On 06/30/2010 12:23 AM, Brett Dennis wrote: > 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? > It depends. If you are issuing BLOCK_PC commands then you are in the clear. If an FS_PC commands then your ULD must take proper care at completion. What ULD are you using? > Thanks in advance. > > Brett Dennis > IBM tape device driver developer Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
