Re: iommu dma mapping alignment requirements

[Steve Wise <swise@opengridcomputing.com>]

Benjamin Herrenschmidt wrote:
> > Sounds good.  Thanks!
> >
> > Note, that these smaller sub-host-page-sized mappings might pollute the 
> > address space causing full aligned host-page-size maps to become 
> > scarce...  Maybe there's a clever way to keep those in their own segment 
> > of the address space?
>
> We already have a large vs. small split in the iommu virtual space to
> alleviate this (though it's not a hard constraint, we can still get
> into the "other" side if the default one is full).
>
> Try that patch and let me know:

Seems to be working!

:)


> Index: linux-work/arch/powerpc/kernel/iommu.c
> ===================================================================
> --- linux-work.orig/arch/powerpc/kernel/iommu.c	2007-12-21 10:39:39.000000000 +1100
> +++ linux-work/arch/powerpc/kernel/iommu.c	2007-12-21 10:46:18.000000000 +1100
> @@ -278,6 +278,7 @@ int iommu_map_sg(struct iommu_table *tbl
>  	unsigned long flags;
>  	struct scatterlist *s, *outs, *segstart;
>  	int outcount, incount, i;
> +	unsigned int align;
>  	unsigned long handle;
>  
>  	BUG_ON(direction == DMA_NONE);
> @@ -309,7 +310,11 @@ int iommu_map_sg(struct iommu_table *tbl
>  		/* Allocate iommu entries for that segment */
>  		vaddr = (unsigned long) sg_virt(s);
>  		npages = iommu_num_pages(vaddr, slen);
> -		entry = iommu_range_alloc(tbl, npages, &handle, mask >> IOMMU_PAGE_SHIFT, 0);
> +		align = 0;
> +		if (IOMMU_PAGE_SHIFT < PAGE_SHIFT && (vaddr & ~PAGE_MASK) == 0)
> +			align = PAGE_SHIFT - IOMMU_PAGE_SHIFT;
> +		entry = iommu_range_alloc(tbl, npages, &handle,
> +					  mask >> IOMMU_PAGE_SHIFT, align);
>  
>  		DBG("  - vaddr: %lx, size: %lx\n", vaddr, slen);
>  
> @@ -572,7 +577,7 @@ dma_addr_t iommu_map_single(struct iommu
>  {
>  	dma_addr_t dma_handle = DMA_ERROR_CODE;
>  	unsigned long uaddr;
> -	unsigned int npages;
> +	unsigned int npages, align;
>  
>  	BUG_ON(direction == DMA_NONE);
>  
> @@ -580,8 +585,13 @@ dma_addr_t iommu_map_single(struct iommu
>  	npages = iommu_num_pages(uaddr, size);
>  
>  	if (tbl) {
> +		align = 0;
> +		if (IOMMU_PAGE_SHIFT < PAGE_SHIFT &&
> +		    ((unsigned long)vaddr & ~PAGE_MASK) == 0)
> +			align = PAGE_SHIFT - IOMMU_PAGE_SHIFT;
> +
>  		dma_handle = iommu_alloc(tbl, vaddr, npages, direction,
> -					 mask >> IOMMU_PAGE_SHIFT, 0);
> +					 mask >> IOMMU_PAGE_SHIFT, align);
>  		if (dma_handle == DMA_ERROR_CODE) {
>  			if (printk_ratelimit())  {
>  				printk(KERN_INFO "iommu_alloc failed, "
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