On Sat, Dec 22, 2007 at 12:15:31PM +0000, rubisher wrote:
> Hello *,
>
> Continuing my blind investigation on ccio-dma stuff, I read those 2
> different comments:
> in include/asm-parisc/scatterlist.h, scartterlist structure is defined like
> this:
> struct scatterlist {
> #ifdef CONFIG_DEBUG_SG
> unsigned long sg_magic;
> #endif
> unsigned long page_link;
> unsigned int offset;
>
> unsigned int length;
>
> /* an IOVA can be 64-bits on some PA-Risc platforms. */
> dma_addr_t iova; /* I/O Virtual Address */
> __u32 iova_length; /* bytes mapped */
> };
>
> in absolute the comment "an IOVA can be 64-bits on some PA-Risc platforms."
> seems ok.
Yes, it's correct. pa8800 allows 64-bit capabale devices to bypass the IOMMU.
But I don't think we allow it yet since we would need to add code that stuffs
the Virtual Index (for DMA to be cache coherent) into the high bits of the IOVA.
> but otoh, include/asm-parisc/types.h, defined dma_addr_t like this:
>
> /* Dma addresses are 32-bits wide. */
>
> typedef u32 dma_addr_t;
> typedef u64 dma64_addr_t;
>
> #endif /* __ASSEMBLY__ */
Yes, this matches the current implementation.
Adding support for "bypass" on zx1 chipsets (pa8800/pa8900 CPUs) will
require something more clever.
> OK it's just a comment but imho there is interesting matter in x86:
>
> typedef u64 dma64_addr_t;
> #if defined(CONFIG_X86_64) || defined(CONFIG_HIGHMEM64G)
> /* DMA addresses come in 32-bit and 64-bit flavours. */
> typedef u64 dma_addr_t;
> #else
> typedef u32 dma_addr_t;
> #endif
>
> But I simply have no idea which "#if defined" would be the most relevant
> for parisc, any idea?
u32 is correct now. u64 could be used for 64-bit builds when someone decides
we should bypass the IOMMU to improve DMA mapping/unmapping performance.
3-4 years ago I saw about 3% better performance for Storage Devices
_with_ the IOMMU enabled. IIRC, it was because of coalescing the longer
SG lists into a single IOMMU entry was more efficient for the PCI device.
Smaller, non-contiguous IOs would benefit from IOMMU bypass - e.g. NIC
workloads.
hth,
grant
>
> Cheers,
> r.
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