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

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Date:   Thu, 15 Oct 2020 14:41:43 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Loic Poulain <loic.poulain@...aro.org>, davem@...emloft.net,
        kuba@...nel.org
Cc:     netdev@...r.kernel.org, hemantk@...eaurora.org,
        manivannan.sadhasivam@...aro.org, eric.dumazet@...il.com
Subject: Re: [PATCH v4 1/2] net: Add mhi-net driver



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.

Also skb_tx_timestamp() would be called multiple times.

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.