Re: Dropping NETIF_F_SG since no checksum feature.

Stephen Hemminger
2006-10-09T17:56:31Z

On Mon, 9 Oct 2006 19:47:05 +0200
"Michael S. Tsirkin" <mst@mellanox.co.il> wrote:

> Hi!
> I'm trying to build a network device driver supporting a very large MTU (around 64K)
> on top of an infiniband connection, and I've hit a couple of issues I'd
> appreciate some feedback on:
>
> 1. On the send side,
> I've set NETIF_F_SG, but hardware does not support checksum offloading,
> and I see "dropping NETIF_F_SG since no checksum feature" warning,
> and I seem to be getting large packets all in one chunk.
> The reason I've set NETIF_F_SG, is because I'm concerned that under real life
> stress Linux won't be able to allocate 64K of continuous memory.
>
> Is this concern of mine valid? I saw in-tree drivers allocating at least 8K.
> What's the best way to enable S/G on send side?
> Is checksum offloading really required for S/G?

Yes, in the current implementation, Linux needs checksum offload. But there
is no reason, your driver can't compute the checksum in software.

> 2. On the receive side, what's the best/right way to create an skb that
> is larger than PAGE_SIZE?
> Do I allocate with alloc_page and fill in nr_frags with skb_fill_page_desc?
> Some drivers seem to fill in frag_list - which is better?
> I see than even skb_put only works properly on linear skb.

Allocating large buffers is problematic on busy systems.
See lastest e1000 or sky2 that use frag_list.

> What are the helpers legal for fragmented skb?
Read the source. Setting up fragmented buffers has less helper
functions, but isn't that hard.

--
Stephen Hemminger <shemminger@osdl.org>