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

[Greg KH <greg@kroah.com>]

On Tue, Mar 12, 2002 at 02:15:05PM -0800, David Brownell wrote:
> > > 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.

It's always been this way, right?

> "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().

Yes, this would be the best way.  I don't think it's a hack, as some
drivers (like the usb-storage one) have said that they really need to
lock the control pipe for a while to prevent things from happening out
of order (for very dumb devices.)

> 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.

Ah, thanks for explaining this, makes more sense now.

greg k-h

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