Implementing tight time constraints for read/write with USB serial (ftdi_sio)

[Greg KH]

On Mon, Sep 03, 2018 at 09:12:35PM +0200, Thomas Bracht Laumann Jespersen wrote:
> Hello all!
> 
> I'm working on a project where we want to integrate into a production line and
> listen on connected devices, and the challenge is that our setup needs to be
> fast enough.

First off, "tight time constraints" and "usb-serial" should never be in
the same sentence, unless you use the words "never will happen" in there
as well :)

There are loads of issues with trying to use USB with anything you wish
to have low-latency or determinism with, and then throw a usb-serial
device into the mix and all bets are off.  Remember there is a little
microcontroller out there at the end of the usb-serial device running a
UART and some unknown firmware blob that can take its time and do
anything it wants at any point in time, which can, and will, cause you
all sorts of headaches as you are finding out.

If you _really_ need to do something like this, use a UART attached to a
PCI card, you will have much better control and be able to actually do
what you are wanting to do here.

Good luck!

greg k-h