netdev - Re: [PATCH net-next v2 1/2] net: mhi-net: Add de-aggeration support

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Date:   Wed, 3 Feb 2021 08:27:49 +0100
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Sean Tranchetti <stranche@...eaurora.org>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
Subject: Re: [PATCH net-next v2 1/2] net: mhi-net: Add de-aggeration support

Hi Willem,

On Tue, 2 Feb 2021 at 23:45, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Tue, Feb 2, 2021 at 11:08 AM Loic Poulain <loic.poulain@...aro.org> wrote:
> >
> > When device side MTU is larger than host side MTU, the packets
> > (typically rmnet packets) are split over multiple MHI transfers.
> > In that case, fragments must be re-aggregated to recover the packet
> > before forwarding to upper layer.
> >
> > A fragmented packet result in -EOVERFLOW MHI transaction status for
> > each of its fragments, except the final one. Such transfer was
> > previoulsy considered as error and fragments were simply dropped.
[...]
> > +static struct sk_buff *mhi_net_skb_agg(struct mhi_net_dev *mhi_netdev,
> > +                                      struct sk_buff *skb)
> > +{
> > +       struct sk_buff *head = mhi_netdev->skbagg_head;
> > +       struct sk_buff *tail = mhi_netdev->skbagg_tail;
> > +
> > +       /* This is non-paged skb chaining using frag_list */
> > +
>
> no need for empty line?
>
> > +       if (!head) {
> > +               mhi_netdev->skbagg_head = skb;
> > +               return skb;
> > +       }
> > +
> > +       if (!skb_shinfo(head)->frag_list)
> > +               skb_shinfo(head)->frag_list = skb;
> > +       else
> > +               tail->next = skb;
> > +
> > +       /* data_len is normally the size of paged data, in our case there is no
>
> data_len is defined as the data excluding the linear len (ref:
> skb_headlen). That is not just paged data, but includes frag_list.

Ok, thanks for clarifying this, I'll remove the comment since it's
then a valid usage.

[...]
> >  static void mhi_net_dl_callback(struct mhi_device *mhi_dev,
> >                                 struct mhi_result *mhi_res)
> >  {
> > @@ -142,19 +175,42 @@ static void mhi_net_dl_callback(struct mhi_device *mhi_dev,
> >         free_desc_count = mhi_get_free_desc_count(mhi_dev, DMA_FROM_DEVICE);
> >
> >         if (unlikely(mhi_res->transaction_status)) {
> > -               dev_kfree_skb_any(skb);
> > -
> > -               /* MHI layer stopping/resetting the DL channel */
> > -               if (mhi_res->transaction_status == -ENOTCONN)
> > +               switch (mhi_res->transaction_status) {
> > +               case -EOVERFLOW:
> > +                       /* Packet can not fit in one MHI buffer and has been
> > +                        * split over multiple MHI transfers, do re-aggregation.
> > +                        * That usually means the device side MTU is larger than
> > +                        * the host side MTU/MRU. Since this is not optimal,
> > +                        * print a warning (once).
> > +                        */
> > +                       netdev_warn_once(mhi_netdev->ndev,
> > +                                        "Fragmented packets received, fix MTU?\n");
> > +                       skb_put(skb, mhi_res->bytes_xferd);
> > +                       mhi_net_skb_agg(mhi_netdev, skb);
> > +                       break;
> > +               case -ENOTCONN:
> > +                       /* MHI layer stopping/resetting the DL channel */
> > +                       dev_kfree_skb_any(skb);
> >                         return;
> > -
> > -               u64_stats_update_begin(&mhi_netdev->stats.rx_syncp);
> > -               u64_stats_inc(&mhi_netdev->stats.rx_errors);
> > -               u64_stats_update_end(&mhi_netdev->stats.rx_syncp);
> > +               default:
> > +                       /* Unknown error, simply drop */
> > +                       dev_kfree_skb_any(skb);
> > +                       u64_stats_update_begin(&mhi_netdev->stats.rx_syncp);
> > +                       u64_stats_inc(&mhi_netdev->stats.rx_errors);
> > +                       u64_stats_update_end(&mhi_netdev->stats.rx_syncp);
> > +               }
> >         } else {
> > +               skb_put(skb, mhi_res->bytes_xferd);
> > +
> > +               if (mhi_netdev->skbagg_head) {
> > +                       /* Aggregate the final fragment */
> > +                       skb = mhi_net_skb_agg(mhi_netdev, skb);
> > +                       mhi_netdev->skbagg_head = NULL;
> > +               }
> > +
> >                 u64_stats_update_begin(&mhi_netdev->stats.rx_syncp);
> >                 u64_stats_inc(&mhi_netdev->stats.rx_packets);
> > -               u64_stats_add(&mhi_netdev->stats.rx_bytes, mhi_res->bytes_xferd);
> > +               u64_stats_add(&mhi_netdev->stats.rx_bytes, skb->len);
>
> might this change stats? it will if skb->len != 0 before skb_put. Even
> if so, perhaps it doesn't matter.

Don't get that point, skb is the received MHI buffer, we simply set
its size because MHI core don't (skb->len is always 0 before put).
Then if it is part of a fragmented transfer we just do the extra
'skb = skb_agg+ skb', so skb->len should always be right here,
whether it's a standalone/linear packet or a multi-frag packet.

Regards,
Loic