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

[Alon Levy <alevy@redhat.com>]

On Wed, Nov 10, 2010 at 04:49:38PM +0100, Markus Armbruster wrote:
> 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?
So the problems with the current commands from my pov:
 - device deletion removes associated chardev
  - no way to do it without removing chardev
  - no way to add chardev later and use it for device add
The outcome of which is that you can't do a guest wise attach/detach
from monitor if your device relies on a chardev association. This
happens with my passthrough ccid device.

> 
> Questions:
> 
> 1. If we add commands so that you can always recreate the configuration,
>    is detach+attach still useful?  Why?
If you make it so you can do a device_del and not remove the chardev, and
later device_add using the already existing chardev, then that will be
equivalent for me.

> 
> 2. Why is this a USB problem, and not a general problem?  In other
>    words, why usb_{detach,attach}, and not device_{detach,attach}?
I guess attach/detach is a don't-free-some-resources del/add. If you
think there are users for a device_attach/detach and it makes sense
conceptually (what's a detach/attach for an ide bus? for a pci it's
pretty clear, for sata, etc.) then you could blow this up to a device
specific callback or something like that (assuming that's how you
would implement this).