Re: [lwip-users] LWIP/WIN32 UDP performance (throughtput)

[Bill Auerbach]

>I've been meaning to talk to you about this issue.
>Since I use DMA, there are hard constraints on the location of the
>buffer.
>It must start at an address multiple of 32 (cache line aligned) and the
>length must be a multiple of 32, and we must purge the appropriate lines
>from cache before the DMA operation.
>
>I suppose I could allocate 32 more bytes to hold the pbuf struct and
>fudge the pointers to make the payload start on the right boundary,
>but you've said that pbuf_header could move the start of payload,
>or something like that... More voodoo stuff.

This is what I've done on 2 platforms - both use DMA and zero-copy (one uses
chained DMA and needs cache maintenance). I've done this "fudging". I
aligned the payload and padded it out to the required size.

>Like someone said on the list, I think lwip wants control over
>the packet buffers, which is a problem when a driver also needs
>control over the buffers.

It's not a problem that can't be overcome with lwIP as is.  If this pbuf
driver management was done from the onset, all the better.  But it wasn't
and they would risk breaking compatibility with a lot of lwIP programs if
pbuf use and the API for it is changed.

Bill