FTDI FT2232 accesses under Linux much slower than under WinXP

Hello,
I've got a problem with the FTDI-chip FT2232 running under Linux:

---------------------------
I have written a small test program for WinXP. The access to the FT2232
in FIFO-Mode occurs as expected.
The porting to the Linux system was also successful.

I use the Evaluationboard "EZ1CUSB 12" of EasyFPGA as hardware. The
board has a USB connector, a FT2232 chip and a FPGA.
Linux system: Suse 10.1, Kernel 2.6.16.13
FTDI driver software: libftd2xx.so.0.4.10

When opening the DEVICE the following parameters are adjusted:
FT_SetUSBParameters (ftHandle, 0 x10000, 0;) // InTransferSize = 64
Kbytes
FT_SetLatencyTimer (ftHandle, 2); // Latency timer = 2 ms

To test the data processing speed I call following function in my
program (ReadBurstFT2232 =):
[1] The host PC claims a larger amount of data (e.g. 16 Kbytes).
Therefore we write an instruction (approx. 5 bytes) with a FT_Write
into the FPGA. The FPGA writes
    the requested data into the FIFO of the FT2232.
[2] Start for the time measurement.
[3] The status of the "Receive queue" is as long as queried with
FT_GetStatus, till 16 Kbytes are available. The number of requests with
FT_GetStatus
    are counted.
[4] end of the time measurement.
[5] The 16 Kbytes read into an Array in my program with FT_Read.

This function "ReadBurstFT2232" is called repeatedly in a loop.
Great differences during the time measurement (= proportionally to the
number of FT_GetStatus requests) occours:
- the first run needs approx. 1000 milliseconds.
- the next 2 -5 runs needs approx. 250 milliseconds
- all further runs needs approx. 30 milliseconds in which randomly runs
occurs from time to time with approx. 150 milliseconds.
I get comparable transmission delays for reputations -- if I call the
loops with the "ReadBurstFT2232" requests.
If I make a delay from approx 1ms [sleep(1)], all runs needs approx.
1000 milliseconds second.
Under Windows the different transmission delays don't occur. Every run
with an amount of data of 16 Kbytes needs approx. 18 milliseconds.
---------------------------

Has anybody got an idea about this issue? Could it perhaps be a
resource scheduling issue of the host PC?

Greetings, Harald

harald <adb.harald@gmx.de> wrote:
> Hello,
> I've got a problem with the FTDI-chip FT2232 running under Linux:

> ---------------------------
> I have written a small test program for WinXP. The access to the FT2232
> in FIFO-Mode occurs as expected.
> The porting to the Linux system was also successful.

> I use the Evaluationboard "EZ1CUSB 12" of EasyFPGA as hardware. The
> board has a USB connector, a FT2232 chip and a FPGA.
> Linux system: Suse 10.1, Kernel 2.6.16.13
> FTDI driver software: libftd2xx.so.0.4.10

> When opening the DEVICE the following parameters are adjusted:
> FT_SetUSBParameters (ftHandle, 0 x10000, 0;) // InTransferSize = 64
> Kbytes
> FT_SetLatencyTimer (ftHandle, 2); // Latency timer = 2 ms

At least try with Latency timer = 1 ms and try again. "Latency timer = 0"
might also be worth a try, but also may make thinhs worse.
-- 
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

harald wrote:
> Has anybody got an idea about this issue? Could it perhaps be a
> resource scheduling issue of the host PC?

Very jumbled post, but it sounds like it might need the
ASYNC_LOW_LATENCY flag of the tty driver. This disables
the Nagle-type algorithm of the tty transmit function.