Date:	Sat, 12 Jan 2008 21:12:43 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Alexander <aledin@...l.ru>, linux-kernel@...r.kernel.org,
	ide <linux-ide@...r.kernel.org>, Jeff Garzik <jeff@...zik.org>,
	Tejun Heo <htejun@...il.com>
Subject: Re: PROBLEM REMAINS: [sata_nv ADMA breaks ATAPI]
	Crash	on	accessing DVD-RAM

On Sat, 2008-01-12 at 19:38 -0600, Robert Hancock wrote:
> James Bottomley wrote:
> > On Sat, 2008-01-12 at 17:04 -0600, Robert Hancock wrote:
> >> I don't think the problem is that there's some buffer which is getting 
> >> allocated above 4GB and never bounced, since the problem goes away if 
> >> ADMA is disabled entirely and the DMA mask remains 32-bit always. My 
> >> guess is something is basing its decision on whether to bounce or not on 
> >> the device DMA mask. That can't possibly work properly for sata_nv since 
> >> the same PCI device has 2 ports, one of which can be in ADMA mode and 
> >> 64-bit capable and the other can be in legacy mode and only 32-bit capable.
> > 
> > Erm, well, you can't decouple them.  Having a differing blk queue bounce
> > mask and device mask is going to cause huge trouble.  The reason is this
> > insidious nasty called swiotlb.  Basically, with it enabled (and again,
> > it can be on ia64 or x86_64), the kernel can bypass the bounce limit
> > safe in the knowledge that swiotlb will fix up behind in the dma_map_
> > Unfortunately, if the device mask doesn't match the queue mask then
> > swiotlb will never kick in and you'll end up with mapped pages beyond
> > the 4GB limit.
> 
> Yuck.. All the IOMMU DMA mapping code checks against the device DMA 
> mask, so it looks like if we get to the point of doing the DMA mapping 
> on >4GB addresses in libata we're screwed with this approach.
> 
> The key problem is that both ata_ports share the same struct device with 
> one DMA mask which really doesn't match what this controller wants. I 
> wonder if we could do a different struct device for each port?
> 
> Other than that, I guess the solutions would be to just set a 32-bit 
> mask on the device if either port has an ATAPI device connected (which 
> is fairly ugly, considering that you could do things like hotplug an 
> ATAPI device when the other port was in use, for example), or do 
> something to prevent requests from reaching this point with >4GB 
> addresses in the first place..

Well ... assuming this is the problem (and perhaps we'd better get the
traces to confirm) there are at least three possible solutions:

     1. As you say, just take the pci device mask down to 32 bits.
     2. Find the problematic allocations and add GFP_DMA32
     3. set the mask on the actual SCSI device rather than the PCI
        device and pass that into dma_map_ (this approach would have to
        get signoff from the arch people; I know it will work on parisc
        and x86 but I'm not sure about any other arch).
 
> >> Tejun, I believe you had a patch that was printing warnings when libata 
> >> tried to program a legacy PRD with an address over 4GB. Could we change 
> >> that to WARN_ON and get someone experiencing this to try it and
> >> see what the stack trace points to?
> > 
> > Unfortunately, the stack trace probably won't help, since the command
> > likely gets issued from the block request function, so the trace won't
> > go back to the culpable initiator; that's why the command would be
> > helpful.
> 
> Well, dumping the ATA command surely isn't helpful, as I'm sure it will 
> be PACKET. I guess we'd have to dump out the actual CDB..

Sorry, when a SCSI person says dump the command, they mean the CDB.

James


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