[NET] back: Add a few comments to the goriest parts of the scatter-gather patch.

Signed-off-by: Keir Fraser <keir@xensource.com>

diff --git a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
index ca712d2a61e35c218478cbcb38d13b6c0b64f7c3..828b771d2938f2de0fcd51bdca9205311217fbb2 100644 (file)
--- a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c
@@ -543,17 +543,14 @@ static gnttab_map_grant_ref_t *netbk_get_requests(netif_t *netif,
        skb_frag_t *frags = shinfo->frags;
        netif_tx_request_t *txp;
        unsigned long pending_idx = *((u16 *)skb->data);
-       int nr_frags = shinfo->nr_frags;
        RING_IDX cons = netif->tx.req_cons + 1;
-       int i;
+       int i, start;
 
-       if ((unsigned long)shinfo->frags[0].page == pending_idx) {
-               frags++;
-               nr_frags--;
-       }
+       /* Skip first skb fragment if it is on same page as header fragment. */
+       start = ((unsigned long)shinfo->frags[0].page == pending_idx);
 
-       for (i = 0; i < nr_frags; i++) {
-               txp = RING_GET_REQUEST(&netif->tx, cons + i);
+       for (i = start; i < shinfo->nr_frags; i++) {
+               txp = RING_GET_REQUEST(&netif->tx, cons++);
                pending_idx = pending_ring[MASK_PEND_IDX(pending_cons++)];
 
                gnttab_set_map_op(mop++, MMAP_VADDR(pending_idx),
@@ -578,10 +575,9 @@ static int netbk_tx_check_mop(struct sk_buff *skb,
        netif_tx_request_t *txp;
        struct skb_shared_info *shinfo = skb_shinfo(skb);
        int nr_frags = shinfo->nr_frags;
-       int start;
-       int err;
-       int i;
+       int i, err, start;
 
+       /* Check status of header. */
        err = mop->status;
        if (unlikely(err)) {
                txp = &pending_tx_info[pending_idx].req;
@@ -595,44 +591,47 @@ static int netbk_tx_check_mop(struct sk_buff *skb,
                grant_tx_handle[pending_idx] = mop->handle;
        }
 
-       start = 0;
-       if ((unsigned long)shinfo->frags[0].page == pending_idx)
-               start++;
+       /* Skip first skb fragment if it is on same page as header fragment. */
+       start = ((unsigned long)shinfo->frags[0].page == pending_idx);
 
        for (i = start; i < nr_frags; i++) {
-               int newerr;
-               int j;
+               int j, newerr;
 
                pending_idx = (unsigned long)shinfo->frags[i].page;
 
+               /* Check error status: if okay then remember grant handle. */
                newerr = (++mop)->status;
                if (likely(!newerr)) {
                        set_phys_to_machine(
-                               __pa(MMAP_VADDR(pending_idx)) >> PAGE_SHIFT,
-                               FOREIGN_FRAME(mop->dev_bus_addr >> PAGE_SHIFT));
+                               __pa(MMAP_VADDR(pending_idx))>>PAGE_SHIFT,
+                               FOREIGN_FRAME(mop->dev_bus_addr>>PAGE_SHIFT));
                        grant_tx_handle[pending_idx] = mop->handle;
-
+                       /* Had a previous error? Invalidate this fragment. */
                        if (unlikely(err))
                                netif_idx_release(pending_idx);
                        continue;
                }
 
+               /* Error on this fragment: respond to client with an error. */
                txp = &pending_tx_info[pending_idx].req;
                make_tx_response(netif, txp->id, NETIF_RSP_ERROR);
                pending_ring[MASK_PEND_IDX(pending_prod++)] = pending_idx;
                netif_put(netif);
 
+               /* Not the first error? Preceding frags already invalidated. */
                if (err)
                        continue;
 
+               /* First error: invalidate header and preceding fragments. */
                pending_idx = *((u16 *)skb->data);
                netif_idx_release(pending_idx);
-
                for (j = start; j < i; j++) {
                        pending_idx = (unsigned long)shinfo->frags[i].page;
                        netif_idx_release(pending_idx);
                }
-               err |= newerr;
+
+               /* Remember the error: invalidate all subsequent fragments. */
+               err = newerr;
        }
 
        *mopp = mop + 1;