[PATCH v2] ui: Print available display backends with '-display help'

[Thomas Huth]

We already print availabled devices with "-device help", or available
backends with "-netdev help" or "-chardev help". Let's provide a way
for the users to query the available display backends, too.

Signed-off-by: Thomas Huth <address@hidden>
---
 v2: Now it should work with modules, too

 include/ui/console.h |  1 +
 ui/console.c         | 15 +++++++++++++++
 vl.c                 |  5 +++++
 3 files changed, 21 insertions(+)

diff --git a/include/ui/console.h b/include/ui/console.h
index f981696848..b7e1a8e6c0 100644
--- a/include/ui/console.h
+++ b/include/ui/console.h
@@ -440,6 +440,7 @@ void qemu_display_register(QemuDisplay *ui);
 bool qemu_display_find_default(DisplayOptions *opts);
 void qemu_display_early_init(DisplayOptions *opts);
 void qemu_display_init(DisplayState *ds, DisplayOptions *opts);
+void qemu_display_help(void);
 
 /* vnc.c */
 void vnc_display_init(const char *id, Error **errp);
diff --git a/ui/console.c b/ui/console.c
index 82d1ddac9c..9556b5d8dd 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -2330,6 +2330,21 @@ void qemu_display_init(DisplayState *ds, DisplayOptions 
*opts)
     dpys[opts->type]->init(ds, opts);
 }
 
+void qemu_display_help(void)
+{
+    int idx;
+
+    printf("Available display backend types:\n");
+    for (idx = DISPLAY_TYPE_NONE; idx < DISPLAY_TYPE__MAX; idx++) {
+        if (!dpys[idx]) {
+            ui_module_load_one(DisplayType_str(idx));
+        }
+        if (dpys[idx]) {
+            printf("%s\n",  DisplayType_str(dpys[idx]->type));
+        }
+    }
+}
+
 void qemu_chr_parse_vc(QemuOpts *opts, ChardevBackend *backend, Error **errp)
 {
     int val;
diff --git a/vl.c b/vl.c
index 86474a55c9..ee15055ba2 100644
--- a/vl.c
+++ b/vl.c
@@ -1869,6 +1869,11 @@ static void parse_display(const char *p)
 {
     const char *opts;
 
+    if (is_help_option(p)) {
+        qemu_display_help();
+        exit(0);
+    }
+
     if (strstart(p, "sdl", &opts)) {
         /*
          * sdl DisplayType needs hand-crafted parser instead of
-- 
2.21.0