netdev - Re: [PATCH net v5 1/2] net: ethernet: cortina: Drop software checksum and TSO

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Date: Fri, 5 Jan 2024 15:36:28 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, Hans Ulli Kroll <ulli.kroll@...glemail.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Household Cang <canghousehold@....com>, Romain Gantois <romain.gantois@...tlin.com>, 
	netdev@...r.kernel.org
Subject: Re: [PATCH net v5 1/2] net: ethernet: cortina: Drop software checksum
 and TSO

On Fri, Jan 5, 2024 at 12:32 PM Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Tue, Jan 02, 2024 at 09:34:25PM +0100, Linus Walleij wrote:
> > @@ -1143,39 +1142,13 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
> >       struct gmac_txdesc *txd;
> >       skb_frag_t *skb_frag;
> >       dma_addr_t mapping;
> > -     unsigned short mtu;
> >       void *buffer;
> > -     int ret;
> > -
> > -     mtu  = ETH_HLEN;
> > -     mtu += netdev->mtu;
> > -     if (skb->protocol == htons(ETH_P_8021Q))
> > -             mtu += VLAN_HLEN;
> >
> > +     /* TODO: implement proper TSO using MTU in word3 */
> >       word1 = skb->len;
> > -     word3 = SOF_BIT;
> > -
> > -     if (word1 > mtu) {
> > -             word1 |= TSS_MTU_ENABLE_BIT;
> > -             word3 |= mtu;
> > -     }
> > +     word3 = SOF_BIT | skb->len;
> >
> > -     if (skb->len >= ETH_FRAME_LEN) {
> > -             /* Hardware offloaded checksumming isn't working on frames
> > -              * bigger than 1514 bytes. A hypothesis about this is that the
> > -              * checksum buffer is only 1518 bytes, so when the frames get
> > -              * bigger they get truncated, or the last few bytes get
> > -              * overwritten by the FCS.
> > -              *
> > -              * Just use software checksumming and bypass on bigger frames.
> > -              */
> > -             if (skb->ip_summed == CHECKSUM_PARTIAL) {
> > -                     ret = skb_checksum_help(skb);
> > -                     if (ret)
> > -                             return ret;
> > -             }
> > -             word1 |= TSS_BYPASS_BIT;
> > -     } else if (skb->ip_summed == CHECKSUM_PARTIAL) {
>
> So are you taking back the statement that "Hardware offloaded
> checksumming isn't working on frames bigger than 1514 bytes"?
>
> Have you increased the interface MTU beyond 1500, and tested with plain
> TCP (no DSA) on top of it? Who will provide the TCP checksum for them now?
>
> I don't understand why you remove the skb_checksum_help() call.
> It doesn't play nice with skb_is_gso() packets, agreed, but you removed
> the TSO netdev feature.

This TSO feature never possibly worked.

This was probably hidden because TCP retransmits non TSO packets eventually.

A TSO enabled driver must use/propagate skb_shinfo(skb)->gso_size
value to the TSO engine on the NIC.
Otherwise, this is absolutely broken.

Please look at my original suggestion. I think the plan is to try to
add back TSO in next release, with proper testing (ie not rely on TCP
resilience)

https://lore.kernel.org/netdev/CANn89iJLfxng1sYL5Zk0mknXpyYQPCp83m3KgD2KJ2_hKCpEUg@mail.gmail.com/