The following code is an example for zero-copy RX ethernet driver: 
typedef struct my_custom_pbuf
{
   void* dma_descriptor;
} my_custom_pbuf_t;
void my_pbuf_free_custom(void* p)
{
  my_custom_pbuf_t* my_puf = (my_custom_pbuf_t*)p;
  
  
  invalidate_cpu_cache(p->payload, p->tot_len);
  free_rx_dma_descriptor(my_pbuf->dma_descriptor);
}
void eth_rx_irq()
{
  dma_descriptor*   dma_desc = get_RX_DMA_descriptor_from_ethernet();
  my_pbuf->p.custom_free_function = my_pbuf_free_custom;
  my_pbuf->dma_descriptor         = dma_desc;
  invalidate_cpu_cache(dma_desc->rx_data, dma_desc->rx_length);
  
     dma_desc->rx_length,
     &my_pbuf->p,
     dma_desc->rx_data,
     dma_desc->max_buffer_size);
  }
}