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Subjectadding 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

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