Re: [Madwifi-devel] Does the mactime point to the first bit of the packet, or the last one?

[Lrj <ren...@gm...>]

I agree that the key problem is when the mactime is taken.

And, I also doubt that if the mactime is the end of the frame, would the
"mactime - skb->len * 8 / ieeerate * 2" be exactly the first bit's arrival?


On 4/11/08, bruno randolf <br...@th...> wrote:
>
> On Friday 11 April 2008 16:31:00 Lrj wrote:
> > Hi,
> >       I did a google search, and find this one:
> > http://madwifi.org/ticket/1429.
> >       According to it, the mactime in the Prism2 header points to the
> end
> > of the packet frame, and for the Radiotp header, if "th->wr_tsft =
> > cpu_to_le64( mactime);" works, the mactime would point to  the end of
> the
> > packet, and if "th->wr_tsft = cpu_to_le64(mactime - skb->len * 8 /
> ieeerate
> > * 2);" works, the mactime would point to the first bit of the pakcet
> > arrives. Am I right? And if so, why we can calculate the time of the
> first
> > bit arrival with the "mactime - skb->len * 8 / ieeerate * 2"?
>
> the problem is that we simply don't know when the mactime is taken.
>
> it seems to be at the end of the packet, as the ticket suggests. but it
> could
> be some time later or earlier so before we make calcluations to adjust the
> time i think we need further experiments or information (atheros?) to see
> when mactime is actually taken. also i'm afraid it could be different for
> different chipset versions.
>
> mactime - skb->len * 8 / ieeerate * 2 would subtract the time it takes to
> send
> all the bits in the packet.
>
> bruno
>
>
>
>
>


-- 
Best wishes,
Lrj, NCI