On 20/08/2020 09:15, Gerd Hoffmann wrote: >> With auto-attach disabled this effectively prevents the device from >> attaching to the USB unless the chardev is opened which feels odd. > > Why? It's not needed to connect at boot btw, the device will be > hotplugged if the chardev is opened.
Yes, I see that - however... >> I would expect that if I add a device to QEMU using -device then it is >> immediately visible in the guest, and if the chardev isn't connected >> then the device should report its status as disconnected as you would >> expect with a real USB to RS232 adapter. > > Huh? How would that work? You can do some guesswork using modem line > status etc, but as far I know _reliable_ connected/disconnected > reporting simply doesn't exist in the world of serial lines ... I'm just comparing the behaviour with a real FTDI adapter: if I plug it in, it's always visible on the guest USB regardless as to whether there is a cable attached - the line status is returned as it is seen on the wire. Whether the guest OS has a concept as to whether a cable is attached is something that is internal to the driver, and completely separate from QEMU's notion as to whether a chardev is attached. For someone who has a reasonable amount of QEMU development experience I was just surprised that I ended up having to going through the source code to understand why my usb-serial device wasn't appearing in the guest. Certainly there is no mention of this requirement in the QEMU documentation. Something else I noticed was that this behaviour is unique to USB: in-built serial ports behave exactly as I described above which matches the real FTDI USB hardware. And another question: if this behaviour is acceptable for USB, should it not be the same for other hotplug buses, e.g. should a PCIe serial card plugged into a PCIe bus plug and unplug itself as the chardev is attached/detached in a similar way? ATB, Mark.
