On Thu, Jun 06, 2013 at 10:50:36AM +0200, Tomaž Šolc wrote:
> 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.
You're absolutely right. This is a known issue (the select overhead)
that was just recently fixed by commit a37025b5c7 ("USB: ftdi_sio: fix
chars_in_buffer overhead") in v3.10-rc3. Care to give v3.10-rc4 a try?
Greg, perhaps we should consider backporting the wait-until-sent
patches (i.e. 0693196fe..4746b6c6e)?
Thanks,
Johan
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
