On Tue, 20 Aug 2002, Terry Lambert wrote:

> I think everyone in this thread needs to read the last instance of
> this same thread, the first time it came up.
> I believe the general consensus was to send the 6, and if it failed,
> retry with the 10, and set a flag so that subsequent requests were
> 10 (this instead of a static quirk table that could find itself out
> of date).

As someone mentioned, some devices choke on the first 6-byte command
and then just don't work anymore even if you start sending the 10-byte
commands from then on.  I have a USB multi-flash (CF/MD, MMC, SD, etc)
card reader that does exactly that.  I have to enable
"kern.cam.da.no_6_byte=1" before try to use the device and everything
works fine.  I also don't have any problems with any of my other SCSI
devices (various SCSI CD-ROMs, a SCSI CD-RW, a SCSI ZIP drive, and
SCSI DDS2 and DDS3 tape drives) when using only 10-byte commands.

What problems would occur if you try 10 first and then 6 if that
fails?  Will the devices that only take 6-byte commands choke
permanently on the first 10-byte command as some of the non-SCSI stuff
does on the 6-byte commands, or would they truncate 4 bytes and treat
it as the wrong command?

I believe someone already proposed this, but since only some very old
SCSI devices won't handle 10-byte commands correctly (correct me if
I'm wrong there) and should affect very few people, how about just
enabling 10-byte commands by default and offering a sysctl to turn on
the 6-byte-then-10-byte method when it is needed?  The benefit of that
should greatly outweigh the drawbacks with the state of the hardware
as it is today.

 Chris Dillon - cdillon(at)wolves.k12.mo.us
 FreeBSD: The fastest and most stable server OS on the planet
 - Available for IA32 (Intel x86) and Alpha architectures
 - IA64, PowerPC, UltraSPARC, ARM, and S/390 under development
 - http://www.freebsd.org

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