On Tue, 2010-04-06 at 12:53 -0600, Grant Likely wrote: > Hold on.... BUFFER_ALIGN is being used to align the DMA buffer on a > cache line boundary. I don't think netdev_alloc_skb() makes any > guarantees about how the start of the IP header lines up against cache > line boundaries. The amount of padding needed is not known until an > skbuff is obtained from netdev_alloc_skb(), and > netdev_alloc_skb_ip_align() can only handle a fixed size padding, > > It doesn't look like netdev_alloc_skb_ip_align() is the right thing in > this regard.
__netdev_alloc_skb reserves NET_SKB_PAD bytes which gets us cacheline alignment on Microblaze. From include/linux/skbuff.h: /* * The networking layer reserves some headroom in skb data (via * dev_alloc_skb). This is used to avoid having to reallocate skb data when * the header has to grow. In the default case, if the header has to grow * 32 bytes or less we avoid the reallocation. * * Unfortunately this headroom changes the DMA alignment of the resulting * network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive * on some architectures. An architecture can override this value, * perhaps setting it to a cacheline in size (since that will maintain * cacheline alignment of the DMA). It must be a power of 2. * * Various parts of the networking layer expect at least 32 bytes of * headroom, you should not reduce this. */ #ifndef NET_SKB_PAD #define NET_SKB_PAD 32 #endif If this doesn't work for some of the PPC variants with larger cache lines, maybe one of the PPC header files needs to define NET_SKB_PAD? And if we want to guard against possible future changes to the default NET_SKB_PAD breaking Microblaze operation, maybe one of its headers should define NET_SKB_PAD as well? ------------------------------------------------------------------------ Steven J. Magnani "I claim this network for MARS! www.digidescorp.com Earthling, return my space modulator!" #include <standard.disclaimer> _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
