Strange dma map/unmap/memcpy dance in ipoib_cm_handle_rx_wc

Hello,

I need a bit of help in understanding the logic behind the following code sequence in ipoib_cm_handle_rx_wc:

newskb = ipoib_cm_alloc_rx_skb(dev, rx_ring, wr_id, frags,

                                       mapping, GFP_ATOMIC);

so Here a new skb is being allocated and pages constituting its fragments are being dma mapped.

ipoib_cm_dma_unmap_rx(priv, frags, rx_ring[wr_id].mapping);

immediately after that the very same mappings are being unmapped. And finally the resulting dma addresses from ipoib_cm_alloc_rx_skb are being copied via this call to memcpy:

memcpy(rx_ring[wr_id].mapping, mapping, (frags + 1) * sizeof *mapping);

Can someone explain what's the logic of doing the memcpy after the mappings have been destroyed, doesn't that mean the dma addresses stored in rx_ring[wr_id].mapping are invalid?

Regards,
Nikolay
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Into the local variable mapping.

No, these are the saved mappings of the skb that has the same ring slot as the one we are going to take.

From our local variable to the saved mapping slot for this ring index.

Doug Ledford <[email protected]>
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