• Andi Kleen's avatar
    [PATCH] x86_64: Add 4GB DMA32 zone · a2f1b424
    Andi Kleen authored
    Add a new 4GB GFP_DMA32 zone between the GFP_DMA and GFP_NORMAL zones.
    As a bit of historical background: when the x86-64 port
    was originally designed we had some discussion if we should
    use a 16MB DMA zone like i386 or a 4GB DMA zone like IA64 or
    both. Both was ruled out at this point because it was in early
    2.4 when VM is still quite shakey and had bad troubles even
    dealing with one DMA zone.  We settled on the 16MB DMA zone mainly
    because we worried about older soundcards and the floppy.
    But this has always caused problems since then because
    device drivers had trouble getting enough DMA able memory. These days
    the VM works much better and the wide use of NUMA has proven
    it can deal with many zones successfully.
    So this patch adds both zones.
    This helps drivers who need a lot of memory below 4GB because
    their hardware is not accessing more (graphic drivers - proprietary
    and free ones, video frame buffer drivers, sound drivers etc.).
    Previously they could only use IOMMU+16MB GFP_DMA, which
    was not enough memory.
    Another common problem is that hardware who has full memory
    addressing for >4GB misses it for some control structures in memory
    (like transmit rings or other metadata).  They tended to allocate memory
    in the 16MB GFP_DMA or the IOMMU/swiotlb then using pci_alloc_consistent,
    but that can tie up a lot of precious 16MB GFPDMA/IOMMU/swiotlb memory
    (even on AMD systems the IOMMU tends to be quite small) especially if you have
    many devices.  With the new zone pci_alloc_consistent can just put
    this stuff into memory below 4GB which works better.
    One argument was still if the zone should be 4GB or 2GB. The main
    motivation for 2GB would be an unnamed not so unpopular hardware
    raid controller (mostly found in older machines from a particular four letter
    company) who has a strange 2GB restriction in firmware. But
    that one works ok with swiotlb/IOMMU anyways, so it doesn't really
    need GFP_DMA32. I chose 4GB to be compatible with IA64 and because
    it seems to be the most common restriction.
    The new zone is so far added only for x86-64.
    For other architectures who don't set up this
    new zone nothing changes. Architectures can set a compatibility
    define in Kconfig CONFIG_DMA_IS_DMA32 that will define GFP_DMA32
    as GFP_DMA. Otherwise it's a nop because on 32bit architectures
    it's normally not needed because GFP_NORMAL (=0) is DMA able
    One problem is still that GFP_DMA means different things on different
    architectures. e.g. some drivers used to have #ifdef ia64  use GFP_DMA
    (trusting it to be 4GB) #elif __x86_64__ (use other hacks like
    the swiotlb because 16MB is not enough) ... . This was quite
    ugly and is now obsolete.
    These should be now converted to use GFP_DMA32 unconditionally. I haven't done
    this yet. Or best only use pci_alloc_consistent/dma_alloc_coherent
    which will use GFP_DMA32 transparently.
    Signed-off-by: default avatarAndi Kleen <ak@suse.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>