usbserial / ftdi_sio (+ others) bug?

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I own a device that implements a data logging interface using the
FT232 USB-serial -chip. Very often it happens that connecting the
associated software with the device requires multiple attempts.
There seems to be two kinds of issues; either the program reports
that it did not receive any data or it reports reading lots of
data, but it was all invalid. I haven't yet looked at the former,
but I did spend some time investigating the latter.

Simple strace of the program startup showed that when connecting
fails, the program gets a lot (hundreds) of binary zeros while
reading the device. I used usbmon to capture the traffic between
the host and the device and the zeros are not strictly speaking
coming from the device. However when this problem happens the
device seems to report quite lot of overruns for a while, which
was a clue. After a somewhat successful attempt to understand
the operation of the tty code in Linux, I have a theory.

The usbserial driver sets the TTY_DRIVER_REAL_RAW flag. Based on
the comment in tty_driver.h this implies that the driver is not
supposed to report any statuses (including overruns) to ldisc
if they are ignored by the application (like they are in this
case). It's just that AFAICS the ftdi_sio subdriver (and many
others) doesn't seem quite honor this, but seems to report any
status unconditionally. Also AFAICS this then means that every
overrun will get converted into single binary zero delivered to
the application(?). If so, this probably isn't what is supposed
to happen and would explain the flood of extraneous zeros the
application was seeing when the connecting failed.

I haven't had yet the time to test this theory, but at least it
seems plausible to me. Any thoughts, anybody?
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