On Wed, Apr 02, 2014 at 11:21:45AM +0200, Arnd Bergmann wrote:
> - As David Laight pointed out earlier, you must also ensure that
> you don't have too much /data/ pending in the descriptor ring
> when you stop the queue. For a 10mbit connection, you have already
> tested (as we discussed on IRC) that 64 descriptors with 1500 byte
> frames gives you a 68ms round-trip ping time, which is too much.
> Conversely, on 1gbit, having only 64 descriptors actually seems
> a little low, and you may be able to get better throughput if
> you extend the ring to e.g. 512 descriptors.
You don't manage that by stopping the queue - there's separate interfaces
where you report how many bytes you've queued (netdev_sent_queue()) and
how many bytes/packets you've sent (netdev_tx_completed_queue()). This
allows the netdev schedulers to limit how much data is held in the queue,
preserving interactivity while allowing the advantages of larger rings.
> > + phys = dma_map_single(&ndev->dev, skb->data, skb->len, DMA_TO_DEVICE);
> > + if (dma_mapping_error(&ndev->dev, phys)) {
> > + dev_kfree_skb(skb);
> > + return NETDEV_TX_OK;
> > + }
> > +
> > + priv->tx_skb[tx_head] = skb;
> > + priv->tx_phys[tx_head] = phys;
> > + desc->send_addr = cpu_to_be32(phys);
> > + desc->send_size = cpu_to_be16(skb->len);
> > + desc->cfg = cpu_to_be32(DESC_DEF_CFG);
> > + phys = priv->tx_desc_dma + tx_head * sizeof(struct tx_desc);
> > + desc->wb_addr = cpu_to_be32(phys);
>
> One detail: since you don't have cache-coherent DMA, "desc" will
> reside in uncached memory, so you try to minimize the number of accesses.
> It's probably faster if you build the descriptor on the stack and
> then atomically copy it over, rather than assigning each member at
> a time.
DMA coherent memory is write combining, so multiple writes will be
coalesced. This also means that barriers may be required to ensure the
descriptors are pushed out in a timely manner if something like writel()
is not used in the transmit-triggering path.
--
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.
--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
