userport: allow plugins to set an error message on init

Author: catharinejm

src/user_pins.h

@@ -99,6 +99,10 @@ typedef struct user_port_t {
 // downgrade, if necessary.
 typedef struct __attribute__((aligned(sizeof(void *)))) user_port_init_args_t {
 	int api_version;
+
+	// Callback for setting an error message from the plugin
+	// Errors longer than 256 chars (including null) will be truncated
+	void (*set_error)(char const *);
 } user_port_init_args_t;
 
 // Populates the provided [user_port_t *]. If any of [read], [write] or [step] is NULL, it

src/via.c

@@ -347,6 +347,18 @@ static user_port_init_t user_port_init = NULL;
 
 static user_port_t user_port;
 
+static char last_user_port_error[256] = "OK";
+
+static void
+set_user_port_error(char const *err)
+{
+	if (err) {
+		strncpy(sizeof(last_user_port_error), last_user_port_error, err);
+	} else {
+		last_user_port_error = "OK";
+	}
+}
+
 void
 via2_init(char const *user_peripheral_plugin_path)
 {
@@ -371,7 +383,7 @@ via2_init(char const *user_peripheral_plugin_path)
 			.api_version = X16_USER_PORT_API_VERSION,
 		};
 		if (user_port_init(&init_args, &user_port) < 0) {
-			fprintf(stderr, "error initializing user peripheral\n");
+			fprintf(stderr, "error initializing user peripheral: %s\n", last_user_port_error);
 			error = true;
 		}
 		else if (user_port.api_version != X16_USER_PORT_API_VERSION) {