netdev - Re: [PATCH v3 net-next 09/12] net: dsa: tag_brcm: let DSA core deal with TX reallocation

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Date:   Tue, 3 Nov 2020 18:15:29 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        Christian Eggers <ceggers@...i.de>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [PATCH v3 net-next 09/12] net: dsa: tag_brcm: let DSA core deal
 with TX reallocation

On Tue, Nov 03, 2020 at 09:04:11AM -0800, Jakub Kicinski wrote:
> In a recent discussion I was wondering if it makes sense to add the
> padding len to struct net_device, with similar best-effort semantics
> to needed_*room. It'd be a u8, so little worry about struct size.

What would that mean in practice? Modify the existing alloc_skb calls
which have an expression e that depends on LL_RESERVED_SPACE(dev), into
max(e, dev->padding_len)? There's a lot of calls to alloc_skb to modify
though...

> You could also make sure DSA always provisions for padding if it has to
> reallocate, you don't need to actually pad:
> 
> @@ -568,6 +568,9 @@ static int dsa_realloc_skb(struct sk_buff *skb, struct net_device *dev)
>                 /* No reallocation needed, yay! */
>                 return 0;
>  
> +       if (skb->len < ETH_ZLEN)
> +               needed_tailroom += ETH_ZLEN;
> +
>         return pskb_expand_head(skb, needed_headroom, needed_tailroom,
>                                 GFP_ATOMIC);
>  }
> 
> That should save the realloc for all reasonable drivers while not
> costing anything (other than extra if()) to drivers which don't care.

DSA does already provision for padding if it has to reallocate, but only
for the case where it needs to add a frame header at the end of the skb
(i.e. "tail taggers"). My question here was whether there would be any
drawback to doing that for all types of switches, including ones that
might deal with padding in some other way (i.e. in hardware).