Re: [PATCH 3/3] x86: restore old GART alloc_coherent behavior

Joerg Roedel
2008-09-24T12:11:46Z

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.

> +
> + flush_gart();

This should depend on need_flush.

> + if (paddr != bad_dma_address) {
> + *dma_addr = paddr;
> + return page_address(page);
> + }
> + __free_pages(page, get_order(size));
> + } else
> + return dma_generic_alloc_coherent(dev, size, dma_addr, flag);
>
> return NULL;
> }
> --
> 1.5.4.2
>
>

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