netdev - Re: [PATCH v3 2/2][BNX2]: Add iSCSI support to BNX2 devices.

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Date:	Thu, 27 Sep 2007 03:31:55 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	FUJITA Tomonori <tomof@....org>
CC:	hare@...e.de, open-iscsi@...glegroups.com, hch@...radead.org,
	davem@...emloft.net, mchristi@...hat.com, netdev@...r.kernel.org,
	anilgv@...adcom.com, talm@...adcom.com, lusinsky@...adcom.com,
	uri@...adcom.com, fujita.tomonori@....ntt.co.jp,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v3 2/2][BNX2]: Add iSCSI support to BNX2 devices.

FUJITA Tomonori wrote:
> Yeah, we could nicely handle lld's restrictions (especially with
> stacking devices). But iommu code needs only max_segment_size and
> seg_boundary_mask, right? If so, the first simple approach to add two
> values to device structure is not so bad, I think.

(replying to slightly older email in the thread)
(added benh, since we've discussed this issue in the past)

dumb question, what happened to seg_boundary_mask?

If you look at drivers/ata/libata-core.c:ata_fill_sg(), you will note 
that we split s/g segments after DMA-mapping.  Looking at libata LLDD's, 
you will also note judicious use of ATA_DMA_BOUNDARY (0xffff).

It was drilled into my head by James and benh that I cannot rely on the 
DMA boundary + block/scsi + dma_map_sg() to ensure that my S/G segments 
never cross a 64K boundary, a legacy IDE requirement.  Thus the 
additional code in ata_fill_sg() to split S/G segments straddling 64K, 
in addition to setting dma boundary to 0xffff.

A key problem I was hoping would be solved with your work here was the 
elimination of that post dma_map_sg() split.

If I understood James and Ben correctly, one of the key problems was 
always in communicating libata's segment boundary needs to the IOMMU layers?

	Jeff

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