Subject: Re: [PATCH net] net: ethernet: ti: cpsw: fix min eth packet size for non-switch use-cases

From: Ben Hutchings <[email-masked]>
Date: Fri, 11 Jun 2021 19:54:49 +0200

On Fri, Jun 11, 2021 at 04:27:32PM +0300, Grygorii Strashko wrote:
[...]
> --- a/drivers/net/ethernet/ti/cpsw_new.c
> +++ b/drivers/net/ethernet/ti/cpsw_new.c
> @@ -918,14 +918,17 @@ static netdev_tx_t cpsw_ndo_start_xmit(struct sk_buff *skb,
>  	struct cpts *cpts = cpsw->cpts;
>  	struct netdev_queue *txq;
>  	struct cpdma_chan *txch;
> +	unsigned int len;
>  	int ret, q_idx;
>  
> -	if (skb_padto(skb, CPSW_MIN_PACKET_SIZE)) {
> +	if (skb_padto(skb, priv->tx_packet_min)) {
>  		cpsw_err(priv, tx_err, "packet pad failed\n");
>  		ndev->stats.tx_dropped++;
>  		return NET_XMIT_DROP;
>  	}
>  
> +	len = skb->len < priv->tx_packet_min ? priv->tx_packet_min : skb->len;
> +
>  	if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP &&
>  	    priv->tx_ts_enabled && cpts_can_timestamp(cpts, skb))
>  		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
> @@ -937,7 +940,7 @@ static netdev_tx_t cpsw_ndo_start_xmit(struct sk_buff *skb,
>  	txch = cpsw->txv[q_idx].ch;
>  	txq = netdev_get_tx_queue(ndev, q_idx);
>  	skb_tx_timestamp(skb);
> -	ret = cpdma_chan_submit(txch, skb, skb->data, skb->len,
> +	ret = cpdma_chan_submit(txch, skb, skb->data, len,
>  				priv->emac_port);
>  	if (unlikely(ret != 0)) {
>  		cpsw_err(priv, tx_err, "desc submit failed\n");

This change is odd because cpdma_chan_submit() already pads the DMA
length.

Would it not make more sense to update cpdma_params::min_packet_size
instead of adding a second minimum?

[...]
> @@ -1686,6 +1690,7 @@ static int cpsw_dl_switch_mode_set(struct devlink *dl, u32 id,
>  
>  			priv = netdev_priv(sl_ndev);
>  			slave->port_vlan = vlan;
> +			priv->tx_packet_min = CPSW_MIN_PACKET_SIZE_VLAN;
>  			if (netif_running(sl_ndev))
>  				cpsw_port_add_switch_def_ale_entries(priv,
>  								     slave);
> @@ -1714,6 +1719,7 @@ static int cpsw_dl_switch_mode_set(struct devlink *dl, u32 id,
>  
>  			priv = netdev_priv(slave->ndev);
>  			slave->port_vlan = slave->data->dual_emac_res_vlan;
> +			priv->tx_packet_min = CPSW_MIN_PACKET_SIZE;
>  			cpsw_port_add_dual_emac_def_ale_entries(priv, slave);
>  		}
>
[...]

What happens if this races with the TX path?  Should there be a
netif_tx_lock() / netif_tx_unlock() around this change?

Ben.