netdev - Re: [RFC 6/9] staging: dpaa2-switch: add .ndo_start

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Date:   Wed, 4 Nov 2020 23:27:00 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Ioana Ciornei <ciorneiioana@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Lunn <andrew@...n.ch>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Ioana Ciornei <ioana.ciornei@....com>
Subject: Re: [RFC 6/9] staging: dpaa2-switch: add .ndo_start_xmit() callback

On Wed, Nov 04, 2020 at 06:57:17PM +0200, Ioana Ciornei wrote:
> From: Ioana Ciornei <ioana.ciornei@....com>
> 
> Implement the .ndo_start_xmit() callback for the switch port interfaces.
> For each of the switch ports, gather the corresponding queue
> destination ID (QDID) necessary for Tx enqueueing.
> 
> We'll reserve 64 bytes for software annotations, where we keep a skb
> backpointer used on the Tx confirmation side for releasing the allocated
> memory. At the moment, we only support linear skbs.
> 
> Signed-off-by: Ioana Ciornei <ioana.ciornei@....com>
> ---
> @@ -554,8 +560,10 @@ static int dpaa2_switch_port_open(struct net_device *netdev)
>  	struct ethsw_core *ethsw = port_priv->ethsw_data;
>  	int err;
>  
> -	/* No need to allow Tx as control interface is disabled */
> -	netif_tx_stop_all_queues(netdev);
> +	if (!dpaa2_switch_has_ctrl_if(port_priv->ethsw_data)) {
> +		/* No need to allow Tx as control interface is disabled */
> +		netif_tx_stop_all_queues(netdev);

Personal taste probably, but you could remove the braces here.

> +	}
>  
>  	/* Explicitly set carrier off, otherwise
>  	 * netif_carrier_ok() will return true and cause 'ip link show'
> @@ -610,15 +618,6 @@ static int dpaa2_switch_port_stop(struct net_device *netdev)
>  	return 0;
>  }
>  
> +static netdev_tx_t dpaa2_switch_port_tx(struct sk_buff *skb,
> +					struct net_device *net_dev)
> +{
> +	struct ethsw_port_priv *port_priv = netdev_priv(net_dev);
> +	struct ethsw_core *ethsw = port_priv->ethsw_data;
> +	int retries = DPAA2_SWITCH_SWP_BUSY_RETRIES;
> +	struct dpaa2_fd fd;
> +	int err;
> +
> +	if (!dpaa2_switch_has_ctrl_if(ethsw))
> +		goto err_free_skb;
> +
> +	if (unlikely(skb_headroom(skb) < DPAA2_SWITCH_NEEDED_HEADROOM)) {
> +		struct sk_buff *ns;
> +
> +		ns = skb_realloc_headroom(skb, DPAA2_SWITCH_NEEDED_HEADROOM);

Looks like this passion for skb_realloc_headroom runs in the company?
Few other drivers use it, and Claudiu just had a bug with that in gianfar.
Luckily what saves you from the same bug is the skb_unshare from right below.
Maybe you could use skb_cow_head and simplify this a bit?

> +		if (unlikely(!ns)) {
> +			netdev_err(net_dev, "Error reallocating skb headroom\n");
> +			goto err_free_skb;
> +		}
> +		dev_kfree_skb(skb);

Please use dev_consume_skb_any here, as it's not error path. Or, if you
use skb_cow_head, only the skb data will be reallocated, not the skb
structure itself, so there will be no consume_skb in that case at all,
another reason to simplify.

> +		skb = ns;
> +	}
> +
> +	/* We'll be holding a back-reference to the skb until Tx confirmation */
> +	skb = skb_unshare(skb, GFP_ATOMIC);
> +	if (unlikely(!skb)) {
> +		/* skb_unshare() has already freed the skb */
> +		netdev_err(net_dev, "Error copying the socket buffer\n");
> +		goto err_exit;
> +	}
> +
> +	if (skb_is_nonlinear(skb)) {
> +		netdev_err(net_dev, "No support for non-linear SKBs!\n");

Rate-limit maybe?
And what is the reason for no non-linear skb's? Too much code to copy
from dpaa2-eth?

> +		goto err_free_skb;
> +	}
> +
> +	err = dpaa2_switch_build_single_fd(ethsw, skb, &fd);
> +	if (unlikely(err)) {
> +		netdev_err(net_dev, "ethsw_build_*_fd() %d\n", err);
> +		goto err_free_skb;
> +	}
> +
> +	do {
> +		err = dpaa2_io_service_enqueue_qd(NULL,
> +						  port_priv->tx_qdid,
> +						  8, 0, &fd);
> +		retries--;
> +	} while (err == -EBUSY && retries);
> +
> +	if (unlikely(err < 0)) {
> +		dpaa2_switch_free_fd(ethsw, &fd);
> +		goto err_exit;
> +	}
> +
> +	return NETDEV_TX_OK;
> +
> +err_free_skb:
> +	dev_kfree_skb(skb);
> +err_exit:
> +	return NETDEV_TX_OK;
> +}
> +
> diff --git a/drivers/staging/fsl-dpaa2/ethsw/ethsw.h b/drivers/staging/fsl-dpaa2/ethsw/ethsw.h
> index bd24be2c6308..b267c04e2008 100644
> --- a/drivers/staging/fsl-dpaa2/ethsw/ethsw.h
> +++ b/drivers/staging/fsl-dpaa2/ethsw/ethsw.h
> @@ -66,6 +66,19 @@
>   */
>  #define DPAA2_SWITCH_SWP_BUSY_RETRIES		1000
>  
> +/* Hardware annotation buffer size */
> +#define DPAA2_SWITCH_HWA_SIZE		64
> +/* Software annotation buffer size */
> +#define DPAA2_SWITCH_SWA_SIZE		64
> +
> +#define DPAA2_SWITCH_TX_BUF_ALIGN	64

Could you align all of these to the "1000" from DPAA2_SWITCH_SWP_BUSY_RETRIES?

> +
> +#define DPAA2_SWITCH_TX_DATA_OFFSET \
> +	(DPAA2_SWITCH_HWA_SIZE + DPAA2_SWITCH_SWA_SIZE)
> +
> +#define DPAA2_SWITCH_NEEDED_HEADROOM \
> +	(DPAA2_SWITCH_TX_DATA_OFFSET + DPAA2_SWITCH_TX_BUF_ALIGN)
> +

Ironically, you create a definition for NEEDED_HEADROOM but you do not
set dev->needed_headroom.