• Ben Hutchings's avatar
    sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate · 950c54df
    Ben Hutchings authored
    efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus
    a software structure, each appropriately aligned, into a single page.
    Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case
    on s390, this assertion fails.
    
    The current scatter buffer size is also not a multiple of 64 or 128,
    which are more common cache line sizes.  If we can make both the start
    and end of a scatter buffer cache-aligned, this will reduce the need
    for read-modify-write operations on inter- processor links.
    
    Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 ==
    1792.  (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE
    also affects user-level networking where a larger amount of
    housekeeping data may be needed.  Although this version of the driver
    does not support user-level networking, I prefer to keep scattering
    behaviour consistent with the out-of-tree version.)
    
    This still doesn't fix the s390 build because like most architectures
    it has NET_IP_ALIGN == 2.  When NET_IP_ALIGN != 0 we cannot achieve
    cache line alignment at either the start or end of a scatter buffer,
    so there is actually no point in padding the buffers to a multiple of
    the cache line size.  All we need is 4-byte alignment of the network
    header, so do that.
    
    Adjust the assertions accordingly.
    Reported-by: default avatarGeert Uytterhoeven <geert@linux-m68k.org>
    Reported-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: default avatarBen Hutchings <bhutchings@solarflare.com>
    Acked-by: default avatarGeert Uytterhoeven <geert@linux-m68k.org>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    950c54df
net_driver.h 39 KB