Subject: Re: [PATCH net-next v5 10/10] net: axienet: Introduce dmaengine support

From: Jakub Kicinski <[email-masked]>
Date: Tue, 8 Aug 2023 15:48:53 -0700

On Mon, 7 Aug 2023 11:21:49 +0530 Radhey Shyam Pandey wrote:
> +struct axi_skbuff {
> +	struct scatterlist sgl[MAX_SKB_FRAGS + 1];
> +	struct dma_async_tx_descriptor *desc;
> +	dma_addr_t dma_address;
> +	struct sk_buff *skb;
> +	struct list_head lh;
> +	int sg_len;
> +} __packed;

Why __packed?

> +static netdev_tx_t
> +axienet_start_xmit_dmaengine(struct sk_buff *skb, struct net_device *ndev)
> +{
> +	struct dma_async_tx_descriptor *dma_tx_desc = NULL;
> +	struct axienet_local *lp = netdev_priv(ndev);
> +	u32 app[DMA_NUM_APP_WORDS] = {0};
> +	struct axi_skbuff *axi_skb;
> +	u32 csum_start_off;
> +	u32 csum_index_off;
> +	int sg_len;
> +	int ret;
> +
> +	sg_len = skb_shinfo(skb)->nr_frags + 1;
> +	axi_skb = kmem_cache_zalloc(lp->skb_cache, GFP_KERNEL);
> +	if (!axi_skb)
> +		return NETDEV_TX_BUSY;

Drop on error, you're not stopping the queue correctly, just drop,
return OK and avoid bugs.

> +static inline int axienet_init_dmaengine(struct net_device *ndev)

Why inline? Please remove the spurious inline annotations.

> +{
> +	struct axienet_local *lp = netdev_priv(ndev);
> +	int i, ret;
> +
> +	lp->tx_chan = dma_request_chan(lp->dev, "tx_chan0");
> +	if (IS_ERR(lp->tx_chan)) {
> +		ret = PTR_ERR(lp->tx_chan);
> +		return dev_err_probe(lp->dev, ret, "No Ethernet DMA (TX) channel found\n");
> +	}
> +
> +	lp->rx_chan = dma_request_chan(lp->dev, "rx_chan0");
> +	if (IS_ERR(lp->rx_chan)) {
> +		ret = PTR_ERR(lp->rx_chan);
> +		dev_err_probe(lp->dev, ret, "No Ethernet DMA (RX) channel found\n");
> +		goto err_dma_request_rx;

name labels after the target, not the source. Makes it a ton easier 
to maintain sanity when changing the code later.

> +	}
> +
> +	lp->skb_cache = kmem_cache_create("ethernet", sizeof(struct axi_skbuff),
> +					  0, 0, NULL);

Why create a cache ?
Isn't it cleaner to create a fake ring buffer of sgl? Most packets will
not have MAX_SKB_FRAGS of memory. On a ring buffer you can use only as
many sg entries as the packet requires. Also no need to alloc/free.

> +	if (!lp->skb_cache) {
> +		ret =  -ENOMEM;

double space, please fix everywhere

> +		goto err_kmem;

> @@ -2140,6 +2432,31 @@ static int axienet_probe(struct platform_device *pdev)
>  		}
>  		netif_napi_add(ndev, &lp->napi_rx, axienet_rx_poll);
>  		netif_napi_add(ndev, &lp->napi_tx, axienet_tx_poll);
> +	} else {
> +		struct xilinx_vdma_config cfg;
> +		struct dma_chan *tx_chan;
> +
> +		lp->eth_irq = platform_get_irq_optional(pdev, 0);

This can't fail?

> +		tx_chan = dma_request_chan(lp->dev, "tx_chan0");
> +
> +		if (IS_ERR(tx_chan)) {

no empty lines between call and its error check, please fix thru out

> +			ret = PTR_ERR(tx_chan);
> +			dev_err_probe(lp->dev, ret, "No Ethernet DMA (TX) channel found\n");
> +			goto cleanup_clk;
> +		}
-- 
pw-bot: cr