netdev - Re: [PATCH v4 1/2] net: Add mhi-net driver

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Date:   Thu, 15 Oct 2020 15:29:24 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, Jakub Kicinski <kuba@...nel.org>,
        netdev@...r.kernel.org, Hemant Kumar <hemantk@...eaurora.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: Re: [PATCH v4 1/2] net: Add mhi-net driver

On Thu, 15 Oct 2020 at 14:41, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 10/15/20 12:31 PM, Loic Poulain wrote:
> > This patch adds a new network driver implementing MHI transport for
> > network packets. Packets can be in any format, though QMAP (rmnet)
> > is the usual protocol (flow control + PDN mux).
> >
> > It support two MHI devices, IP_HW0 which is, the path to the IPA
> > (IP accelerator) on qcom modem, And IP_SW0 which is the software
> > driven IP path (to modem CPU).
> >
> >
> > +static int mhi_ndo_xmit(struct sk_buff *skb, struct net_device *ndev)
> > +{
> > +     struct mhi_net_dev *mhi_netdev = netdev_priv(ndev);
> > +     struct mhi_device *mdev = mhi_netdev->mdev;
> > +     int err;
> > +
> > +     skb_tx_timestamp(skb);
> > +
> > +     /* mhi_queue_skb is not thread-safe, but xmit is serialized by the
> > +      * network core. Once MHI core will be thread save, migrate to
> > +      * NETIF_F_LLTX support.
> > +      */
> > +     err = mhi_queue_skb(mdev, DMA_TO_DEVICE, skb, skb->len, MHI_EOT);
> > +     if (err == -ENOMEM) {
> > +             netif_stop_queue(ndev);
>
> If you return NETDEV_TX_BUSY, this means this skb will be requeues,
> then sent again right away, and this will potentially loop forever.

The TX queue is stopped in that case, so the net core will not loop, right?

>
> Also skb_tx_timestamp() would be called multiple times.

OK so I'm going to remove that, maybe the MHI layer should mark
timestamp instead.

>
> I suggest you drop the packet instead.
>
> > +             return NETDEV_TX_BUSY;
> > +     } else if (unlikely(err)) {
> > +             net_err_ratelimited("%s: Failed to queue TX buf (%d)\n",
> > +                                 ndev->name, err);
> > +             mhi_netdev->stats.tx_dropped++;
> > +             kfree_skb(skb);
> > +     }
> > +
> > +     return NETDEV_TX_OK;
> > +}
> > +
> > +static void mhi_ndo_get_stats64(struct net_device *ndev,
> > +                             struct rtnl_link_stats64 *stats)
> > +{
> > +     struct mhi_net_dev *mhi_netdev = netdev_priv(ndev);
> > +
> > +     stats->rx_packets = mhi_netdev->stats.rx_packets;
> > +     stats->rx_bytes = mhi_netdev->stats.rx_bytes;
> > +     stats->rx_errors = mhi_netdev->stats.rx_errors;
> > +     stats->rx_dropped = mhi_netdev->stats.rx_dropped;
> > +     stats->tx_packets = mhi_netdev->stats.tx_packets;
> > +     stats->tx_bytes = mhi_netdev->stats.tx_bytes;
> > +     stats->tx_errors = mhi_netdev->stats.tx_errors;
> > +     stats->tx_dropped = mhi_netdev->stats.tx_dropped;
> > +}
>
> This code is not correct on a 32bit kernel, since u64 values
> can not be safely be read without extra care.
>
>