• David Dillow's avatar
    IB: Increase DMA max_segment_size on Mellanox hardware · 7f9e5c48
    David Dillow authored
    By default, each device is assumed to be able only handle 64 KB chunks
    during DMA. By giving the segment size a larger value, the block layer
    will coalesce more S/G entries together for SRP, allowing larger
    requests with the same sg_tablesize setting.  The block layer is the
    only direct user of it, though a few IOMMU drivers reference it as
    well for their *_map_sg coalescing code. pci-gart_64 on x86, and a
    smattering on on sparc, powerpc, and ia64.
    
    Since other IB protocols could potentially see larger segments with
    this, let's check those:
    
     - iSER is fine, because you limit your maximum request size to 512
       KB, so we'll never overrun the page vector in struct iser_page_vec
       (128 entries currently). It is independent of the DMA segment size,
       and handles multi-page segments already.
    
     - IPoIB is fine, as it maps each page individually, and doesn't use
       ib_dma_map_sg().
    
     - RDS appears to do the right thing and has no dependencies on DMA
       segment size, but I don't claim to have done a complete audit.
    
     - NFSoRDMA and 9p are OK -- they do not use ib_dma_map_sg(), so they
       doesn't care about the coalescing.
    
     - Lustre's ko2iblnd does not care about coalescing -- it properly
       walks the returned sg list.
    
    This patch ups the value on Mellanox hardware to 1 GB, which matches
    reported firmware limits on mlx4.
    Signed-off-by: default avatarDavid Dillow <dillowda@ornl.gov>
    Signed-off-by: default avatarRoland Dreier <roland@purestorage.com>
    7f9e5c48
mthca_main.c 36.5 KB