Sense fetching [Was: cdrtools /devel ...]
Joerg.Schilling at fokus.fraunhofer.de
Mon Nov 8 17:23:31 UTC 2010
Alexander Motin <mav at FreeBSD.org> wrote:
> > Your patch to libscg looks definitely OK if we only look at the new corrected
> > kernel driver behavior.
> > There is a problem:
> > In case that there is a sense data residual > 0, libscg will asume that there
> > is less sense data that really present in case that a "new" libscg is runnung
> > on an old kernel.
> > Given the fact that many drives will probably only return 18 bytes of sense
> > data, this will happen every time libscg is told to fetch more sense than the
> > drive is willing to return.
> > Is there a way to distinct an old kernel from a new one?
> I don't see the problem. Previous kernel in most cases reported
> sesnse_resid == 0, lying that there is more sense data then really is.
> New one should report real (often positive) value. In both cases
> sesnse_resid value measured from the value submitted to the kernel.
Did the old kernel return a zero sense_resid for any implemented SCSI
transport? Libscg is a generic SCSI transport library and cdrecord is just one
user of this lib.
> > I see no advantage in removing the call to fillbytes().
> scgcheck tests how much sense data received by counting 0x00 and 0xff
> bytes. Zero-filling response buffer breaks that check. Though I have no
> idea if other crdtools' applications depend on these zeros. There could
> be some internal inconsistency.
O, I need to check other low level transport adaptation layers and think about
> > Did you test a modified libscg on an unmodified kernel?
> Unmodified kernel by default doesn't return any sense data at all for
> new CAM-based ATA -- this changes should be invariant. New scgcheck runs
> same bad as old. It just can't become worse. :)
> Legacy atapicam wrapper ignores sense_len on input and doesn't fill
> sense_resig on output -- I haven't tested, but it also should be invariant.
I am not only talking about ATAPI but abut SCSI in general.
Do you know the CAM behavior for other SCSI transports?
EMail:joerg at schily.isdn.cs.tu-berlin.de (home) Jörg Schilling D-13353 Berlin
js at cs.tu-berlin.de (uni)
joerg.schilling at fokus.fraunhofer.de (work) Blog: http://schily.blogspot.com/
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