Hello, I wrote:
All the S/G counts printed out were divisible by 4 (36 for
INQUIRY and 96 for REQUSET SENSE). It's the *actual* byte count for
the REQUEST SENSE that's no divisible. The SCSI/ATAPI devices are
free to sent less data than requested on non block transfer commands.
That is just fine - if the sg list is not corrupt or being mishandled
and
the atapi pio code is not buggy.
RTFS a bit and it becomes obvious that the core libata code has a bug:
Oh, I have already... and saw where the issue could be. It just
wasn't obvious why 32-bit PIO triggered it.
Got it now, however the issue doesn't seem as evident simple to me...
From libata-sff.c:
/* consumed can be larger than count only for the last
transfer */
WARN_ON_ONCE(qc->cursg && count != consumed);
The big clue turns out to be that the code doesn't match the comment.
Next note the check on qc->cursg. If my input sg list is a 36 byte
single
sg entry then qc->cursg should be NULL by the WARN_ON() - but it isn't.
I think it's still not NULL because qc->cursg_ofs == sg->length
check was *not* true right above, hence sg_next() wasn't called...
If qc->cursg is NULL when the sg_next() is run then we don't warn
because
we are quite happy with the last segment being padded or underrunning.
I don't think that sg_next() is called on an underrun segment. And
here lies the mistake.
What we actually want to explode on is a case where we transfer more
bytes than are wanted and where there are more sg entries to perform
- at
that point we would corrupt.
So at least one failure case is
Core code issues an SG list for 96 bytes
Drive indicates it wishes to return 18 bytes
data_xfer transfers 18 bytes + 2 padding (correctly) -> 20 bytes
Correctly indeed? I'm not at all sure it's correct to read an extra
16-bit word off the device when it thinks it's already done with the
data transfer. This is not the same as to read 16-bit word and ignore
its MSB as it happened. The same concern about the writes... Note that
the IDE code doesn't do this...
MBR, Sergei
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