ucm: allow passing variables through ucm open string

It is useful to pass information like application capabilities
to the UCM configuration parser. Those variables are prefixed
with '@' for the configuration files.

An example:

   "<<<v1='a b c',x=12>>>hw:1"

Variables can substituted in the configuration:

   "${var:@v1}" -> "a b c"
   "${var:@x}" -> 12

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

src/ucm/main.c

@@ -1356,6 +1356,57 @@ static int set_device(snd_use_case_mgr_t *uc_mgr,
 	return err;
 }
 
+/**
+ * \brief Parse open arguments
+ * \param uc_mgr Use case manager
+ * \param name name of card to open
+ * \return the rest of the card name to open
+ */
+const char *parse_open_variables(snd_use_case_mgr_t *uc_mgr, const char *name)
+{
+	const char *end, *id;
+	char *args, *var;
+	snd_config_t *cfg, *n;
+	snd_config_iterator_t i, next;
+	char vname[128];
+	size_t l;
+	int err;
+
+	end = strstr(name, ">>>");
+	if (end == NULL)
+		return name;
+	l = end - name - 3;
+	args = alloca(l + 1);
+	strncpy(args, name + 3, l);
+	args[l] = '\0';
+
+	err = snd_config_load_string(&cfg, args, 0);
+	if (err < 0) {
+		uc_error("error: open arguments are not valid (%s)", args);
+		goto skip;
+	}
+
+	/* set arguments */
+	snd_config_for_each(i, next, cfg) {
+		n = snd_config_iterator_entry(i);
+		err = snd_config_get_id(n, &id);
+		if (err < 0)
+			goto skip;
+		err = snd_config_get_ascii(n, &var);
+		if (err < 0)
+			goto skip;
+		snprintf(vname, sizeof(vname), "@%s", id);
+		err = uc_mgr_set_variable(uc_mgr, vname, var);
+		free(var);
+		if (err < 0)
+			goto skip;
+	}
+
+	snd_config_delete(cfg);
+skip:
+	return end + 3;
+}
+
 /**
  * \brief Init sound card use case manager.
  * \param uc_mgr Returned use case manager pointer
@@ -1388,6 +1439,9 @@ int snd_use_case_mgr_open(snd_use_case_mgr_t **uc_mgr,
 		mgr->suppress_nodev_errors = 1;
 	}
 
+	if (card_name[0] == '<' && card_name[1] == '<' && card_name[2] == '<')
+		card_name = parse_open_variables(mgr, card_name);
+
 	err = uc_mgr_card_open(mgr);
 	if (err < 0) {
 		uc_mgr_free(mgr);

src/ucm/parser.c

@@ -278,6 +278,10 @@ static int evaluate_regex(snd_use_case_mgr_t *uc_mgr,
 		err = snd_config_get_id(n, &id);
 		if (err < 0)
 			return err;
+		if (id[0] == '@') {
+			uc_error("error: value names starting with '@' are reserved for application variables");
+			return -EINVAL;
+		}
 		err = uc_mgr_define_regex(uc_mgr, id, n);
 		if (err < 0)
 			return err;
@@ -327,6 +331,11 @@ static int evaluate_define(snd_use_case_mgr_t *uc_mgr,
 		free(var);
 		if (err < 0)
 			return err;
+		if (id[0] == '@') {
+			free(s);
+			uc_error("error: value names starting with '@' are reserved for application variables");
+			return -EINVAL;
+		}
 		err = uc_mgr_set_variable(uc_mgr, id, s);
 		free(s);
 		if (err < 0)