Hello, On Monday, July 11, 2011 9:01 PM Janusz Krzysztofik wrote: > 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. Defining ARCH_HAS_HOLES_MEMORYMODEL changes the behavior of valid_pfn() macro/function, which is used in the ioremap(). When defined, valid_pfn() checks if the selected pfn is inside system memory or not (using memblock information). If the area is removed with memblock_remove(), then a check with valid_pfn() fails and ioremap() doesn't complain about mapping system memory. > 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. Please check again with 3.0-rc1. ARCH_HAS_HOLES_MEMORYMODEL solution was suggested by Russell. It looks like this is the correct solution for this problem, because I don't believe that ioremap() will be removed from dma_declare_coherent() anytime soon. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
