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Date: Tue, 10 May 2022 15:37:24 +0300 From: Vladimir Oltean <olteanv@...il.com> To: Felix Fietkau <nbd@....name> Cc: Sean Wang <sean.wang@...iatek.com>, Landen Chao <Landen.Chao@...iatek.com>, DENG Qingfang <dqfext@...il.com>, Andrew Lunn <andrew@...n.ch>, Vivien Didelot <vivien.didelot@...il.com>, Florian Fainelli <f.fainelli@...il.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Matthias Brugger <matthias.bgg@...il.com>, netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets On Tue, May 10, 2022 at 11:40:13AM +0200, Felix Fietkau wrote: > Padding for transmitted packets needs to account for the special tag. > With not enough padding, garbage bytes are inserted by the switch at the > end of small packets. I don't think padding bytes are guaranteed to be zeroes. Aren't they discarded? What is the issue? > > Fixes: 5cd8985a1909 ("net-next: dsa: add Mediatek tag RX/TX handler") > Signed-off-by: Felix Fietkau <nbd@....name> > --- > net/dsa/tag_mtk.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/net/dsa/tag_mtk.c b/net/dsa/tag_mtk.c > index 415d8ece242a..1d1f9dbd9e93 100644 > --- a/net/dsa/tag_mtk.c > +++ b/net/dsa/tag_mtk.c > @@ -25,6 +25,14 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb, > u8 xmit_tpid; > u8 *mtk_tag; > > + /* The Ethernet switch we are interfaced with needs packets to be at > + * least 64 bytes (including FCS) otherwise their padding might be > + * corrupted. With tags enabled, we need to make sure that packets are > + * at least 68 bytes (including FCS and tag). > + */ > + if (__skb_put_padto(skb, ETH_ZLEN + MTK_HDR_LEN, false)) > + return NULL; > + > /* Build the special tag after the MAC Source Address. If VLAN header > * is present, it's required that VLAN header and special tag is > * being combined. Only in this way we can allow the switch can parse > -- > 2.36.1 >
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