usb: usbtest: Add TEST 29, toggle sync, Clear toggle between bulk writes

[Felipe Balbi <felipe.balbi@linux.intel.com>]

Hi,

Alan Stern <stern@rowland.harvard.edu> writes:
> On Thu, 14 Dec 2017, Mathias Nyman wrote:
>
>> Clear Feature Endpoint Halt should reset the data toggle even if the
>> endpoint isn't halted. Host should manage to clear the host side data
>> toggle to keep in sync with the device.
>> 
>> Test by sending a "3 data packet URB" before and after clearing the halt.
>> this should create a toggle sequence with two consecutive DATA0 packets.
>> 
>> A successful test sequence looks like this
>>     ClearFeature(ENDPOINT_HALT) - initial toggle clear
>>   DATA0 (max packet sized)
>>   DATA1 (max packet sized)
>>   DATA0 (zero length packet)
>>     ClearFeature(ENDPOINT_HALT) - resets toggle
>>   DATA0 (max packet sized), if clear halt fails then toggle is DATA1
>>   DATA1 (max packet sized)
>>   DATA0 (zero length packet)
>> 
>> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
>
> This test is a little unusual in that it doesn't contain a way to
> detect failures.  That is, you can't tell from the test results whether
> the device and the host behaved the way they are supposed to (in the
> case of the host, you could use a USB analyzer to find out).

wouldn't we get EPIPE in cases where host's and device's data
toggle/seqNum go out of sync?

> Also, some devices don't handle Clear-Halt requests properly if the 
> endpoint isn't already halted.  Presumably people wouldn't use one of 
> those devices for this test!  :-)

It's mandatory for devices to support Clear-Halt as means to reset data
toggle/SeqNum. If such controllers exist, they aren't certifiable IIRC.