> On Aug 12, 2015, at 11:53 AM, Siva Velusamy <siva.velus...@gmail.com> wrote:
>
> Hello!
>
> On OS X 10.10.4, we see that ReadPipe returns a stall, but when
> looking at the USB packets captured (via Total Phase Beagle), there
> are no stalls.
>
> Are there scenarios where this is possible? And is there any info on
> how to recover from such a stall?
>
> For reference, the code does something like this:
>
> result = (*handle->interface)->ReadPipe(handle->interface,
> handle->bulkIn, buf, &numBytes);
> if (kIOUSBPipeStalled == result) {
> DBG(" Pipe stalled, clearing stall.\n");
>
> and that print statement is hit. More application specific information
> and the USB capture are attached to
> https://code.google.com/p/android/issues/detail?id=182151, comment #8
> if anyone is interested.
What’s the actual error reported by USB?
STALL has two meanings. One a pipe will STALL when any error happens involving
it. This prevents any further transactions happening on the pipe, until the
host driver has done something to recover from the error. The host driver needs
to clear the STALL, then it can carry on.
The other is the STALL PID handshake. When a STALL PID is retuned by a device
it’s telling the host it doesn’t understand the command it was sent. A STALL
PID also has the effect of the STALLIng the pipe so the host driver has to do
something about it.
You can have a pipe STALL without a STALL PID being involved. You need to see
the exact error for some clue as to what’s going on. Installing the logging
family (if available) and looking at the Prober log may also be illuminating
(and/or confusing).
Add: I see you did mention the exact error: kIOUSBPipeStalled
As USB.h says:
#define kIOUSBPipeStalled iokit_usb_err(0x4f) // 0xe000404f Pipe has stalled,
error needs to be cleared
The pipe previously STALLed and the host driver has not yet cleared the STALL.
So this call isn’t the problem, a previous one is.
USB.h also notes that a "generic IOKit error instead of a USB specific error”
is returned for a STALL PID, but notes that kIOUSBPipeStalled is returned. So
the error code is ambiguous.
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