Move away logic which attaches frontend device to a mux
from `char-fe.c` to actual `char-mux.c` implementation
and make it a separate function.

No logic changes are made.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
---
 chardev/char-fe.c          |  9 +--------
 chardev/char-mux.c         | 17 +++++++++++++++++
 chardev/chardev-internal.h |  2 ++
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/chardev/char-fe.c b/chardev/char-fe.c
index 69b47d16bdfa..3b8771ca2ac4 100644
--- a/chardev/char-fe.c
+++ b/chardev/char-fe.c
@@ -197,16 +197,9 @@ bool qemu_chr_fe_init(CharBackend *b, Chardev *s, Error **errp)
         if (CHARDEV_IS_MUX(s)) {
             MuxChardev *d = MUX_CHARDEV(s);
 
-            if (d->mux_cnt >= MAX_MUX) {
-                error_setg(errp,
-                           "too many uses of multiplexed chardev '%s'"
-                           " (maximum is " stringify(MAX_MUX) ")",
-                           s->label);
+            if (!mux_chr_attach_frontend(d, b, &tag, errp)) {
                 return false;
             }
-
-            d->backends[d->mux_cnt] = b;
-            tag = d->mux_cnt++;
         } else if (s->be) {
             error_setg(errp, "chardev '%s' is already in use", s->label);
             return false;
diff --git a/chardev/char-mux.c b/chardev/char-mux.c
index b2d7abf2fc01..9294f955462e 100644
--- a/chardev/char-mux.c
+++ b/chardev/char-mux.c
@@ -301,6 +301,23 @@ static void mux_chr_update_read_handlers(Chardev *chr)
                                   chr->gcontext, true, false);
 }
 
+bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
+                             unsigned int *tag, Error **errp)
+{
+    if (d->mux_cnt >= MAX_MUX) {
+        error_setg(errp,
+                   "too many uses of multiplexed chardev '%s'"
+                   " (maximum is " stringify(MAX_MUX) ")",
+                   d->parent.label);
+        return false;
+    }
+
+    d->backends[d->mux_cnt] = b;
+    *tag = d->mux_cnt++;
+
+    return true;
+}
+
 void mux_set_focus(Chardev *chr, unsigned int focus)
 {
     MuxChardev *d = MUX_CHARDEV(chr);
diff --git a/chardev/chardev-internal.h b/chardev/chardev-internal.h
index ab93f6ea1720..8126ce180690 100644
--- a/chardev/chardev-internal.h
+++ b/chardev/chardev-internal.h
@@ -59,6 +59,8 @@ DECLARE_INSTANCE_CHECKER(MuxChardev, MUX_CHARDEV,
 #define CHARDEV_IS_MUX(chr)                             \
     object_dynamic_cast(OBJECT(chr), TYPE_CHARDEV_MUX)
 
+bool mux_chr_attach_frontend(MuxChardev *d, CharBackend *b,
+                             unsigned int *tag, Error **errp);
 void mux_set_focus(Chardev *chr, unsigned int focus);
 void mux_chr_send_all_event(Chardev *chr, QEMUChrEvent event);
 
-- 
2.34.1