On Fri, 18 May 2007, David Brownell wrote:
> On Friday 18 May 2007, jidong xiao wrote:
> > Regarding this, I have one question:
> > "except at the beginning and the end, scatter-gather buffers follow page
> > boundaries", why? i.e. Is there any specific reason that sg buffers should
> > follow page boundaries? Then why the beinging and the end are exceptions?
>
> You should ask the person who wrote that comment; I can't
> make sense of it myself. What's "follow" etc.
I wrote it. Stating it more verbosely:
Each entry in a scatter-gather list maps a group of physically
contiguous pages. The buffer itself occupies only a portion of
those pages, as given by the .offset and .length fields in the
entry.
For each entry in the list (except possibly the first),
.offset is 0 and hence the buffer starts at a page boundary.
For each entry in the list (except possibly the last),
.offset + .length is equal to the total size of the pages
mapped by the entry, and hence the buffer ends at a page
boundary.
Alan Stern
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