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

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Date:	Mon, 14 Mar 2011 12:46:52 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	KyongHo Cho <pullip.cho@...sung.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	k.debski@...sung.com, linux-samsung-soc@...r.kernel.org,
	강민규 <mk7.kang@...sung.com>,
	linux-kernel@...r.kernel.org,
	대인기 <inki.dae@...sung.com>,
	kyungmin.park@...sung.com, kgene.kim@...sung.com,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 3/7] ARM: Samsung: update/rewrite Samsung SYSMMU
	(IOMMU) driver

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.)

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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