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SubjectRe: Mark Russinovich's reponse Was: [OT] Comments to WinNT Mag !! (fwd)
On Sat, 8 May 1999, Chris Wedgwood wrote:

> Could someone please clue me in on how this works? I don't understand
> how we can do zero copy without hardware checksumming... and in the
> cases we do, surely the overhead of twiddling this hardware is
> comparable to copying 1500 bytes of data? (For HIPPI I can see it
> might make sense).

As Alan said, you get it mostly for free. in the case of the 905b (aside
from sun's stuff, the only 100mb pci card I know of that can do hw csum)
its merely some bits to set in the descriptors that we pass the card
anyway. but once you do this you can start passing the card iovec style
packets. this makes frag calculation simple iovec math rather than
copy/csum operations, and in the easiest sendfile() case we can dma pages
right out from the page cache, no memory sitting around in skbs for us to
touch/manage/etc.

of course, the book keeping is not exactly trivial.

-- zach

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