[PATCH v9 10/19] docs/system/devices/usb: Note the automatic hub change

[Jamin Lin <jamin_lin@aspeedtech.com>]

The automatic hub is now off for a new machine type. Say how to get a
hub back: plug in a usb-hub, or turn auto-hub back on for the
controller.

TODO: The text says 12.0. That is right once the compat entries move
to hw_compat_11_2.

Suggested-by: Junjie Cao <junjie.cao@linux.dev>
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
---
 docs/system/devices/usb.rst | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/docs/system/devices/usb.rst b/docs/system/devices/usb.rst
index dc694d23c2..dc73c4048e 100644
--- a/docs/system/devices/usb.rst
+++ b/docs/system/devices/usb.rst
@@ -3,8 +3,21 @@ USB emulation
 
 QEMU can emulate a PCI UHCI, OHCI, EHCI or XHCI USB controller. You can
 plug virtual USB devices or real host USB devices (only works with
-certain host operating systems). QEMU will automatically create and
-connect virtual USB hubs as necessary to connect multiple USB devices.
+certain host operating systems).
+
+A machine type older than 12.0 plugs a virtual USB hub into the last
+free port, so that more devices fit. A newer one does not, and a
+device that finds no free port fails.
+
+There are two ways to get a hub back. One is to add a hub yourself.
+Put it on the command line before the other USB devices::
+
+    -device usb-hub
+
+The other is to turn the automatic hub back on. Use the name of your
+USB controller, for example::
+
+    -global piix3-usb-uhci.auto-hub=on
 
 USB controllers
 ~~~~~~~~~~~~~~~
-- 
2.43.0