Bug in split transactions on Raspberry Pi

[Alan Stern]

On Wed, 27 Jan 2016, Doug Anderson wrote:

> Alan,
> 
> On Wed, Jan 27, 2016 at 1:34 PM, Alan Stern <stern at rowland.harvard.edu> wrote:
> > On Wed, 27 Jan 2016, Doug Anderson wrote:
> >
> >> This patch should fix ya.
> >>
> >> FIXUP: FROMLIST: usb: dwc2: host: Manage frame nums better in scheduler
> >> https://chromium-review.googlesource.com/324185
> >
> > Hmmm.  That fixed the problem of the polls occuring too frequently, but
> > now I see again intervals that are larger than 256 ms.  In the most
> > recent test there are two intervals of 512 ms and one of 2048 ms.
> 
> OK, good to know.  Ugh.  I'll have to see if I can reproduce that.  If
> I had to guess, though, I'd say that you're probably running into high
> interrupt latency problems.

Quite possibly.  Would that delay the transfers by a full period or 
only by one frame?

>  Those problems would be worse on the
> Raspberry Pi than on my system due to the significantly slower
> processor.
> 
> Can you confirm that these problems also were introduced by my series?
>  AKA: you never saw > 256 ms polls before my series and now you see
> them?

No, these problems were also present in the kernel without your 
patches.

> Turning on scheduler tracing would probably be quite helpful at this
> point.  You can see
> <https://chromium-review.googlesource.com/#/c/314254/>.  If printing
> even non-verbose traces to the console is too much, you could make
> everything "trace_printk".  You might also want
> <https://chromium-review.googlesource.com/#/c/323308/>.

Later...

Alan Stern