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

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Date:   Thu, 15 Oct 2020 20:56:20 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Loic Poulain <loic.poulain@...aro.org>
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 10/15/20 3:29 PM, Loic Poulain wrote:
> 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?

-ENOMEM suggests a memory allocation failed.

What is going to restart the queue when memory is available ?

-ENOMEM seems weird if used for queue being full.

> 
>>
>> 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.

Yes, probably.