linux-kernel - Re: [PATCH]: PCI: GART iommu alignment fixes

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Message-Id: <20080722001634L.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 22 Jul 2008 00:16:14 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	prarit@...hat.com
Cc:	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH]: PCI: GART iommu alignment fixes

On Mon, 21 Jul 2008 10:15:27 -0400
Prarit Bhargava <prarit@...hat.com> wrote:

> pci_alloc_consistent/dma_alloc_coherent is supposed to return size aligned
> addresses.
> 
> From Documentation/DMA-mapping.txt:
> 
> "pci_alloc_consistent returns two values: the virtual address which you
> can use to access it from the CPU and dma_handle which you pass to the
> card.
> 
> The cpu return address and the DMA bus master address are both
> guaranteed to be aligned to the smallest PAGE_SIZE order which
> is greater than or equal to the requested size.  This invariant
> exists (for example) to guarantee that if you allocate a chunk
> which is smaller than or equal to 64 kilobytes, the extent of the
> buffer you receive will not cross a 64K boundary."
> 
> Fix the GART's alloc_iommu code to return a size aligned address.  Also fix
> an incorrect alignment calculation in the iommu-helper code.
> 
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> 
> diff --git a/arch/x86/kernel/pci-gart_64.c b/arch/x86/kernel/pci-gart_64.c
> index faf3229..329718e 100644
> --- a/arch/x86/kernel/pci-gart_64.c
> +++ b/arch/x86/kernel/pci-gart_64.c
> @@ -96,11 +96,12 @@ static unsigned long alloc_iommu(struct device *dev, int size)
>  
>  	spin_lock_irqsave(&iommu_bitmap_lock, flags);
>  	offset = iommu_area_alloc(iommu_gart_bitmap, iommu_pages, next_bit,
> -				  size, base_index, boundary_size, 0);
> +				  size, base_index, boundary_size, size - 1);
>  	if (offset == -1) {
>  		need_flush = 1;
>  		offset = iommu_area_alloc(iommu_gart_bitmap, iommu_pages, 0,
> -					  size, base_index, boundary_size, 0);
> +					  size, base_index, boundary_size,
> +					  size - 1);
>  	}

This affects map_sg and map_single unnecessarily.


>  	if (offset != -1) {
>  		set_bit_string(iommu_gart_bitmap, offset, size);
> diff --git a/lib/iommu-helper.c b/lib/iommu-helper.c
> index a3b8d4c..b970f1b 100644
> --- a/lib/iommu-helper.c
> +++ b/lib/iommu-helper.c
> @@ -16,7 +16,7 @@ again:
>  	index = find_next_zero_bit(map, size, start);
>  
>  	/* Align allocation */
> -	index = (index + align_mask) & ~align_mask;
> +	index = (index + align_mask + 1) & ~align_mask;

Doesn't look right. What we do here is __ALIGN_MASK().
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