Re: USB network gadget / DWC3 issue

[Ferry Toth <fntoth@gmail.com>]

Hi,

Op 30-03-2021 om 18:17 schreef Felipe Balbi:
> Hi,
>
> Andy Shevchenko <andy.shevchenko@gmail.com> writes:
>> Hi!
>>
>> I have a platform with DWC3 in Dual Role mode. Currently I'm
>> experimenting on v5.12-rc5 with a few patches (mostly configuration)
>> applied [1]. I'm using Debian Unstable on the host machine and
>> BuildRoot with the above mentioned kernel on the target.
>>
>> **So, scenario 0:
>> 1. Run iperf3 -s on target
>> 2. Run iperf3 -c ... -t 0 on the host
>> 3.  0.00-10.36  sec   237 MBytes   192 Mbits/sec                  receiver
>>
>> **Scenario 1:
>> 1. Now, detach USB cable, wait for several seconds, attach it back,
>> repeat above:
>> 0.00-9.94   sec   209 MBytes   176 Mbits/sec                  receiver
>>
>> Note the bandwidth drop (177 vs. 192).
>>
>> (Repeating scenario 1 will give now the same result)
>>
>> **Scenario 2.
>> 1. Detach USB cable, attach a device, for example USB stick,
>> 2. See it being enumerated and detach it.
>> 3. Attach cable from host
>> 4 .   0.00-19.36  sec   315 MBytes   136 Mbits/sec                  receiver
>>
>> Note even more bandwidth drop!
>>
>> (Repeating scenario 1 keeps the same lower bandwidth)
>>
>> NOTE, sometimes on this scenario after several seconds the target
>> simply reboots (w/o any logs [from kernel] printed)!
>>
>> So, any pointers on how to debug and what can be a smoking gun here?
>>
>> Ferry reported this in [2]. There are different kernel versions and
>> tools to establish the connection (like connman vs. none in my case).
>>
>> [1]: https://github.com/andy-shev/linux/
>> [2]: https://github.com/andy-shev/linux/issues/31
> dwc3 tracepoints should give some initial hints. Look at packets sizes
> and period of transmission. From dwc3 side, I can't think of anything we
> would do to throttle the transmission, but tracepoints should tell a
> clearer story.
>
My testing (but yes, with difference kernel and network managed by 
connman) shows:

1) on cold boot eem network gadget works fine

2) after unplug or warm reboot (which is also an unplug) it's broken, 
speed is lost (|12.0 Mbits/sec from 200Mb/s normally)|, packets lost, no 
configuration received from dhcp, occasional reboot, only way to fix is 
cold boot

3) if before unplug `connmanctl disable gadget`, on replugging and 
enabling it works fine

My theory is that some HW register is disturbed on a surprise unplug, 
but not reset on plug or warm boot. But on cold boot is cleared. Maybe 
that can help to narrow down tracepoints?