Hi!

I've FINALLY find out why 2.2.x kernel always fail on first time access
to my scsi DAT 20/40 [1][2] .

 2.2.x: sense key Illegal Request, then halt with I/O error
 2.4.x: sense key Unit Attention,  but works

After too many hours spent reading linux scsi code I've decided to apply
a "brute force" method to discover why 2.4.x works and why 2.2.x not.
I've dicotomically test too many kernels from 2.3.51 to 2.4.17
to catch the breakpoint, and boom it's on patch-2.4.0-test2, before
that release we have the same error as 2.2.x .
Reading the scsi changes of patch-2.4.0-test2 [3] I've also find
that the "TEST_UNIT_READ removal" from scsi_scan.c cure the problem!

I've made a patch [4] against 2.2.20 that can be applied without
major problems also to 2.2.19,2.2.21pre2.

Comments, bug reports, suggestions welcome,
luca

[1] http://marc.theaimsgroup.com/?l=linux-scsi&m=100876196731096&w=2
[2] http://marc.theaimsgroup.com/?l=linux-scsi&m=100902910327577&w=2
[3] http://www.uwsg.iu.edu/hypermail/linux/kernel/0005.3/0688.html
[4] see attach