Thank you Peter and sorry for the delay !

You put me in the correct direction and the problem is now resolved :)

I wrongly assumed that attaching the device to the guest in "preboot mode" should be only made in the "serial" way because I supposed that we can not attach the device as a "hostdev" before the host really detect the device as a USB device listed by "lsusb" which can take a moment.

But simply attach with "hostdev" with the idVendor/idProduct given in "preboot mode" did the trick :

    echo    "<hostdev mode='subsystem' type='usb' managed='yes'> \
                <source> \
                    <vendor id='0x091e'/> \
                    <product id='0x0003'/> \
                </source> \
            </hostdev>" > serial_device.xml

From the host (here Debian 11) when I plug the GPS, I see this :

root@host:~# journalctl -kf
...
févr. 04 13:11:46 host kernel: usb 2-11: new high-speed USB device number 34 using xhci_hcd
févr. 04 13:11:46 host kernel: usb 2-11: New USB device found, idVendor=091e, idProduct=0003, bcdDevice= 0.01
févr. 04 13:11:46 host kernel: usb 2-11: New USB device strings: Mfr=0, Product=0, SerialNumber=0
févr. 04 13:11:46 host kernel: garmin_gps 2-11:1.0: Garmin GPS usb/tty converter detected
févr. 04 13:11:46 host kernel: usb 2-11: Garmin GPS usb/tty converter now attached to ttyUSB0
févr. 04 13:11:54 host kernel: usb 2-11: USB disconnect, device number 34
févr. 04 13:11:54 host kernel: garmin_gps ttyUSB0: Garmin GPS usb/tty converter now disconnected from ttyUSB0
févr. 04 13:11:54 host kernel: garmin_gps 2-11:1.0: device disconnected
...

From a guest (here Debian 11 too) when the script attach the GPS, I see this :

root@guest:~# journalctl -kf
...
févr. 04 13:25:58 guest kernel: usb 2-6: new high-speed USB device number 8 using ehci-pci
févr. 04 13:25:58 guest kernel: usb 2-6: New USB device found, idVendor=091e, idProduct=0003, bcdDevice= 0.01
févr. 04 13:25:58 guest kernel: usb 2-6: New USB device strings: Mfr=0, Product=0, SerialNumber=0
févr. 04 13:25:58 guest kernel: garmin_gps 2-6:1.0: Garmin GPS usb/tty converter detected
févr. 04 13:25:58 guest kernel: usb 2-6: Garmin GPS usb/tty converter now attached to ttyUSB0
févr. 04 13:26:03 guest kernel: usb 2-6: USB disconnect, device number 8
févr. 04 13:26:03 guest kernel: garmin_gps ttyUSB0: Garmin GPS usb/tty converter now disconnected from ttyUSB0
févr. 04 13:26:03 guest kernel: garmin_gps 2-6:1.0: device disconnected
...

So the device is correctly transferred, and it is confirmed as the "preboot tools" detect the device in a "Windows 7" guest

I want to add that, as far as I know we can not use the "virt-manager" GUI to do the same, for two reasons :
- the USB host device window do not refresh itself when the items are added or removed
- the delay to attach this device is pretty short (5 to 8 seconds) and just after, from the guest we must trigger the tools which can manage the device in "preboot mode" (typically a flash tool)

That is why the solution I found, is to automate this with a script which detects the kernel events and automatically attach the devices.


The host versions :
root@host:~# cat /etc/os-release | grep -E "^(NAME|VERSION)=" | tr '\n' ' ' ; uname -a ; kvm --version | grep version ; libvirtd --version ; virt-manager --version
NAME="Debian GNU/Linux" VERSION="11 (bullseye)"
Linux host 5.10.0-8-amd64 #1 SMP Debian 5.10.46-4 (2021-08-03) x86_64 GNU/Linux
QEMU emulator version 5.2.0 (Debian 1:5.2+dfsg-11)
libvirtd (libvirt) 7.0.0
3.2.0


As I want to post the complete method I used, I will certainly make a feedback in this thread later ;) 

Le 25/01/2023 à 15:24, Peter Krempa a écrit :
On Tue, Jan 24, 2023 at 00:10:46 +0100, lnj.sam@gmail.com wrote:

Hello everyone and best wishes for 2023 :)

I have an old *Garmin Drive Smart 50 GPS* and I want to be able to attach it
when it is in preboot mode to a VM hosted by a *Debian 11 host*.

>From what I understand, the preboot mode allows us to flash a firmware
before the GPS actually starts (used among other things when the GPS is soft
bricked).

When I connect the GPS to the host it gives:

[...]


My questions :

Q1 : Is it possible ?

It shpi;d be/


Q2 : Am I doing it the right way ?

I don't think so. You are using a serial device emulated which is then
connected to the host side USB-tty device.

A better way is simply to use USB-assignment/passthrough and have the
guest OS handle also the USB side


Q3 : Is there a method to pass such a device directly (pass-through) ?

To pass a USB device use the following XML: 

    <hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
        <vendor id='0x1a86'/>
        <product id='0x55d4'/>
      </source>
      <address type='usb' bus='0' port='4'/>
    </hostdev>

Obviously tweak vendor/product.

Alternatively when you are using virt-manager GUI there's a menu to pass
any device, or by default when you have a VM window active and plug in a
USB device it's passed to the VM. (It can be disabled though).