On Thu, Jun 06, 2013 at 11:58:56AM +0200, Johan Hovold wrote:
> 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)?
Yes, that's a good idea, I'll do that for the next round of stable
updates, after this next release tomorrow.
thanks,
greg k-h
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