I probably missed some emails ?
In any case i was only concerned about the additional copy
done by m_devget when the controller can already DMA into
an mbuf, and there are no alignment constraints.
cheers
luigi
> Andrew Gallatin wrote:
> > > I disagree with this code; the elemenets in the header
> > > are referenced multiple times. If you are doing the
> > > checksum check, you might as well be relocating the data,
> > > as well. The change I would make would be to integrate
> > > the checksum calculation with the m_devget(), to ensure
> > > a single pass, in the case that m_devget() must be used
> > > to get aligned packet payload, and the checksum has not
> > > been offloaded to hardware.
> >
> > Interesting idea... However, what if you're a bridge or a router?
> > You've just done a whole lot of work for nothing. I imagine its just
> > this case that Luigi cares about.
> >
> > If you want to integrate a checksum & a copy, it should really be done
> > at the copyout() stage.
>
> You're missing the point. You do the m_devget() only when
> you decide to do a checksum, which means you've decided to handle
> the packet yourself. The alignment is done via a copy of the
> header field; specifically, a byte copy of the protocol type,
> when you decide how to handle it.
>
> In the case of the bridge, it's very easy: you don't care about
> the contents of the packet, unless it's destined for you. For
> a router, you are operating above layer 2, so you _do_ care,
> and must hterefore do the checksum to be correct (since you
> should not reference the field contents without knowledge that
> the checksum is correct). The Cisco approach of ignoring the
> checksums is all well and good, if the work is being done in
> hardware, but for most software, it very definitely cares (it's
> more overhead to relay bad packets onto your network, even if
> you assume it's OK for a router to do that).
>
> -- Terry
>
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