Re: usbserial / ftdi_sio (+ others) bug?

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On 10/29/2014 04:51 AM, Johan Hovold wrote:
> [ +CC: Peter, linux-serial ]
> 
> On Wed, Oct 29, 2014 at 10:07:26AM +0200, Janne Huttunen wrote:
>> 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.

So you're only getting status and not data.

> 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?
> 
> You are correct. The usb-serial drivers, and at least some serial
> drivers, fail to implement TTY_DRIVER_REAL_RAW correctly in that they
> do not honour ((IGNBRK || (!BRKINT && !PARMRK)) && (IGNPAR || !INPCK)).

These settings are a constant source of bugs in serial drivers.
We really need to abstract the way these settings are processed;
even the 8250 driver is getting this wrong.

Regards,
Peter Hurley
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