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? -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
