From: Sergei Shtylyov
> Sent: 25 July 2015 21:42
> Renesas Ethernet AVB controller requires that all data are aligned on 4-byte
> boundary. While it's easily achievable for the RX data with the help of
> skb_reserve() (we even align on 128-byte boundary as recommended by the
> manual),
> we can't do the same with the TX data, and it always comes unaligned from
> the networking core. Originally we solved it an easy way, copying all packet
> to a preallocated aligned buffer; however, it's enough to copy only up to
> 3 first bytes from each packet, doing the transfer using 2 TX descriptors
> instead of just 1. Here's an implementation of the new TX algorithm that
> significantly reduces the driver's memory requirements.
>
...
> - buffer = PTR_ALIGN(priv->tx_buffers[q][entry], RAVB_ALIGN);
> - memcpy(buffer, skb->data, skb->len);
> - desc = &priv->tx_ring[q][entry];
> - desc->ds_tagl = cpu_to_le16(skb->len);
> - dma_addr = dma_map_single(&ndev->dev, buffer, skb->len, DMA_TO_DEVICE);
> + buffer = PTR_ALIGN(priv->tx_align[q], DPTR_ALIGN) +
> + entry / NUM_TX_DESC * DPTR_ALIGN;
The above would be clearer if tx_align was char[DPTR_ALIGN][].
> + len = PTR_ALIGN(skb->data, DPTR_ALIGN) - skb->data;
> + memcpy(buffer, skb->data, len);
Does this imply there has been an skb_linearize() ???
The old version didn't really need it (it was doing a copy anyway).
> + dma_addr = dma_map_single(&ndev->dev, buffer, len, DMA_TO_DEVICE);
> if (dma_mapping_error(&ndev->dev, dma_addr))
> goto drop;
> +
> + desc = &priv->tx_ring[q][entry];
> + desc->ds_tagl = cpu_to_le16(len);
> + desc->dptr = cpu_to_le32(dma_addr);
> +
> + buffer = skb->data + len;
> + len = skb->len - len;
> + dma_addr = dma_map_single(&ndev->dev, buffer, len, DMA_TO_DEVICE);
> + if (dma_mapping_error(&ndev->dev, dma_addr))
> + goto unmap;
> +
> + desc++;
> + desc->ds_tagl = cpu_to_le16(len);
What happens if a fragment is less than DPTR_ALIGN bytes ???
Actually is looks like you relying on having a linear skb.
On systems with expensive dma_map it might be worth copying
small fragments.
David
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