• Michael Hennerich's avatar
    Blackfin arch: Add assembly function insl_16 · 5c91fb90
    Michael Hennerich authored
    /*
     * CPUs often take a performance hit when accessing unaligned memory
     * locations. The actual performance hit varies, it can be small if the
     * hardware handles it or large if we have to take an exception and fix
     * it
     * in software.
     *
     * Since an ethernet header is 14 bytes network drivers often end up
     * with
     * the IP header at an unaligned offset. The IP header can be aligned by
     * shifting the start of the packet by 2 bytes. Drivers should do this
     * with:
     *
     * skb_reserve(NET_IP_ALIGN);
     *
     * The downside to this alignment of the IP header is that the DMA is
     * now
     * unaligned. On some architectures the cost of an unaligned DMA is high
     * and this cost outweighs the gains made by aligning the IP header.
     *
     * Since this trade off varies between architectures, we allow
     * NET_IP_ALIGN
     * to be overridden.
     */
    
    This new function insl_16 allows to read form 32-bit IO and writes to
    16-bit aligned memory. This is useful in above described scenario -
    In particular with the AXIS AX88180 Gigabit Ethernet MAC.
    Once the device is in 32-bit mode, reads from the RX FIFO always
    decrements 4bytes.
    While on the other side the destination address in SDRAM is always
    16-bit aligned.
    If we use skb_reserve(0) the receive buffer is 32-bit aligned but later
    we hit a unaligned exception in the IP code.
    Signed-off-by: default avatarMichael Hennerich <michael.hennerich@analog.com>
    Signed-off-by: default avatarBryan Wu <bryan.wu@analog.com>
    5c91fb90
io.h 5.84 KB