RT and tty communication

Hi,

I'm currently testing a RT application that communicates with sensor
hardware using a tty serial port. The application polls the tty port
(non-blocking) in a user space thread (priority 80) for incoming data.
On a raspberry PI3 (ttyAMA0), with kernel 4.4.9-rt17, response times <
650usec are achieved, which is fine.

Any version equal or above 4.4.12-rt20 will increase response peaks to
(about 0.1%) > 2msec, with some peaks > 5msec.

cyclictest doesn't reproduce this, so I suspected a change in the tty
driver, or related to raspberry pi.

I found that, by reverting the attached patch, response times return
to < 650usec. I suspect it has something to do with a change in the
read poll.

I'd like to help solving this issue, can anyone give me some pointers
on how to continue?

Regards,
Niels Kolthoff