Hi
I have noticed that the ftdi_sio serial driver in recent kernel versions
has very bad performance when used through the Python's serial library.
As a test case I have a custom device that will send a continuous block
of 5k characters once every few seconds over a RS-232 line (115200 baud)
to an Olimex programmer (based on FT2232C, also tried one with FT2232H).
Programmer is connected to a Linux system where a simple Python script
reads the device:
import serial
comm = serial.Serial("/dev/ttyUSB0", 115200)
while True:
line = comm.readline()
With kernels before 3.7.0 the script reads uncorrupted data while using
newer kernels (including 3.9.4) the Python script sees heavy byte loss.
"top" shows an 95% "idle" CPU. Only very slow transmissions (on the
order of tens of bytes per second) will come through uncorrupted.
Using git-bisect, I have found the commit that introduced this problem:
6f602912c9d0c84c2edbd446dd9f72660b701605
usb: serial: ftdi_sio: Add missing chars_in_buffer function
This might also be related with the unusual way Python serial library
reads the device. It uses select() with no timeout and single byte
read()s in a loop. strace output:
select(4, [3], [], [], NULL) = 1 (in [3])
read(3, "D", 1) = 1
select(4, [3], [], [], NULL) = 1 (in [3])
read(3, "E", 1) = 1
...
With sufficiently large read()s the byte loss can be eliminated.
With the commit above, each select() now causes an additional round trip
over USB to read the state of the hardware buffer. It's possible that
constant status querying triggers some bug in the hardware or the query
is simply too slow and causes overflows in the hardware buffer.
Thanks for your help
Tomaž
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