netdev - Re: [PATCH net-next 1/2] net: typhoon: implement ndo_features

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Date:   Fri, 4 Feb 2022 20:26:58 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] net: typhoon: implement ndo_features_check method

On Fri, Feb 4, 2022 at 7:52 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu,  3 Feb 2022 10:02:26 -0800 Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Instead of disabling TSO if MAX_SKB_FRAGS > 32, implement
> > ndo_features_check() method for this driver.
> >
> > If skb has more than 32 frags, use the following heuristic:
> >
> > 1) force GSO for gso packets.
> > 2) Otherwise force linearization.
> >
> > Most locally generated packets will use a small number
> > of fragments anyway.
> >
> > For forwarding workloads, we can limit gro_max_size at ingress,
> > we might also implement gro_max_segs if needed.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > ---
> >  drivers/net/ethernet/3com/typhoon.c | 23 ++++++++++++++++++-----
> >  1 file changed, 18 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/3com/typhoon.c b/drivers/net/ethernet/3com/typhoon.c
> > index 8aec5d9fbfef2803c181387537300502a937caf0..67b1a1f439d841ed0ed0f620e9477607ac6e2fae 100644
> > --- a/drivers/net/ethernet/3com/typhoon.c
> > +++ b/drivers/net/ethernet/3com/typhoon.c
> > @@ -138,11 +138,6 @@ MODULE_PARM_DESC(use_mmio, "Use MMIO (1) or PIO(0) to access the NIC. "
> >  module_param(rx_copybreak, int, 0);
> >  module_param(use_mmio, int, 0);
> >
> > -#if defined(NETIF_F_TSO) && MAX_SKB_FRAGS > 32
> > -#warning Typhoon only supports 32 entries in its SG list for TSO, disabling TSO
> > -#undef NETIF_F_TSO
> > -#endif
> > -
> >  #if TXLO_ENTRIES <= (2 * MAX_SKB_FRAGS)
> >  #error TX ring too small!
> >  #endif
> > @@ -2261,9 +2256,27 @@ typhoon_test_mmio(struct pci_dev *pdev)
> >       return mode;
> >  }
> >
> > +#if MAX_SKB_FRAGS > 32
> > +static netdev_features_t typhoon_features_check(struct sk_buff *skb,
> > +                                             struct net_device *dev,
> > +                                             netdev_features_t features)
> > +{
> > +     if (skb_shinfo(skb)->nr_frags > 32) {
> > +             if (skb_is_gso(skb))
> > +                     features &= ~NETIF_F_GSO_MASK;
> > +             else
> > +                     features &= ~NETIF_F_SG;
>
> Should we always clear SG? If we want to make the assumption that
> non-gso skbs are never this long (like the driver did before) then
> we should never clear SG. If we do we risk one of the gso-generated
> segs will also be longer than 32 frags.

If I read the comment (deleted in this patch), it seems the 32 limits
is about TSO only ?

#warning Typhoon only supports 32 entries in its SG list for TSO, disabling TSO

This is why I chose this implementation.

>
> Thought I should ask.
>
> > +     }
> > +     return features;
>
> return vlan_features_check(skb, features) ?

Hmm... not sure why we duplicate vlan_features_check() &
vxlan_features_check() in all ndo_features_check() handlers :/

>
> > +}
> > +#endif
> > +
> >  static const struct net_device_ops typhoon_netdev_ops = {
> >       .ndo_open               = typhoon_open,
> >       .ndo_stop               = typhoon_close,
> > +#if MAX_SKB_FRAGS > 32
> > +     .ndo_features_check     = typhoon_features_check,
> > +#endif
> >       .ndo_start_xmit         = typhoon_start_tx,
> >       .ndo_set_rx_mode        = typhoon_set_rx_mode,
> >       .ndo_tx_timeout         = typhoon_tx_timeout,
>