Re: [Linaro-mm-sig] [PATCH 6/8] drivers: add Contiguous Memory Allocator

[Janusz Krzysztofik]

Dnia poniedziaÅek, 11 lipca 2011 o 15:47:32 Marek Szyprowski napisaÅ(a):
> Hello,
> 
> On Saturday, July 09, 2011 4:57 PM Janusz Krzysztofik	wrote:
> > On Wed, 6 Jul 2011 at 16:59:45 Arnd Bergmann wrote:
> > > On Wednesday 06 July 2011, Nicolas Pitre wrote:
> > > > On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:
> > > > > Another issue is that when a platform has restricted DMA
> > > > > regions, they typically don't fall into the highmem zone. 
> > > > > As the dmabounce code allocates from the DMA coherent
> > > > > allocator to provide it with guaranteed DMA-able memory,
> > > > > that would be rather inconvenient.
> > > > 
> > > > Do we encounter this in practice i.e. do those platforms
> > > > requiring large contiguous allocations motivating this work
> > > > have such DMA restrictions?
> > > 
> > > You can probably find one or two of those, but we don't have to
> > > optimize for that case. I would at least expect the maximum size
> > > of the allocation to be smaller than the DMA limit for these,
> > > and consequently mandate that they define a sufficiently large
> > > CONSISTENT_DMA_SIZE for the crazy devices, or possibly add a
> > > hack to unmap some low memory and call
> > > dma_declare_coherent_memory() for the device.
> > 
> > Once found that Russell has dropped his "ARM: DMA: steal memory for
> > DMA coherent mappings" for now, let me get back to this idea of a
> > hack that would allow for safely calling
> > dma_declare_coherent_memory() in order to assign a device with a
> > block of contiguous memory for exclusive use.
> 
> We tested such approach and finally with 3.0-rc1 it works fine. You
> can find an example for dma_declare_coherent() together with
> required memblock_remove() calls in the following patch series:
> http://www.spinics.net/lists/linux-samsung-soc/msg05026.html
> "[PATCH 0/3 v2] ARM: S5P: Add support for MFC device on S5PV210 and
> EXYNOS4"
> 
> > Assuming there should be no problem with successfully allocating a
> > large continuous block of coherent memory at boot time with
> > dma_alloc_coherent(), this block could be reserved for the device.
> > The only problem is with the dma_declare_coherent_memory() calling
> > ioremap(), which was designed with a device's dedicated physical
> > memory in mind, but shouldn't be called on a memory already
> > mapped.
> 
> All these issues with ioremap has been finally resolved in 3.0-rc1.
> Like Russell pointed me in
> http://www.spinics.net/lists/arm-kernel/msg127644.html, ioremap can
> be fixed to work on early reserved memory areas by selecting
> ARCH_HAS_HOLES_MEMORYMODEL Kconfig option.

I'm not sure. Recently I tried to refresh my now 7 months old patch in 
which I used that 'memblock_remove() then dma_declare_coherent_memery()' 
method[1]. It was different from your S5P MFC example in that it didn't 
punch any holes in the system memory, only stole a block of SDRAM from 
its tail. But Russell reminded me again: "we should not be mapping SDRAM 
using device mappings."[2]. Would defining ARCH_HAS_HOLES_MEMORYMODEL 
(even if it was justified) make any diference in my case? I don't think 
so. Wnat I think, after Russell, is that we still need that obligatory 
ioremap() removed from dma_declare_coherent_memory(), or made it 
optional, or a separate dma_declare_coherent_memory()-like function 
without (obligatory) ioremap() provided by the DMA API, in order to get 
the dma_declare_coherent_memery() method being accepted without any 
reservations when used inside arch/arm, I'm afraid.

Thanks,
Janusz

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/034644.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2011-June/052488.html
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