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

[Joerg Roedel]

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@xxxxxxxxxxxxx>
> ---
>  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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