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    SubjectRe: DMA memory limitation?
    Vasu Varma P V wrote:
    >
    > Hi,
    >
    > Is there any limitation on DMA memory we can allocate using
    > kmalloc(size, GFP_DMA)? I am not able to acquire more than
    > 14MB of the mem using this on my PCI SMP box with 256MB ram.
    > I think there is restriction on ISA boards of 16MB.
    > Can we increase it ?

    You can allocate a lot more memory for your pci activities.
    No problem there. Just drop the "GFP_DMA" and you'll get
    up to 1G or so.

    You shouldn't use GFP_DMA because PCI cards don't need that.
    Only ISA cards needs GFP_DMA because they can't use more
    than 16M. So obviously GFP_DMA is limited to
    16M because it is really ISA_DMA.

    PCI don't need such special tricks, so don't use GFP_DMA!
    Your PCI cards is able to DMA into any memory, including
    the non-GFP_DMA memory.

    > but we have a macro in include/asm-i386/dma.h,
    > MAX_DMA_ADDRESS (PAGE_OFFSET+0x1000000).
    >
    > if i change it to a higher value, i am able to get more dma
    > memory. Is there any way i can change this without compiling
    > the kernel?
    >
    No matter what you do, DON'T change that. Yeah, you'll get
    a bigger GFP_DMA pool, but that'll break each and every
    ISA card that tries to allocate GFP_DMA memory. You
    achieve exactly the same effect for your PCI card by ditching
    the GFP_DMA parameter, but then you achieve it without breaking
    ISA cards.

    Helge Hafting
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