Re: [linux-usb-devel] [PATCH] fix some hubs and hid devices at startup

[David Brownell <david-b@pacbell.net>]

> > It doesn't make sense to me that if more delays are added in
> > the kernel code, userland needs to see even more.
> 
> I agree, but Olaf and others prove that this works.

There was that hypothesis that it's really control traffic
interference ... I notice that usb_control_msg() chose
against the "keep the setup data in struct usb_device
and lock it with a semaphore" strategy (it kmallocs
the setup buffer), so I could see how that'd happen.

"usbmodules" asks the device for its config descriptor,
rather than reading /proc/bus/usb/BBB/DDD, so HCDs
that don't handle control queuing (I think OHCI behaves
there, and EHCI, but not the UHCIs) might suffer from
concurrent control messages from it and from HID ...

I think the best that can be done for now would be a
hack:  add a lock in usb_device that must be held
by whoever submits a control request, and released
when that request completes.  The non-hack solution
is to support control queuing in all HCDs.

What kind of lock?  Maybe that binary semaphore.
Code that needs to issue control requests in_interrupt()
would need to be modified, as would usb_control_msg().


> I'm more puzzled why usbmodules is needing to talk to a device.  I
> thought usbmodules was only for cold-plug stuff.  Have any ideas?

Usbmodules is used to cope with /sbin/hotplug not providing
a complete interface descriptor dump:  it only shows the first
of N inferface descriptors.  With coldplug, it doesn't run, and
doesn't provide any descriptors at all -- so it must be used.

However, even for the hotplug case "usbmodules" gives a
more complete module list.  In the case of an audio device
with a sound/ISO interface and a control/HID interface, it'll
pick up the HID.

- Dave



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