Description of problem:  The RI serial status line for a USB to Serial adapter
with an (apparently) older Prolific PL2303 chipset is not supported - it appears
 as permanently asserted (SET).  (The other input status lines CD, DSR, and CTS
are properly supported.)

Version-Release number of selected component (if applicable):
Linux version 2.6.18-1.2868.fc6
(but also was a problem with earlier kernels, e.g., FC-3, final kernel update.)

The USB to Serial adapter tested with this chipset is the Sabrent SBT-USC1M.
  http://www.sabrent.com/products/specs/SBT-USC1M.htm
purchased from ByteRunner as their SKU: CBL-USB-CS1B       
http://www.byterunner.com/byterunner/category=USB+to+Single+Serial+Adapters/exact_match=exact

How reproducible:
Always, for this particular Prolific chipset version.  The attached test program
"adapter_test.c" toggles the DTR output status line and monitors the state of
the four input status lines, one or more of which is assumed to be jumpered to
the DTR line.  

Steps to Reproduce:
1. Compile the attached test program :
  $ cc -Wall adapter_test.c -o adapter_test

2. Plug the USB to Serial adapter into a USB port on the PC.  Verify the
assigned device port by running 'dmesg'.  E.g., /dev/ttyUSB0

3. Connect a jumper between the DTR and RI pins (DB-9 pins 4 and 9) on the
adapter.  Optionally additionally jumper the DTR pin to the CD, DSR, and CTS
input pins (pins 1, 6, and 8).

4. Run the test program:
 $ ./adapter_test  /dev/ttyUSB0 
  
Actual results (with all four input status pins jumpered to DTR):
Jumpered pin  4   to    9    1    6    8
Status Line: DTR  =>   RI   CD   DSR  CTS
             ---       ---  ---  ---  ---
             clr  =>   SET  clr  clr  clr 
             SET  =>   SET  SET  SET  SET 
             clr  =>   SET  clr  clr  clr 
             SET  =>   SET  SET  SET  SET 

Expected results:
Jumpered pin  4   to    9    1    6    8
Status Line: DTR  =>   RI   CD   DSR  CTS
             ---       ---  ---  ---  ---
             clr  =>   clr  clr  clr  clr 
             SET  =>   SET  SET  SET  SET 
             clr  =>   clr  clr  clr  clr 
             SET  =>   SET  SET  SET  SET 

Additional info:

USB to Serial adapters with an apparently later version of the Prolific PL2303
perform properly as expected, e.g., the ByteRunner SKU Y-105.

Sabrent did not respond to my request for the specific Prolific chipset version
in their product, however the Sabrent adapter appears to use the same Prolific
chipset as an adapter purchased in mid-2004 from Sewell Development Corp, for
two reasons: 1. Both have basic support under the Linux 2.4 kernel (newer PL2303
chipsets aren't supported by the 2.4 kernel). 2. Both exhibit the same RI problem.

Sewell describes the chipset change in their adapter in Oct 2004 from the PL2303
to the PL2303HX here: http://sewelldirect.com/UsbToSerialSupportLinux.asp

Output from dmesg when adapter is plugged into the USB port:
------------------
usb 1-2: new full speed USB device using uhci_hcd and address 3
usb 1-2: configuration #1 chosen from 1 choice
pl2303 1-2:1.0: pl2303 converter detected
usb 1-2: pl2303 converter now attached to ttyUSB0
-------------------

Note: Under kernel 2.4 (Red Hat 9, final update) support for both the Sybrent
and Sewell adapter Prolific chipsets was limited to the RX, TX, DTR, and RTS
lines.  None of the four input status lines respond at all to the test described
above, i.e., the test program displays 'clr' for these lines regardless of the
state of the DTR line.