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Date: Sat, 17 Oct 2020 15:11:52 -0700 From: Florian Fainelli <f.fainelli@...il.com> To: Vladimir Oltean <vladimir.oltean@....com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org> Cc: "andrew@...n.ch" <andrew@...n.ch>, "vivien.didelot@...il.com" <vivien.didelot@...il.com>, "kuba@...nel.org" <kuba@...nel.org>, Christian Eggers <ceggers@...i.de>, Kurt Kanzenbach <kurt@...utronix.de> Subject: Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure On 10/17/2020 3:01 PM, Vladimir Oltean wrote: > On Sun, Oct 18, 2020 at 12:36:00AM +0300, Vladimir Oltean wrote: >> diff --git a/net/dsa/slave.c b/net/dsa/slave.c >> index d4326940233c..790f5c8deb13 100644 >> --- a/net/dsa/slave.c >> +++ b/net/dsa/slave.c >> @@ -548,6 +548,36 @@ netdev_tx_t dsa_enqueue_skb(struct sk_buff *skb, struct net_device *dev) >> } >> EXPORT_SYMBOL_GPL(dsa_enqueue_skb); >> >> +static int dsa_realloc_skb(struct sk_buff *skb, struct net_device *dev) > > I forgot to actually pad the skb here, if it's a tail tagger, silly me. > The following changes should do the trick. > >> +{ >> + struct net_device *master = dsa_slave_to_master(dev); > > The addition of master->needed_headroom and master->needed_tailroom used > to be here, that's why this unused variable is here. > >> + struct dsa_slave_priv *p = netdev_priv(dev); >> + struct dsa_slave_stats *e; >> + int headroom, tailroom; > int padlen = 0, err; >> + >> + headroom = dev->needed_headroom; >> + tailroom = dev->needed_tailroom; >> + /* For tail taggers, we need to pad short frames ourselves, to ensure >> + * that the tail tag does not fail at its role of being at the end of >> + * the packet, once the master interface pads the frame. >> + */ >> + if (unlikely(tailroom && skb->len < ETH_ZLEN)) >> + tailroom += ETH_ZLEN - skb->len; > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > padlen = ETH_ZLEN - skb->len; > tailroom += padlen; >> + >> + if (likely(skb_headroom(skb) >= headroom && >> + skb_tailroom(skb) >= tailroom) && >> + !skb_cloned(skb)) >> + /* No reallocation needed, yay! */ >> + return 0; >> + >> + e = this_cpu_ptr(p->extra_stats); >> + u64_stats_update_begin(&e->syncp); >> + e->tx_reallocs++; >> + u64_stats_update_end(&e->syncp); >> + >> + return pskb_expand_head(skb, headroom, tailroom, GFP_ATOMIC); > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > err = pskb_expand_head(skb, headroom, tailroom, GFP_ATOMIC); > if (err < 0 || !padlen) > return err; > > return __skb_put_padto(skb, padlen, false); >> +} >> + >> static netdev_tx_t dsa_slave_xmit(struct sk_buff *skb, struct net_device *dev) >> { >> struct dsa_slave_priv *p = netdev_priv(dev); >> @@ -567,6 +597,11 @@ static netdev_tx_t dsa_slave_xmit(struct sk_buff *skb, struct net_device *dev) >> */ >> dsa_skb_tx_timestamp(p, skb); >> >> + if (dsa_realloc_skb(skb, dev)) { >> + kfree_skb(skb); >> + return NETDEV_TX_OK; >> + } >> + >> /* Transmit function may have to reallocate the original SKB, >> * in which case it must have freed it. Only free it here on error. >> */ >> @@ -1802,6 +1837,14 @@ int dsa_slave_create(struct dsa_port *port) >> slave_dev->netdev_ops = &dsa_slave_netdev_ops; >> if (ds->ops->port_max_mtu) >> slave_dev->max_mtu = ds->ops->port_max_mtu(ds, port->index); >> + /* Try to save one extra realloc later in the TX path (in the master) >> + * by also inheriting the master's needed headroom and tailroom. >> + * The 8021q driver also does this. >> + */ > > Also, this comment is bogus given the current code. It should be removed > from here, and... > >> + if (cpu_dp->tag_ops->tail_tag) >> + slave_dev->needed_tailroom = cpu_dp->tag_ops->overhead; >> + else >> + slave_dev->needed_headroom = cpu_dp->tag_ops->overhead; > ...put here, along with: > slave_dev->needed_headroom += master->needed_headroom; > slave_dev->needed_tailroom += master->needed_tailroom; Not positive you need that because you may be account for more head or tail room than necessary. For instance with tag_brcm.c and systemport.c we need 4 bytes of head room for the Broadcom tag and an additional 8 bytes for pushing the transmit status block descriptor in front of the Ethernet frame about to be transmitted. These additional 8 bytes are a requirement of the DSA master here and exist regardless of DSA being used, but we should not be propagating them to the DSA slave. -- Florian
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