With kernel 2.6.19 no sg devices for devices that return PQ=1, PDT=0x1f

[Martin Petermann <martin@xxxxxxxxxxxxxxxxxx>]

With kernel 2.6.19 a change was introduced that no sg device was 
generated if PQ=1, PDT=0x1f was returned from the particular device:

commit 84961f28e9d13a4b193d0c8545f3c060c1890ff3
Author: dave wysochanski <davidw@xxxxxxxxxx>
Date:   Wed Aug 9 14:56:32 2006 -0400

    [SCSI] Don't add scsi_device for devices that return PQ=1, PDT=0x1f

Before it was possible on Linux 390 in user space to a e.g. LUN 0 to a 
port and to receive a generic device:

t6345056:/sys/bus/ccw/devices/0.0.5922/0x500507630313c562 # ll
total 0
-r--r--r-- 1 root root 4096 Aug  4 12:07 access_denied
-rw-r--r-- 1 root root 4096 Aug  4 12:07 failed
-r--r--r-- 1 root root 4096 Aug  4 12:07 in_recovery
-r--r--r-- 1 root root 4096 Aug  4 12:07 status
--w------- 1 root root 4096 Aug  4 12:07 uevent
--w------- 1 root root    0 Aug  4 13:46 unit_add
--w------- 1 root root    0 Aug  5 14:24 unit_remove
t6345056:/sys/bus/ccw/devices/0.0.5922/0x500507630313c562 # echo 0 > 
unit_add
t6345056:/sys/bus/ccw/devices/0.0.5922/0x500507630313c562 # ll
total 0
drwxr-xr-x 2 root root    0 Aug  5 14:25 0x0000000000000000
-r--r--r-- 1 root root 4096 Aug  4 12:07 access_denied
-rw-r--r-- 1 root root 4096 Aug  4 12:07 failed
-r--r--r-- 1 root root 4096 Aug  4 12:07 in_recovery
-r--r--r-- 1 root root 4096 Aug  4 12:07 status
--w------- 1 root root 4096 Aug  4 12:07 uevent
--w------- 1 root root    0 Aug  5 14:25 unit_add
--w------- 1 root root    0 Aug  5 14:24 unit_remove
t6345056:/sys/bus/ccw/devices/0.0.5922/0x500507630313c562 # lsscsi -g
[0:0:0:0]    no dev  IBM      2107900          2.27  -         /dev/sg0

After this fix there is no /dev/sg0 device generated.

We are utilizing the possibility to create such a device for the 
sg_utils commands in the case no other LUN has been attached to a port.

I do not want to put this fix into question. I would like to know if 
someone has an idea how to workaround this problem and to generate a 
generic device in user space using kernel 2.6.19 or a later version.

Regards
   Martin

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