Re: false positive -ENOSPC in ohci-q

On Monday 26 February 2007 8:33 am, Andrea Arcangeli wrote:
> On Mon, Feb 26, 2007 at 08:07:59AM -0800, David Brownell wrote:
> > On Monday 26 February 2007 7:11 am, Andrea Arcangeli wrote:
> > > Hello,
> > > 
> > > until I applied the below patch I couldn't use the usb camera at the
> > > same time of the usb modem (both usb1 devices). The camera returned
> > > -ENOSPC. Now they both work fine at the same time (camera managed by
> > > zoneminder and running all the time and the modem giving me the
> > > internet link, both without any apparent problem).
> > 
> > NAK unless you can demonstrate a bug in the logic you disabled.
> 
> demonstrate a bug? I thought I already did.

No, you didn't demonstrate that the code implements the 90% limit
on periodic transfer bandwidth (from the USB spec) incorrectly.

What's puzzling is that the modem seems to require periodic transfers.
At all.

 
> Now if this is a hardware bug in the modem or the camera, I still
> would need a runtime tweak to disable that heuristic 

That's not a heuristic ... as I assume you know.  No guesswork
or nondeterministic behavior involved; just straightforward
implementation of a bog-standard USB requirement.  Which nobody
else has *EVER* complained about, note.


> that clearly 
> can choke in practice with real life usage (I can also imagine
> hardware advertising a peak value, but if run in certain modes, like
> with low res, generating lower max-values,

Then it would use a different configuration, different ISO endpoints,
different bandwidth reservation.


> so a runtime tweak to tell 
> the heuristic to go away still sounds useful even if this wouldn't be
> a hardware issue and if the heuristic would be legitimate at only 90%).
> 
> > Looks more like your config is one that's disallowed.
> 
> I tried all possibile config and tweaks, so I doubt you can blame on me.

Sure I can.  Does the camera work by itself?  Evidently.  How about
the modem?  Ditto.  The config that's disallowed would be both of
them working at the same time.  Bug == trying that config.

People normally stick usb 1.1 webcams on their own USB bus, so that
these kinds of problems can't show up.

- Dave

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