Re: [Libusb-devel] libusb_bulk_transfer timeout (Win7 x64)

[Alan S. <st...@ro...>]

On Tue, 17 Jan 2012, Johannes Stezenbach wrote:

> On Tue, Jan 17, 2012 at 11:42:45AM -0500, Alan Stern wrote:
> > On Tue, 17 Jan 2012, Johannes Stezenbach wrote:
> > 
> > > On Tue, Jan 17, 2012 at 10:42:53AM -0500, Alan Stern wrote:
> > > > On Tue, 17 Jan 2012, Johannes Stezenbach wrote:
> > > > > 
> > > > > Among other things, SET_CONFIGURATION resets the data toggle of
> > > > > the endpoint to DATA0.
> > > > 
> > > > Why would anybody want to do that?  Isn't it easier and simpler just to 
> > > > keep using the endpoint's current data toggle value?
> > > 
> > > USB spec section 9.1.1.5 "Configured" requires it.
> > 
> > You misunderstood.  I wasn't asking why you would want to put the 
> > device into the Configured state; I was asking why you would want to 
> > reset the endpoint's data toggle.
> 
> That's what I mean, 9.1.1.5 requires the data toggle reset
> when setting the configuration.

Sorry, I can't follow what you're saying at all.

Yes, 9.1.1.5 requires the device to reset its data toggles when it
receives a Set-Config request.  That doesn't explain why you would want
to send a Set-Config request in the first place, or why you would want 
to reset the data toggles.

> > Although, come to think of it, why _would_ you want to put the device 
> > into the Configured state?  The only reason I can think of would be if 
> > the operating system didn't already take care of that for you.
> 
> It doesn't matter who calls SET_CONFIGURATION, what matters is
> that HCD and device agree, otherwise the HCD will signal
> an error.  I mention it because I spent some time debugging
> FX2 firmware and this was one of the issues causing mysterious
> error returns.

How would the HCD and the device get out of agreement to begin with?  
If they agree all along, there's no reason to change the toggle 
setting.

> However, my device is self powered, if the device is bus powered
> and the configuration is set once after plug-in by the OS then you'll
> never see this issue.

Even with a self-powered device, how would the HCD and the device get
out of agreement on a toggle value?  A bug in the device firmware,
perhaps?  I have noticed that many firmwares seem to have trouble
handling Set-Interface requests.  They crash, or they forget to clear
the data toggles (in spite of the fact that 9.1.1.5 requires them to do
so).

Alan Stern