Copy SKB to USART DMA - c

Question

Hello i am writing a network device driver. I already setup the USART.
i registrated the Network device in the Probe funktion like this:
my_net = alloc_etherdev(sizeof(struct stm32_port));
if (my_net == NULL){
return ENOMEM;
}
/* Set MAC address */
eth_hw_addr_random(my_net);
my_net -> netdev_ops = &my_net_ops;
SET_NETDEV_DEV(my_net, &pdev->dev);
return register_netdev(my_net);
now in the ndo_start_xmit methode my kernal always gets a panic when i try to copy the socked puffer to the CPU dma adress
static int my_net_send(struct sk_buff *skb, struct net_device *ndev)
{
//struct stm32_port *priv = netdev_priv(ndev);
//unsigned int entry = TX_BUF_L;
//unsigned int entry = tp->cur_tx;
struct stm32_port *lp = netdev_priv(ndev);
struct sk_buff *sk_buff;
unsigned pktlen = skb->len;
/* adjust the packet length to min. required
* and hope that the buffer is large enough
* to provide some random data.
*/
if (pktlen < ETH_ZLEN)
{
if (skb_padto(skb, ETH_ZLEN))
return NETDEV_TX_OK;
pktlen = ETH_ZLEN;
}
//pr_info("do_start_xmit(skb=%p, ndev=%p) len=%u\n", skb, ndev, pktlen);
netif_stop_queue(ndev);
//send
if (lp->tx_dma_busy)
return NETDEV_TX_BUSY;
//pr_info("copy to usart: %s", skb->data);
sk_buff = skb_get(skb);
pr_info("copy to usart: %s", sk_buff->data);
memcpy(&lp->tx_buf[0], sk_buff, pktlen);
//dma_addr_t dma_addr = dma_map_single(ndev->d, skb->data, skb->len, DMA_TO_DEVICE);
ndev->stats.tx_packets++;
ndev->stats.tx_bytes += pktlen;
dev_kfree_skb (skb);
netif_start_queue(ndev);
return NETDEV_TX_OK;
}
struct stm32_port {
struct uart_port port;
struct clk *clk;
struct stm32_usart_info *info;
struct dma_chan *rx_ch; /* dma rx channel */
dma_addr_t rx_dma_buf; /* dma rx buffer bus address */
unsigned char *rx_buf; /* dma rx buffer cpu address */
struct dma_chan *tx_ch; /* dma tx channel */
dma_addr_t tx_dma_buf; /* dma tx buffer bus address */
unsigned char *tx_buf; /* dma tx buffer cpu address */
u32 cr1_irq; /* USART_CR1_RXNEIE or RTOIE */
u32 cr3_irq; /* USART_CR3_RXFTIE */
int last_res;
bool tx_dma_busy; /* dma tx busy */
bool hw_flow_control;
bool fifoen;
int wakeirq;
int rdr_mask; /* receive data register mask */
struct mctrl_gpios *gpios; /* modem control gpios */
};
Whats the correct way to copy the socked puffer. Sorry i am new to device drivers.