Re: [REGRESSION] usb: gadget: f_fs: Allow scatter-gather buffers

[John Stultz]

On Mon, May 20, 2019 at 9:23 AM Yang, Fei <fei.yang@xxxxxxxxx> wrote:
>
> >> One question that comes to my mind is this: Does the USB transmission
> >> stall (e.g. endpoint stall) or not? In other words, is adb connection
> >> broken because USB stops transmitting anything, or because the data is
> >> transmitted but its integrity is broken during transmission and that
> >> causes adb/adbd confusion which results in stopping their operation?
> >> Does anything keep happening on FunctionFS when adb connection is
> >> broken?
> >
> >Any discoveries about the problem?
>
> In my debugging, I'm seeing a lot of requests queued up through ffs_epfile_io (returning -EIOCBQUEUED), but
> only a few of them came back through ffs_epfile_async_io_complete -> ffs_user_copy_worker.
> I donât think there is a USB transmission stall though, because if I manually disable io_data->use_sg, everything
> goes back to normal. So it looks more likely to be a buffer handling problem in the DWC3 driver.

Yea, I also did reconfirm that reverting 772a7a724f6, or setting
gadget->sg_supported to false makes the isssue go away.

And after spending a bunch of time trying to trace through the code
last week, in particular the sg_supported checks, but I'm not seeing
anything that is standing out with the f_fs logic.

I'd start to agree it might be a buffer handling problem in dwc3, but
it feels odd that I'm also seeing this w/ dwc2 hardware as well. Maybe
the  same bug was copied into both drivers?

I'll try to dig a little on that theory today.

thanks
-john