
 	default:
-		usage_with_options(config_usage, config_options);
+		if (argc)
+			for (i = 0; argv[i]; i++)
+				ret = show_spec_config(set, argv[i]);
+		else
+			usage_with_options(config_usage, config_options);
 	}
 
 	perf_config_set__delete(set);