Dnia poniedziałek, 11 lipca 2011 o 15:47:32 Marek Szyprowski napisał(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.h... [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2011-June/052488.html