adding preallocated data to skb

A number of other OSs I worked with (namely windows and VxWorks) has this sometimes very convinient feature - you can allocate dma buffers and then, after DMA receive is finished, you can allocate an empty mblock (which is a VxWorks skb) and just add preallocated data to this empty mblock.

This is extremely convinient, especially for fragmented packets.

I could not find anything similar in Linux, however it seems to me that it should be quite streightforward to implement.

All that needs to be done is to break alloc_skb() into two functions, similar to the following :
(it's just a C pseudo code, it does not compile - only demonstrates an idea)

struct sk_buff *alloc_empty_skb() {
struct sk_buff *skb;

   if (in_interrupt() && (gfp_mask & __GFP_WAIT)) {
       static int count = 0;
       if (++count < 5) {
           printk(KERN_ERR "alloc_skb called nonatomically "
                  "from interrupt %p\n", NET_CALLER(size));
            BUG();
       }
       gfp_mask &= ~__GFP_WAIT;
   }

   /* Get the HEAD */
   skb = skb_head_from_pool();
   if (skb == NULL) {
       skb = kmem_cache_alloc(skbuff_head_cache, gfp_mask & ~__GFP_DMA);
       if (skb == NULL)
           goto nohead;
   }


   atomic_set(&skb->users, 1);

return skb;

nohead:
return NULL;
}

int skb_add_data(u8 * data, struct sk_buff * skb, int size) {
if (data == NULL)

       goto nodata;

/* XXX: does not include slab overhead */
skb->truesize = size + sizeof(struct sk_buff);

   /* Load the data pointers. */
   skb->head = data;
   skb->data = data;
   skb->tail = data;
   skb->end = data + size;

   /* Set up other state */
   skb->len = 0;
   skb->cloned = 0;
   skb->data_len = 0;

atomic_set(&(skb_shinfo(skb)->dataref), 1);
skb_shinfo(skb)->nr_frags = 0;
skb_shinfo(skb)->frag_list = NULL;
return 0;
}

Any ideas why it should/should not work ?
Anybody ever implemented anything similar ?

Alexander Sirotkin
SW Engineer

Texas Instruments
Broadband Communications Israel (BCIL)
Tel:  +972-9-9706587
________________________________________________________________________
"Those who do not understand Unix are condemned to reinvent it, poorly."
     -- Henry Spencer