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

Russell King - ARM Linux ? ?:

On Tue, Mar 15, 2011 at 10:45:50AM +0900, InKi Dae wrote:

quoted

2011/3/14 Russell King - ARM Linux [off-list ref]:

quoted

On Mon, Mar 14, 2011 at 09:37:51PM +0900, KyongHo Cho wrote:

quoted

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.

sorry but I couldn't understand exactly what you said. could you give me 
your answer one more time?
does non-system RAM mean reserved memory regions? if not, is it 
arbitrary virtual address space that isn't kernel or user virtual 
address space and is the space for iommu based deivce?

quoted

actually, we have memory mapping to arbitrary device virtual address
space, not kernel virtual address space.

quoted

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.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel at lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel