On Thu, Oct 11 2007, Tejun Heo wrote:
quoted text
> Torsten Kaiser wrote:
> > Looking closer at
> > http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=commitdiff;h=ec6...
> > the change to libata.h seems bogus :
> > 
> > in ata_qc_first_sg:
> > old                                new
> > return qc->__sg                    return qc->__sg
> > qc->__sg - qc->__sg == 0           qc->n_iter=0
> > -> sg - qc->__sg corresponds to qc->n_iter
> > 
> > in ata_qc_next_sg:
> > sg++;                              sg_next(sg); qc->n_iter++;
> > sg - qc->__sg < qc->n_elem         qc->n_iter < qc->nelem
> > -> sg - qc->__sg corresponds to qc->n_iter
> > 
> > but in ata_sg_is_last:
> > (sg - qc->__sg) +1 == qc->n_elem   qc->n_iter == qc->n_elem
> > if sg - qc->__sg corresponds to qc->n_iter then shoudn't it be
> > qc->n_iter+1 == qc->n_elem?
> > 
> > That missing +1 would explain, why the SGE_TRM never gets set.
> 
> Thanks a lot for tracking this down.  Does changing the above code fix
> your problem?
> 
> Jens, Torsten's analysis looks correct && depending on qc state (n_iter)
> during iteration doesn't look like a good idea.  Those iterators are not
> supposed to have side effects.  Would it be difficult to implement
> sg_last() test?

This is the old ata_sg_is_last:

static inline int
ata_sg_is_last(struct scatterlist *sg, struct ata_queued_cmd *qc)
{
        if (sg == &qc->pad_sgent)
                return 1;
        if (qc->pad_len)
                return 0;
        if (((sg - qc->__sg) + 1) == qc->n_elem)
                return 1;
        return 0;
}

and the new one:

static inline int
ata_sg_is_last(struct scatterlist *sg, struct ata_queued_cmd *qc)
{
        if (sg == &qc->pad_sgent)
                return 1;
        if (qc->pad_len)
                return 0;
        if (qc->n_iter == qc->n_elem)
                return 1; 
        return 0;
}

->n_iter is how ata_qc_next_sg() walks over the sglist, I don't
understand your reference to why depending on that during iteration
would be bad?

So we could add a test for sg_last() there, but that would turn sg table
iteration into an O(N^2) operation for those drivers that use
ata_sg_is_last() with chained sg tables. I'd much rather just get rid of
ata_sg_is_last(), it's only used to mark end-of-table entries for
hardware. That logic can be performed cheaper.

Torsten, your analysis does look correct. Does it work with this simple
patch?

diff --git a/include/linux/libata.h b/include/linux/libata.h
index 2784163..0152bf4 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -947,7 +947,7 @@ ata_sg_is_last(struct scatterlist *sg, struct ata_queued_cmd *qc)
 		return 1;
 	if (qc->pad_len)
 		return 0;
-	if (qc->n_iter == qc->n_elem)
+	if ((qc->n_iter + 1) == qc->n_elem)
 		return 1;
 	return 0;
 }

-- 
Jens Axboe

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