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

Douglas Gilbert <dougg@xxxxxxxxxx> · Wed, 06 Aug 2008 11:06:20 +0200

Swen Schillig wrote:

On Tuesday 05 August 2008 18:51, James Bottomley wrote:

On Tue, 2008-08-05 at 14:40 +0200, Martin Petermann wrote:

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.

First of all, why is the device returning PQ=1 PTD=0x1f? this should
mean its not connected and probably doesn't exist... ie inaccessible
without some unspecified action being taken.  If you can use it, it's
clearly not behaving like a PQ=1 LUN.  Perhaps the simplest thing would
be for something in s390 to fix up the inquiry data ... or we could
allow you could have a script to force it to appear (as in if you send a
specific scan for this one LUN we could override the catch in the code
that throws it out again).

James

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James

I think Martin is saying that this LUN is a non existent one which is just used for scanning
all available (existing) LUNs on the remote storage port.
That's why PQ=1 PTD=0x1f are returned and correct !

So what's required here is the possibility to add a "dummy" LUN which can be used just for this purpose.
Not sure whether this is covered by anything in the standard

That sounds like a job for the REPORT LUNS well known logical
unit (WLUN or W-LUN). I implemented one in the scsi_debug driver.
See the description of the "no_lun_0" parameter in
http://sg.torque.net/sg/sdebug26.html

You might try:
  echo "- - 49409" > /sys/class/scsi_host/host<n>/scan
where "host<n>" is the controller in question. Then
see if a WLUN has appeared as a sg device node.

Doug Gilbert

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