perf config: Add '--system' and '--user' options to select which config file is used

The '--system' option means $(sysconfdir)/perfconfig and '--user' means
$HOME/.perfconfig. If none is used, both system and user config file are
read. E.g.:

# perf config [<file-option>] [options]

With an specific config file:

# perf config --user | --system

or both user and system config file:

# perf config

Signed-off-by: Taeung Song <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-config.txt

···
 SYNOPSIS
 --------
 [verse]
-'perf config' -l | --list
+'perf config' [<file-option>] -l | --list
 
 DESCRIPTION
 -----------
···
 --list::
 	Show current config variables, name and value, for all sections.
 
+--user::
+	For writing and reading options: write to user
+	'$HOME/.perfconfig' file or read it.
+
+--system::
+	For writing and reading options: write to system-wide
+	'$(sysconfdir)/perfconfig' or read it.
+
 CONFIGURATION FILE
 ------------------
 
···
 The '$HOME/.perfconfig' file is used to store a per-user configuration.
 The file '$(sysconfdir)/perfconfig' can be used to
 store a system-wide default configuration.
+
+When reading or writing, the values are read from the system and user
+configuration files by default, and options '--system' and '--user'
+can be used to tell the command to read from or write to only that location.
 
 Syntax
 ~~~~~~

tools/perf/builtin-config.c

···
 #include "util/util.h"
 #include "util/debug.h"
 
+static bool use_system_config, use_user_config;
+
 static const char * const config_usage[] = {
-	"perf config [options]",
+	"perf config [<file-option>] [options]",
 	NULL
 };
 
···
 static struct option config_options[] = {
 	OPT_SET_UINT('l', "list", &actions,
 		     "show current config variables", ACTION_LIST),
+	OPT_BOOLEAN(0, "system", &use_system_config, "use system config file"),
+	OPT_BOOLEAN(0, "user", &use_user_config, "use user config file"),
 	OPT_END()
 };
 
···
 int cmd_config(int argc, const char **argv, const char *prefix __maybe_unused)
 {
 	int ret = 0;
+	char *user_config = mkpath("%s/.perfconfig", getenv("HOME"));
 
 	argc = parse_options(argc, argv, config_options, config_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
+
+	if (use_system_config && use_user_config) {
+		pr_err("Error: only one config file at a time\n");
+		parse_options_usage(config_usage, config_options, "user", 0);
+		parse_options_usage(NULL, config_options, "system", 0);
+		return -1;
+	}
+
+	if (use_system_config)
+		config_exclusive_filename = perf_etc_perfconfig();
+	else if (use_user_config)
+		config_exclusive_filename = user_config;
 
 	switch (actions) {
 	case ACTION_LIST:
···
 			parse_options_usage(config_usage, config_options, "l", 1);
 		} else {
 			ret = perf_config(show_config, NULL);
-			if (ret < 0)
+			if (ret < 0) {
+				const char * config_filename = config_exclusive_filename;
+				if (!config_exclusive_filename)
+					config_filename = user_config;
 				pr_err("Nothing configured, "
-				       "please check your ~/.perfconfig file\n");
+				       "please check your %s \n", config_filename);
+			}
 		}
 		break;
 	default:

tools/perf/util/cache.h

···
 #define PERF_TRACEFS_ENVIRONMENT "PERF_TRACEFS_DIR"
 #define PERF_PAGER_ENVIRONMENT "PERF_PAGER"
 
+extern const char *config_exclusive_filename;
+
 typedef int (*config_fn_t)(const char *, const char *, void *);
 extern int perf_default_config(const char *, const char *, void *);
 extern int perf_config(config_fn_t fn, void *);
···
 extern int perf_config_bool(const char *, const char *);
 extern int config_error_nonbool(const char *);
 extern const char *perf_config_dirname(const char *, const char *);
+extern const char *perf_etc_perfconfig(void);
 
 char *alias_lookup(const char *alias);
 int split_cmdline(char *cmdline, const char ***argv);

tools/perf/util/config.c

···
 static int config_linenr;
 static int config_file_eof;
 
-static const char *config_exclusive_filename;
+const char *config_exclusive_filename;
 
 static int get_next_char(void)
 {
···
 	return ret;
 }
 
-static const char *perf_etc_perfconfig(void)
+const char *perf_etc_perfconfig(void)
 {
 	static const char *system_wide;
 	if (!system_wide)