Subject: Re: [PATCH net-next] net: hns3: Add support to change MTU in hardware & netdev

From: Andrew Lunn <[email-masked]>
Date: Fri, 18 Aug 2017 15:31:28 +0200

On Fri, Aug 18, 2017 at 12:35:58PM +0100, Salil Mehta wrote:
> This patch adds the following support to the HNS3 driver:
> 1. Support to change the Maximum Transmission Unit of a
>    netdevice and of a port in hardware .
> 2. Initializes the supported MTU range for the netdevice.
> 
> Signed-off-by: lipeng <[email-masked]>
> Signed-off-by: Salil Mehta <[email-masked]>
> ---
>  .../net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c | 46 ++++++++++++++++++++++
>  .../net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.h |  1 +
>  2 files changed, 47 insertions(+)
> 
> diff --git a/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c b/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c
> index e731f87..8e3711e 100644
> --- a/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c
> +++ b/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c
> @@ -1278,11 +1278,53 @@ static int hns3_ndo_set_vf_vlan(struct net_device *netdev, int vf, u16 vlan,
>  	return ret;
>  }
>  
> +static int hns3_nic_change_mtu(struct net_device *netdev, int new_mtu)
> +{
> +	struct hns3_nic_priv *priv = netdev_priv(netdev);
> +	struct hnae3_handle *h = priv->ae_handle;
> +	bool if_running = netif_running(netdev);
> +	int ret;
> +
> +	/* no change in MTU */
> +	if (new_mtu == netdev->mtu)
> +		return 0;

Hi Salil

It is worth reading the core code:

http://elixir.free-electrons.com/linux/latest/source/net/core/dev.c#L6713

 +
> +	if (!h->ae_algo->ops->set_mtu)
> +		return -ENOTSUPP;
> +
> +	/* if this was called with netdev up then bring netdevice down */
> +	if (if_running) {
> +		(void)hns3_nic_net_stop(netdev);
> +		msleep(100);
> +	}
> +
> +	ret = h->ae_algo->ops->set_mtu(h, new_mtu);
> +	if (ret) {
> +		netdev_err(netdev, "failed to change MTU in hardware %d\n",
> +			   ret);
> +		return ret;
> +	}
> +
> +	/* assign newly changed mtu to netdevice as well */
> +	netdev->mtu = new_mtu;

http://elixir.free-electrons.com/linux/latest/source/net/core/dev.c#L6698

> +
> +	/* if the netdev was running earlier, bring it up again */
> +	if (if_running) {
> +		if (hns3_nic_net_open(netdev)) {
> +			netdev_err(netdev, "MTU, couldnt up netdev again\n");
> +			ret = -EINVAL;
> +		}
> +	}
> +
> +	return ret;
> +}
> +
>  static const struct net_device_ops hns3_nic_netdev_ops = {
>  	.ndo_open		= hns3_nic_net_open,
>  	.ndo_stop		= hns3_nic_net_stop,
>  	.ndo_start_xmit		= hns3_nic_net_xmit,
>  	.ndo_set_mac_address	= hns3_nic_net_set_mac_address,
> +	.ndo_change_mtu		= hns3_nic_change_mtu,
>  	.ndo_set_features	= hns3_nic_set_features,
>  	.ndo_get_stats64	= hns3_nic_get_stats64,
>  	.ndo_setup_tc		= hns3_nic_setup_tc,
> @@ -2752,6 +2794,10 @@ static int hns3_client_init(struct hnae3_handle *handle)
>  		goto out_reg_netdev_fail;
>  	}
>  
> +	/* MTU range: 68 - 9706 */
> +	netdev->min_mtu = ETH_MIN_MTU;

http://elixir.free-electrons.com/linux/latest/source/net/ethernet/eth.c#L361

> +	netdev->max_mtu = HNS3_MAX_MTU - (ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN);
> +
>  	return ret;
>  
>  out_reg_netdev_fail:
> diff --git a/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.h b/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.h
> index a6e8f15..7e87461 100644
> --- a/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.h
> +++ b/drivers/net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.h
> @@ -76,6 +76,7 @@ enum hns3_nic_state {
>  #define HNS3_RING_NAME_LEN			16
>  #define HNS3_BUFFER_SIZE_2048			2048
>  #define HNS3_RING_MAX_PENDING			32768
> +#define HNS3_MAX_MTU				9728

It seems odd that it does not already exists somewhere. The core does
not enforce the MTU.  You could be passed a frame which is bigger. So
you should check before trying to pass something to the hardware which
the hardware cannot handle.

    Andrew