Re: Length and alignment in scsi_execute_async and blk_rq_map_user

[Boaz Harrosh <bharrosh@panasas.com>]

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