Re: Unaligned scatter-gather buffers and usb-storage

[Jens Axboe <axboe@suse.de>]

On Tue, Nov 18 2003, Patrick Mansfield wrote:
> On Tue, Nov 18, 2003 at 10:20:17AM -0500, Alan Stern wrote:
> 
> > Alignment to a 512-byte boundary would suffice and page alignment would be
> > more than enough.  The cleanest way to fix the problem would be to have st
> > and sg refuse to do direct I/O when the user's buffer isn't page aligned.  
> > That's probably too draconian (but it would allow us to give the user
> > process an appropriate error code right away).
> 
> The above restriction sounds reasonable to me, but I do not have
> experience in this area. I thought that the raw and direct block
> interfaces already had such restrictions.
> 
> Jens or Doug G can you comment on this?
> 
> You could add a scsi_host fields or bits specifying alignment
> restrictions, and enforce them as needed in upper level drivers.

The queue already has such a restriction embedded, see bio_map_user()
and queue_dma_alignment().

-- 
Jens Axboe