hw/usb: Do not build USB subsystem if not required

If the Kconfig 'USB' value is not selected, it is pointless to
build the USB core components. Add a stub for the HMP commands
and usbdevice_create() which is called by usb_device_add in
softmmu/vl.c.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210424224110.3442424-3-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

MAINTAINERS

@@ -1804,6 +1804,7 @@ USB
 M: Gerd Hoffmann <kraxel@redhat.com>
 S: Maintained
 F: hw/usb/*
+F: stubs/usb-dev-stub.c
 F: tests/qtest/usb-*-test.c
 F: docs/usb2.txt
 F: docs/usb-storage.txt

hw/usb/meson.build

@@ -1,17 +1,14 @@
 hw_usb_modules = {}
 
 # usb subsystem core
-softmmu_ss.add(files(
+softmmu_ss.add(when: 'CONFIG_USB', if_true: files(
   'bus.c',
   'combined-packet.c',
   'core.c',
-  'pcap.c',
-  'libhw.c'
-))
-
-softmmu_ss.add(when: 'CONFIG_USB', if_true: files(
   'desc.c',
   'desc-msos.c',
+  'libhw.c',
+  'pcap.c',
 ))
 
 # usb host adapters

stubs/meson.build

@@ -50,6 +50,7 @@ if have_block
 endif
 if have_system
   stub_ss.add(files('semihost.c'))
+  stub_ss.add(files('usb-dev-stub.c'))
   stub_ss.add(files('xen-hw-stub.c'))
 else
   stub_ss.add(files('qdev.c'))

stubs/usb-dev-stub.c

@@ -0,0 +1,25 @@
+/*
+ * QEMU USB device emulation stubs
+ *
+ * Copyright (C) 2021 Philippe Mathieu-Daudé <f4bug@amsat.org>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/error-report.h"
+#include "sysemu/sysemu.h"
+#include "monitor/monitor.h"
+#include "hw/usb.h"
+
+USBDevice *usbdevice_create(const char *driver)
+{
+    error_report("Support for USB devices not built-in");
+
+    return NULL;
+}
+
+void hmp_info_usb(Monitor *mon, const QDict *qdict)
+{
+    monitor_printf(mon, "Support for USB devices not built-in\n");
+}