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Subject: Re: [RFC] Implement dma_boundary in the generic device
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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Date: Mon, 01 Mar 2004 22:46:42 +1100
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To: Matthew Wilcox <willy@debian.org>
Cc: James Bottomley <James.Bottomley@steeleye.com>, Linux Arch list <linux-arch@vger.kernel.org>, Jeff Garzik <jgarzik@pobox.com>
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Ok, I have a ppc64 implementation that appear to work after a quick
test, I'll have to hammer it more tomorrow though and will send a patch.

One thing we discussed on irc but I want to raise here so we all agree
about the "limitation" and can wire it deep into the documentation:

The boundary stuff is only respected as long as the segment doesn't
start with an "offset". If I have a 64k boundary and get passed a
64k segment but which start with an "offset" in the first page, I
will _not_ break it up. Same as without iommu, I don't break
existing segments. I just prevent merging and align big segments
properly so they stay within the boundary.

Oh ... and I would still have liked (both at the struct device and
the BIO layer) a separate "max_seg_size" .... I just figured out that
stupid DBDMA controller used all over the place on Macs (though only
for CD/DVD ATA/100 and sound driver on G5s with an iommu) has a limit
of 64k ... -1 ! (0xffff is the max, 0 does NOT mean 64k shit !)

Right now, I work around that in IDE since I can allocate a larger
DMA descriptor table that the normal IDE PRD, so I allocate 512 of
them... (drivers/ide doesn't have hooks to let the low lever driver
set different queus sizes, sg list sizes, boundaries etc... I have
to mimmic the "legacy" ones).

But maybe it's not worth bothering...

Ben.