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Date: Tue, 2 Feb 2021 17:45:09 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Loic Poulain <loic.poulain@...aro.org>
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
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.
nit: previously
>
> This change adds re-aggregation mechanism using skb chaining, via
> skb frag_list.
>
> A warning (once) is printed since this behavior usually comes from
> a misconfiguration of the device (e.g. modem MTU).
>
> Signed-off-by: Loic Poulain <loic.poulain@...aro.org>
Only one real question wrt stats. Otherwise looks good to me, thanks.
> ---
> v2: use zero-copy skb chaining instead of skb_copy_expand.
>
> drivers/net/mhi_net.c | 79 ++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 69 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/mhi_net.c b/drivers/net/mhi_net.c
> index 4f512531..be39779 100644
> --- a/drivers/net/mhi_net.c
> +++ b/drivers/net/mhi_net.c
> @@ -32,6 +32,8 @@ struct mhi_net_stats {
> struct mhi_net_dev {
> struct mhi_device *mdev;
> struct net_device *ndev;
> + struct sk_buff *skbagg_head;
> + struct sk_buff *skbagg_tail;
> struct delayed_work rx_refill;
> struct mhi_net_stats stats;
> u32 rx_queue_sz;
> @@ -132,6 +134,37 @@ static void mhi_net_setup(struct net_device *ndev)
> ndev->tx_queue_len = 1000;
> }
>
> +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.
> + * paged data (nr_frags = 0), so it represents the size of chained skbs,
> + * This way, net core will consider skb->frag_list.
> + */
> + head->len += skb->len;
> + head->data_len += skb->len;
> + head->truesize += skb->truesize;
> +
> + mhi_netdev->skbagg_tail = skb;
> +
> + return mhi_netdev->skbagg_head;
> +}
> +
> 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.
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