Another ISP1761 / FTDI / serial problem

[Arvid Brodin]

Hi!

I get weird problems when I use a null modem cable between the two ports 
on a FT232BM (2-port FTDI USB-to-serial converter) with an isp1761 host 
controller. The same thing does not happen with an EHCI host controller. 
Lengthy details follows:

With the EHCI host controller:
=============================

[Terminal 1]
$ cat /dev/ttyUSB1

[Terminal 2]
$ echo "Something seems wrong here" > /dev/ttyUSB0
$ echo "Something seems wrong here" > /dev/ttyUSB0

[Terminal 1]
Something seems wrong here

Something seems wrong here

^C
$

Observe the extra new line after the strings. I'm not sure if this is a 
bug in the ftdi driver, some other part of the serial subsystem, or if 
it is the result of some terminal setting; with a USB analyzer I observe 
the following sequence of packets (#-# == device-endpoint):

3-2 OUT: "Something seems wrong here"
3-2 OUT: 0x0D 0x0A
4-1 IN: 0x01 0x60 'S'
4-1 IN: 0x01 0x60 'o'
...
4-1 IN: 0x01 0x60 0x0D
4-1 IN: 0x01 0x60 0x0A
4-2 OUT: 'S' # The echo from the reading side starts here
4-2 OUT: 'o'
...
4-2 OUT: 0x0D 0x0A
4-2 OUT: 0x0D 0x0A # The newline pair is echoed twice - perhaps both 
'CR' and 'NL' are converted to "CRNL" in the echo?

Anyway, since 3-1 is never read, no feedback loop is created by the two 
echoing devices.


With the isp1761 host controller, driver from 2.6.35.7 backported to 
2.6.27.54 and modified for platform endianness:
=============================

[Terminal 1]
$ cat /dev/ttyUSB1

[Terminal 2]
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0

[Terminal 1]
This is isp1761 and something is wrong

This is isp1761 and something is wrong

This is isp1761 and something is wrong

This is isp1761 and something is wrong

This is isp1761 and something is wrong

and something is wrong
(blank line x7)
This is isp1761 and something is wrong

^C
# echo "This is isp1761 and something is wrong" > /dev/ttyUSB0
# cat /dev/ttyUSB1
  is wrong
(blank line x30 ca)
TThis is isp1761 and something is wrong

^C
#

The exact look of the error differs, but it is quite easy to trigger by 
echoing text several times in a row; also breaking the "cat" execution, 
echoing some more text, and then starting cat again seems to trigger the 
error often.

On the bus the traffic looks something like this:

7-2 OUT: "This is isp1761 and something is wrong"
7-2 OUT: 0x0D
7-2 OUT: 0x0A (NAK) # The CRNL pair is split here and the NL is not 
directly accepted by the device
7-2 OUT: 0x0A (NAK)
Following this is lots and lots of "0x0A NAKs" intermixed with traffic 
that looks quite like the EHCI case except that much of the traffic has 
to be re-sent lots of times before it is accepted. The 0x0A is finally 
accepted after all text has been received by 8-1 and echoed from 8-2. 
This exact pattern is repeated every time text is written to the serial 
port.

Device 8 receives the characters and echoes them back to 7-1 as in the 
EHCI case. But with isp1761, after a while device 7 for some reason 
starts echoing parts of the received echo-text back. My guess is that 
this should not happen since device 7 is never read from. Anyway, this 
creates a feedback loop which would be the source of the spurious 
characters in the "cat" run above and that sometimes is powerful enough 
to make the terminal unresponsible.

I've been trying to figure this thing out for a while now but quite 
frankly I'm stumped; I can't figure out where and why things goes wrong. 
Any ideas are welcome! Perhaps the NAK problem/bug triggers a 
problem/bug somewhere in linux' serial terminal code?

The terminal settings are the defaults and are the same during all 
examples above on both serial ports:

$ stty -a -F /dev/ttyUSB0
speed 9600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; 
eol2 = <undef>; swtch = <undef>;
start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; 
flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon 
-ixoff -iuclc -ixany -imaxbel
-iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 
vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop 
-echoprt echoctl echoke

$ stty -g -F /dev/ttyUSB0
500:5:cbd:8a3b:3:1c:7f:15:4:0:1:0:11:13:1a:0:12:f:17:16:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0

--
Arvid Brodin
Enea Services Stockholm AB