Re: [Thumbgps-devel] USB handshake signals and Linux

[Patrick Maupin <pmaupin@gmail.com>]

On Wed, Mar 14, 2012 at 5:49 AM, Eric S. Raymond <esr@thyrsus.com> wrote:
> Since the Plane Jane design concept depends on the host system being
> able to see USB events corresponding to serial handshaking state
> changes, I went looking for information on which handshake signals
> from which serial-to-USB adapters actually propagate these signals
> through. I focused on Linux becase that's the kernel for the
> bufferbloat deployment.

There is another question, which is about the internal latency inside
the USB adapter.  In general (e.g. for character data) the USB
adapters will deliberately add some latency so as to make better use
of internal buffering resources.  (Once an adapter sends a packet of
characters up to the host, it has to wait for an ACK before it can
reuse that buffer -- it might need to resend for a NAK.)  So if the
host polls with an interrupt packet, and there is data at the USB
device but it reasonably expects that more data is coming, it might
not send the packet up right away.

The FTDI devices have a latency timer that allows you to control this
somewhat.  But even better, if we're going to use the PPS signal, they
essentially guarantee that they will send ASAP on modem control signal
change.  We might need to characterize "ASAP" but I would hope that it
doesn't vary too much.

I know nothing about the Prolific device; you might look for what they
have to say about latency.