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

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Date:   Fri, 4 Feb 2022 19:52:29 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/2] net: typhoon: implement ndo_features_check
 method

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.

Thought I should ask.

> +	}
> +	return features;

return vlan_features_check(skb, features) ?

> +}
> +#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,