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    SubjectRe: [PATCH v3] Fix dma_mapping_error for 32bit x86
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    On Mon, 1 Dec 2008 12:34:51 +0100
    Joerg Roedel <joro@8bytes.org> wrote:

    > On Sat, Nov 29, 2008 at 01:46:27PM +0100, Thomas Bogendoerfer wrote:
    > > Devices like b44 ethernet can't dma from addresses above 1GB. The driver
    > > handles this cases by falling back to GFP_DMA allocation. But for detecting
    > > the problem it needs to get an indication from dma_mapping_error.
    > > The bug is triggered by using a VMSPLIT option of 2G/2G.
    >
    > Looks like your system uses swiotlb as the dma_ops backend. Its the only
    > implementation providing the ops->mapping_error callback and does not
    > use bad_dma_address as the error value.

    I think that you misunderstand the problem.

    He uses X86_32 so swiotlb should not be used (which is available on only
    X86_64 and IA64 for now).

    b44 needs an address under 1GB so it sets device->dma_mask to
    DMA_30BIT_MASK. With VMSPLIT option of 2G/2G, I guess that b44 could
    get addresses above 1GB from the networking subsystem. In such case,
    nommu_map_single returns bad_dma_address properly, but on X86_32,
    dma_mapping_error always returns 0 (success). So b44 wrongly thinks
    that the address is under 1GB.

    This patch fixes dma_mapping_error() to check a passed address
    properly (compares it with bad_dma_address).

    As I already wrote, the current git needs a patch modified slightly:

    http://marc.info/?l=linux-kernel&m=122797163405377&w=2


    > > Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
    >
    > Acked-by: Joerg Roedel <joro@8bytes.org>
    >
    > >
    > > diff -ru orig/linux-2.6.27.4/include/asm-x86/dma-mapping.h linux-2.6.27.4/include/asm-x86/dma-mapping.h
    > > --- orig/linux-2.6.27.4/include/asm-x86/dma-mapping.h 2008-10-26 00:05:07.000000000 +0200
    > > +++ linux-2.6.27.4/include/asm-x86/dma-mapping.h 2008-10-26 11:06:14.000000000 +0100
    > > @@ -74,15 +74,13 @@
    > > /* Make sure we keep the same behaviour */
    > > static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
    > > {
    > > -#ifdef CONFIG_X86_32
    > > - return 0;
    > > -#else
    > > +#ifdef CONFIG_X86_64
    > > struct dma_mapping_ops *ops = get_dma_ops(dev);
    > > if (ops->mapping_error)
    > > return ops->mapping_error(dev, dma_addr);
    > >
    > > - return (dma_addr == bad_dma_address);
    > > #endif
    > > + return (dma_addr == bad_dma_address);
    > > }
    > >
    > > #define dma_alloc_noncoherent(d, s, h, f) dma_alloc_coherent(d, s, h, f)
    > > --
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