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SubjectRe: Scatter-gather list constraints
On Sat, 21 Jun 2008 17:50:13 -0400 (EDT)
Alan Stern <> wrote:

> > >> I don't think the block layer knows about such kinds of restrictions.
> > >
> > > Evidently not. Is it feasible to add such knowledge to the block
> > > layer?
> >
> > You would need to ask Jens, but I would assume he would ask:
> > - Is it common?
> This is the only situation I know about. But of course it will become
> more and more common as wireless USB devices spread into use.
> > - Is it performance critical?
> For people using wireless USB drives, yes.
> > and presumably the answer to both would be "no" ?
> In theory this could be fixed at the host controller level, by making
> packets span S-G elements. But this would be a very large and
> difficult change. Altering the block layer should be a lot easier (he
> said, secure in his blind ignorance). For example, the DMA alignment
> restriction could be made to apply to the _end_ of each S-G element as
> well as the _beginning_, except for the last element in the list.

I don't think that the block layer has the DMA alignment concept in FS
I/O path. And I think that you need kinda the DMA padding instead the
DMA alignment though again The block layer doesn't have the DMA
padding concept in FS I/O path. And the DMA padding applies to only
the last SG element.

I guess that it's pretty hard to implement such a strange restriction
in the block layer cleanly.

The iSER driver has a strange restriction too. I think that as iSER
does, bouncing is a better option, though adding some generic
mechanism to reserve buffer in the block layer might be nice, I

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