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
[off-list ref] wrote:

On Tue, Feb 2, 2021 at 11:08 AM Loic Poulain [off-list ref] wrote:

quoted

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.

[...]

quoted

+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?

quoted

+       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.

[...]

quoted

 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