[PATCH v3] ui: Print available display backends with '-display help'

[Thomas Huth <thuth@redhat.com>]

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.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v3: Mention -display help in the qemu-doc, too (as suggested by Philippe)

 include/ui/console.h |  1 +
 qemu-options.hx      |  3 ++-
 ui/console.c         | 15 +++++++++++++++
 vl.c                 |  5 +++++
 4 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/ui/console.h b/include/ui/console.h
index 281f9c145b..f35b4fc082 100644
--- a/include/ui/console.h
+++ b/include/ui/console.h
@@ -442,6 +442,7 @@ 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/qemu-options.hx b/qemu-options.hx
index e9d6231438..d593931664 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1586,7 +1586,8 @@ STEXI
 @item -display @var{type}
 @findex -display
 Select type of display to use. This option is a replacement for the
-old style -sdl/-curses/... options. Valid values for @var{type} are
+old style -sdl/-curses/... options. Use @code{-display help} to list
+the available display types. Valid values for @var{type} are
 @table @option
 @item sdl
 Display video output via SDL (usually in a separate graphics
diff --git a/ui/console.c b/ui/console.c
index ac79d679f5..69339b028b 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -2333,6 +2333,21 @@ 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
-- 
2.18.1