linux-kernel - Re: sata_sil24 broken since 2.6.23-rc4-mm1

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Date:	Thu, 11 Oct 2007 12:25:49 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Torsten Kaiser <just.for.lkml@...glemail.com>
CC:	Jens Axboe <jens.axboe@...cle.com>, Jeff Garzik <jeff@...zik.org>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: sata_sil24 broken since 2.6.23-rc4-mm1

Torsten Kaiser wrote:
> Looking closer at
> http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=commitdiff;h=ec6fdded4d76aa54aa57341e5dfdd61c507b1dcd
> 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?

> And it would fit the symptoms, that the boot would fail at random. If
> the "correct" garbage was in place to where the sglist runs off it
> hangs the drive.
> And that would even fit the two different errors that I only got one time each:
> * a completely illegal access (PCI master abort while fetching SGT)
> * wrong alignment of the SGT (SGT no on qword boundary)
> At that that times the garbage seemed to point invalid addresses.
> 
> But I'm still not understanding, how the kernel could only fail
> sometimes at bootup, but after that working without any visible
> errors? Is the sil-chip rather intelligent about detecting corrupted
> sglists and silently ignoring them?

I have no idea why it fails only sometimes.

-- 
tejun
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