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    SubjectRe: [PATCH 3/3] x86: restore old GART alloc_coherent behavior
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    On Wed, 24 Sep 2008 14:11:27 +0200
    Joerg Roedel <joerg.roedel@amd.com> wrote:

    > On Wed, Sep 24, 2008 at 08:48:37PM +0900, FUJITA Tomonori wrote:
    > > Currently, GART alloc_coherent tries to allocate pages with GFP_DMA32
    > > for a device having dma_masks > 24bit < 32bits. If GART gets an
    > > address that a device can't access to, GART try to map the address to
    > > a virtual I/O address that the device can access to.
    > >
    > > But Andi pointed out, "The GART is somewhere in the 4GB range so you
    > > cannot use it to map anything < 4GB. Also GART is pretty small."
    > >
    > > http://lkml.org/lkml/2008/9/12/43
    > >
    > > That is, it's possible that GART doesn't have virtual I/O address
    > > space that a device can access to. The above behavior doesn't work for
    > > a device having dma_masks > 24bit < 32bits.
    > >
    > > This patch restores old GART alloc_coherent behavior (before the
    > > alloc_coherent rewrite).
    >
    > Patchset looks good in principle. But I have a small question, see
    > below.
    >
    > >
    > > Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
    > > ---
    > > arch/x86/kernel/pci-gart_64.c | 43 +++++++++++++++++++---------------------
    > > 1 files changed, 20 insertions(+), 23 deletions(-)
    > >
    > > diff --git a/arch/x86/kernel/pci-gart_64.c b/arch/x86/kernel/pci-gart_64.c
    > > index 7e08e46..25c94fb 100644
    > > --- a/arch/x86/kernel/pci-gart_64.c
    > > +++ b/arch/x86/kernel/pci-gart_64.c
    > > @@ -487,31 +487,28 @@ static void *
    > > gart_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_addr,
    > > gfp_t flag)
    > > {
    > > - void *vaddr;
    > > dma_addr_t paddr;
    > > unsigned long align_mask;
    > > - u64 dma_mask = dma_alloc_coherent_mask(dev, flag);
    > > -
    > > - vaddr = (void *)__get_free_pages(flag | __GFP_ZERO, get_order(size));
    > > - if (!vaddr)
    > > - return NULL;
    > > -
    > > - paddr = virt_to_phys(vaddr);
    > > - if (is_buffer_dma_capable(dma_mask, paddr, size)) {
    > > - *dma_addr = paddr;
    > > - return vaddr;
    > > - }
    > > -
    > > - align_mask = (1UL << get_order(size)) - 1;
    > > -
    > > - *dma_addr = dma_map_area(dev, paddr, size, DMA_BIDIRECTIONAL,
    > > - align_mask);
    > > - flush_gart();
    > > -
    > > - if (*dma_addr != bad_dma_address)
    > > - return vaddr;
    > > -
    > > - free_pages((unsigned long)vaddr, get_order(size));
    > > + struct page *page;
    > > +
    > > + if (force_iommu && !(flag & GFP_DMA)) {
    > > + flag &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);
    > > + page = alloc_pages(flag | __GFP_ZERO, get_order(size));
    > > + if (!page)
    > > + return NULL;
    > > +
    > > + align_mask = (1UL << get_order(size)) - 1;
    > > + paddr = dma_map_area(dev, page_to_phys(page), size,
    > > + DMA_BIDIRECTIONAL, align_mask);
    >
    > Can't we check if a mapping is required before calling dma_map_area?
    > Your recently introduced IOMMU helper functions should make that easy
    > and GART address space is a rare resource. AFAIR this is also the
    > behaviour of the old generic dma_alloc_coherent function.

    I think that the behavior of the old generic dma_alloc_coherent
    function and GART is different.

    The old GART code does virtual mappings only with force_iommu option
    enabled. The old GART code always do virtual mappings with force_iommu
    option enabled (unless GFP_DMA is set).


    > > +
    > > + flush_gart();
    >
    > This should depend on need_flush.

    Theoretically, yes, I think. But this patch restores the old GART code
    behavior.


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