On 04/27/2016 03:48 AM, Nikolay Borisov wrote:
> 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.

Into the local variable mapping.

> ipoib_cm_dma_unmap_rx(priv, frags, rx_ring[wr_id].mapping);
> 
> immediately after that the very same mappings are being unmapped.

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

> 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);

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

> 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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-rdma" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 


-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
              GPG KeyID: 0E572FDD