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From:   Alexander Lobakin <alexandr.lobakin@intel.com>
To:     Felix Fietkau <nbd@nbd.name>
Cc:     Alexander Lobakin <alexandr.lobakin@intel.com>,
        netdev@vger.kernel.org, John Crispin <john@phrozen.org>,
        Sean Wang <sean.wang@mediatek.com>,
        Mark Lee <Mark-MC.Lee@mediatek.com>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/5] net: ethernet: mtk_eth_soc: work around issue with sending small fragments
Date:   Thu, 24 Nov 2022 18:54:10 +0100
Message-Id: <20221124175410.5684-1-alexandr.lobakin@intel.com>
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From: Felix Fietkau <nbd@nbd.name>
Date: Wed, 23 Nov 2022 10:57:52 +0100

> When frames are sent with very small fragments, the DMA engine appears to
> lock up and transmit attempts time out. Fix this by detecting the presence
> of small fragments and use skb_gso_segment + skb_linearize to deal with
> them

Nit: all of your commit messages don't have a trailing dot (.), not
sure if it's important, but my eye is missing it definitely :D

skb_gso_segment() and skb_linearize() are slow as hell. I think you
can do it differently. I guess only the first (head) and the last
frag can be so small, right?

So, if a frag from shinfo->frags is less than 16, get a new frag of
the minimum acceptable size via netdev_alloc_frag(), copy the data
to it and pad the rest with zeroes. Then increase skb->len and
skb->data_len, skb_frag_unref() the current, "invalid" frag and
replace the pointer to the new frag. I didn't miss anything I
believe... Zero padding the tail is usual thing for NICs. skb frag
substitution is less common, but should be legit.

If skb_headlen() is less than 16, try doing pskb_may_pull() +
__skb_pull() at first. The argument would be `16 - headlen`. If
pskb_may_pull() returns false, then yeah, you have no choice other
than segmenting and linearizing ._.

> 
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
>  drivers/net/ethernet/mediatek/mtk_eth_soc.c | 36 +++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)

[...]

>  	if (unlikely(atomic_read(&ring->free_count) <= ring->thresh))
>  		netif_tx_stop_all_queues(dev);
> -- 
> 2.38.1

Thanks,
Olek