Hello,
Apologies for posting twice (messed up on the subject line of the first post...)
In kernel 2.6.26, the implementation of __blk_rq_map_user changed and
now includes a call to bio_copy_user:
uaddr = (unsigned long) ubuf;
alignment = queue_dma_alignment(q) | q->dma_pad_mask;
if (!(uaddr & alignment) && !(len & alignment))
bio = bio_map_user(q, NULL, uaddr, len, reading);
else
bio = bio_copy_user(q, uaddr, len, reading);
bio_copy_user uses the HBA's scatter-gather pages for the copy. The
problem I face is that if the data is not aligned properly, or the len
parameter does not divide properly, and the transfer is greater than
the number of scatter-gather pages, the IO operation will fail. I am
considering increasing the size of my data buffer to meet the
length-alignment requirement and changing the 'len' parameter passed
into blk_rq_map_user to include a larger size than my CDB transfer
length. This way it will hit the bio_map_user branch instead of the
bio_copy_user branch and I won't have to worry about insufficient
sg-pages. The tape device should still honor the CDB write length, so
unless there are other limitations imposed by the Linux kernel or SCSI
subsystem, I think this is a workable solution. Does anyone in this
list know if there are requirements against sending a larger buffer
length than CDB transfer length?
Thanks in advance.
Brett Dennis
IBM tape device driver developer
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