chardev: Add 'help' option to print all available chardev backend types

Signed-off-by: NLin Ma <lma@suse.com>
Message-Id: <20160816171352.17021-1-lma@suse.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -39,6 +39,7 @@
 #include "io/channel-file.h"
 #include "io/channel-tls.h"
 #include "sysemu/replay.h"
+#include "qemu/help_option.h"
 
 #include <zlib.h>
 
@@ -3879,16 +3880,26 @@ CharDriverState *qemu_chr_new_from_opts(QemuOpts *opts,
     const char *id = qemu_opts_id(opts);
     char *bid = NULL;
 
-    if (id == NULL) {
-        error_setg(errp, "chardev: no id specified");
-        goto err;
-    }
-
     if (qemu_opt_get(opts, "backend") == NULL) {
         error_setg(errp, "chardev: \"%s\" missing backend",
                    qemu_opts_id(opts));
         goto err;
     }
+
+    if (is_help_option(qemu_opt_get(opts, "backend"))) {
+        fprintf(stderr, "Available chardev backend types:\n");
+        for (i = backends; i; i = i->next) {
+            cd = i->data;
+            fprintf(stderr, "%s\n", cd->name);
+        }
+        exit(!is_help_option(qemu_opt_get(opts, "backend")));
+    }
+
+    if (id == NULL) {
+        error_setg(errp, "chardev: no id specified");
+        goto err;
+    }
+
     for (i = backends; i; i = i->next) {
         cd = i->data;
 

qemu-options.hx

@@ -2152,6 +2152,7 @@ The general form of a character device option is:
 ETEXI
 
 DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
+    "-chardev help\n"
     "-chardev null,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
     "-chardev socket,id=id[,host=host],port=port[,to=to][,ipv4][,ipv6][,nodelay][,reconnect=seconds]\n"
     "         [,server][,nowait][,telnet][,reconnect=seconds][,mux=on|off]\n"
@@ -2217,6 +2218,8 @@ Backend is one of:
 @option{spiceport}.
 The specific backend will determine the applicable options.
 
+Use "-chardev help" to print all available chardev backend types.
+
 All devices must have an id, which can be any string up to 127 characters long.
 It is used to uniquely identify this device in other command line directives.