Re: [Qemu-devel] [PATCH 0/3] add usb_detach and usb_attach (v3)

[Markus Armbruster <armbru@redhat.com>]

Sorry for coming so late to this thread...

Alon Levy <alevy@redhat.com> writes:

> On Thu, Oct 21, 2010 at 08:13:19AM -0500, Anthony Liguori wrote:
>> On 10/21/2010 08:03 AM, Gerd Hoffmann wrote:
>> >On 10/21/10 08:36, Alon Levy wrote:
>> >>v2->v3 changes:
>> >>  * add configure parameter
>> >>  * fix docs
>> >>
>> >>v2 message:
>> >>This patchset uses id like device_del for attaching/detaching usb
>> >>devices. The first two patches ready the way:
>> >>  1. makes qdev_find_recursive non static and in qdev.h
>> >>  2. adds a usb_device_by_id which goes over the usb buses calling
>> >>   qdev_find_recursive
>> >>  3. adds the commands that use usb_device_by_id
>> >>
>> >>Alon Levy (3):
>> >>   qdev: make qdev_find_recursive public
>> >>   usb: add public usb_device_by_id
>> >>   monitor: add usb_attach and usb_detach (v2)
>> >>
>> >
>> >Acked-by: Gerd Hoffmann <kraxel@redhat.com>
>> 
>> Okay, I am still confused about the use-case for this and I don't
>> see any further explanation in the commit messages.  I've seen
>> "debugging" but can you be a bit more specific about which cases
>> it's needed for?
>> 
>
> I use it for debugging the usb-ccid device. I think it's useful for
> any other usb device tests as well. The existing commands are not
> good enough to do a remove/insert of a usb device, since deleting
> a device also deletes any chardev associated with it, and there is
> no monitor command to add a chardev. Also sometimes you don't want
> to close the chardev, just have the guest see a removal/reinsert of
> the device.
[...]

Let's see whether I get you: detach removes the device, but doesn't
destroy it.  The only thing you can do with a detached device is attach
it.  Detach+attach is basically the same as del+add with the same
configuration.  Except shortcomings in our command set make it
impossible to recreate the configuration sometimes.  Correct?

Questions:

1. If we add commands so that you can always recreate the configuration,
   is detach+attach still useful?  Why?

2. Why is this a USB problem, and not a general problem?  In other
   words, why usb_{detach,attach}, and not device_{detach,attach}?