Re: [PATCH] scsi_debug: disable clustering

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sun, 2008-02-17 at 23:52 +0900, FUJITA Tomonori wrote:
> On Sun, 17 Feb 2008 07:28:48 -0700
> Matthew Wilcox <matthew@wil.cx> wrote:
> 
> > On Sun, Feb 17, 2008 at 08:18:11AM -0600, James Bottomley wrote:
> > > No, he means that kmap_atomic can only map a page of data.  This makes
> > > single page only sg list entries and input assumption into this loop.
> > > with ENABLE_CLUSTERING, that's potentially not true.   Of course, this
> > > accidentally works most of the time because of the way kmap functions.
> > 
> > Ah, right.  I'm on the verge of releasing a ram-based scsi driver I've
> > been working on ... this loop should work fine with clustering as it
> > takes account of the sg potentially having multiple pages:
> > 
> >         scsi_for_each_sg(cmnd, sg, scsi_sg_count(cmnd), i) {
> >                 struct page *sgpage = sg_page(sg);
> >                 unsigned int to_off = sg->offset;
> >                 unsigned int sg_copy = sg->length;
> >                 if (sg_copy > len)
> >                         sg_copy = len;
> >                 len -= sg_copy;
> 
> stex driver has a similar function to copies data between a buffer and
> a scatter list. I think that scsi_kmap_atomic_sg is a bit primitive
> (and not very popular).  I'll send a patch to add a helper function to
> scsi_lib.c that copies data between a buffer and a scatter list. It
> would be useful for several drivers.

Actually, if you're going to sweep up them all, libata also does this.

However, mapping and copying data isn't a SCSI specific function, it's
one any virtual block driver should do, so I think block might be the
correct location for such a function.

James