FUJITA Tomonori wrote:
CC'ed Jens, James, and linux-scsi.
On Thu, 27 Sep 2007 03:31:55 -0400
Jeff Garzik <jeff@xxxxxxxxxx> wrote:
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?
I'll work on it too after finishing max_seg_size.
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).
I know the workaround since I fixed libata's sg chaining patch.
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.
I think that the block layer can handle both max_segment_size and
seg_boundary_mask properly (and SCSI-ml just uses the block layer). So
if we fix iommu, then we can remove a workaround to fix sg lists in
llds.
A key problem I was hoping would be solved with your work here was the
elimination of that post dma_map_sg() split.
Yeah, that's my goal too.
Great :) Well, I'm generally happy with your max-seg-size stuff (sans
the minor nits I pointed out in another email).
Thanks for pursuing this,
Jeff
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