Re: [PATCH 1/2 v2] mv643xx_eth: use sw csum for big packets

[Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>]

On Sun, Jun 20, 2010 at 06:49:56AM -0500, saeed bishara wrote:

> >> >> >> @@ -2666,6 +2668,9 @@ static int
> >> >> >mv643xx_eth_shared_probe(struct platform_device *pdev)
> >> >> >>          * Detect hardware parameters.
> >> >> >>          */
> >> >> >>         msp->t_clk = (pd != NULL && pd->t_clk != 0) ? pd->t_clk
> >> >> >: 133000000;
> >> >> >> +       msp->tx_csum_limit = pd->tx_csum_limit ?
> >> >> >pd->tx_csum_limit : 9 * 1024;
> >> >> >> +       /* add header count so we can compare against skb->len */
> >> >> >> +       msp->tx_csum_limit += ETH_HLEN;
> >> >> >>         infer_hw_params(msp);
> >> >> >
> >> >> >Is the limit 9 * 1024 + 14 for the whole packet, or 9 *
> >> >1024 for the IP
> >> >> >part?
> >> >>
> >> >> the limit is for the IP part, but I thought that adding the
> >> >header length, then comparing agains skb->len will be the
> >> >same. What do you suggest?
> >> >
> >> >Right, but for the header length you take 14, while if to include VLAN
> >> >tags or DSA tags the header might actually be longer -- how does that
> >> >affect the ability of the hardware to compute the checksum?
> >> >
> >> >I.e. is the restriction "total packet length must be < N + 14 bytes"
> >> >or is it "the IP part must be < N bytes and it doesn't matter whether
> >> >there are VLAN tags or not"?
> >>
> >> It's the second option: "the IP part must be < N" regardless to the L2 header.
> >
> > Then I suppose you want to do something like:
> >
> >        int hdr_len;
> >        int tag_bytes;
> >
> >        hdr_len = (void *)ip_hdr(skb) - (void *)skb->data;
> >        tag_bytes = hdr_len - 4;
>
> that should be 14 instead of 4, right?

Yes, sorry.


> >        if (skb->len - hdr_len > the limit || unlikely(tag_bytes & ~12)) {
> >
> > (And then keep the limit as maximum number of bytes in the packet
> > starting from the IP header part.)
>
> ok. here is the updated patch:
> >From 009530d4220dd710737d5a357629edcbc102727a Mon Sep 17 00:00:00 2001
> From: Saeed Bishara <saeed@xxxxxxxxxxx>
> Date: Sun, 6 Jun 2010 14:40:49 +0300
> Subject: [PATCH 1/2 v3] mv643xx_eth: use sw csum for big packets
> 
> Some controllers (KW, Dove) limits the TX IP/layer4 checksum
> offloading to a max size.
> 
> Signed-off-by: Saeed Bishara <saeed@xxxxxxxxxxx>

Looks good!

Acked-by: Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>
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