Re: [PATCH 3/7] ARM: Samsung: update/rewrite Samsung SYSMMU (IOMMU) driver

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Mar 15, 2011 at 10:45:50AM +0900, InKi Dae wrote:
> 2011/3/14 Russell King - ARM Linux <linux@arm.linux.org.uk>:
> > On Mon, Mar 14, 2011 at 09:37:51PM +0900, KyongHo Cho wrote:
> >> I have also noticed that dma_map_single/page/sg() can map physical
> >> memory into an arbitrary device address region.
> >> But it is not enough solution for various kinds of IOMMUs.
> >> As Kukjin Kim addressed, we need to support larger page size than 4KB
> >> because we can reduce TLB miss when we have larger page size.
> >>
> >> Our IOMMU(system mmu) supports all page size of ARM architecture
> >> including 16MB, 1MB, 64KB and 4KB.
> >> Since the largest size supported by buddy system of 32-bit architecture is 4MB,
> >> our system support all page sizes except 16MB.
> >> We proved that larger page size is helpful for DMA performance
> >> significantly (more than 10%, approximately).
> >> Big page size is not a problem for peripheral devices
> >> because their address space is not suffer from external fragmentation.
> >
> > 1. dma_map_single() et.al. is used for mapping *system* *RAM* for devices
> >   using whatever is necessary.  It must not be used for trying to setup
> >   arbitary other mappings.
> >
> > 2. It doesn't matter where the memory for dma_map_single() et.al. comes
> >   from provided the virtual address is a valid system RAM address or
> >   the struct page * is a valid struct page in the memory map (iow, you
> >   can't create this yourself.)
> 
> You mean that we cannot have arbitrary virtual address mapping for
> iommu based device?

No.  I mean exactly what I said - I'm talking about the DMA API in the
above two points.  The implication is that you can not create arbitary
mappings of non-system RAM with the DMA API.

> actually, we have memory mapping to arbitrary device virtual address
> space, not kernel virtual address space.
> 
> >
> > 3. In the case of an IOMMU, the DMA API does not limit you to only using
> >   4K pages to setup the IOMMU mappings.  You can use whatever you like
> >   provided the hardware can cope with it.  You can coalesce several
> >   existing entries together provided you track what you're doing and can
> >   undo what's been done when the mapping is no longer required.
> >
> > So really there's no reason not to use 64K, 1M and 16M IOMMU entries if
> > that's the size of buffer which has been passed to the DMA API.
> >
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