Re: SG_IO problem on tape devices

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2008-06-03 at 09:59 +0200, Arne Wiebalck wrote: 
> Hi all,
> 
> I got a problem using the SG_IO ioctl with our tape drives. 
> In order to examine errors on our drives in more detail, I 
> would like to make the sense bytes available to the 
> application using the REQUEST_SENSE command.
> 
> However, there's a discrepancy between the kernel output in
> /var/log/messages and what I see using SG_IO within my 
> application: while the kernel sees
> 
> kernel: st0: Error with sense data: scsi1 : destination target 0, lun 0
> kernel:         command = Space 01 00 0d d5 00
> kernel: Info fld=0x1, Current st0: sense key Medium Error
> kernel: Additional sense: Read retries exhausted
> kernel: Raw sense data:0xf0 0x00 0x03 0x00 0x00 0x00 0x01 0x12 0x00 0x00
> 0x00 0x00 0x11 0x01 0x00 0x00 0x00 0x00 0x37 0xf7 0x10 0x01 0x00 0x00
> 0xf7 0x37
> 
> (which is realistic) the ioctl reports something like
> 
> 70 00 00 00 00 00 00 12  00 00 00 00 00 00 00 00
> 00 00 00 00 00 00 00 00  10 02
> 
> (since the first bit is not set, the sense bytes are not even valid,
> as far as I understand).
> 
> So, could it be that the sense bytes are already cleared when I request
> them? They cleared/set by the next SCSI cmd, I assume? But shouldn't
> they be valid even then?
> 
> I also tried the sg3-utils to query the drive's sense bytes, and they
> report (almost) the same sense bytes as SG_IO inside my application
> does. Sending an INQUIRY cmd using sg3_utils works fine, btw.
> 
> Maybe that's all a plain usage error, so please find the code I use
> below.
> 
> Anything obvious I am doing wrong here? All comments appreciated. 

Yes: SCSI automatically requests sense in response to a check condition.
So, the sense should be attached to the SG_IO command that receives the
error.  You can't do an additional request sense for it because the
sense has already been cleared by the automatic request sense the
mid-layer did.

Does the sense data really not get returned by the SG_IO command that
actually encounters the check condition return?

James