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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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