Messages in this thread | | | Date | Fri, 4 Dec 2015 17:15:54 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] drivers: dma-coherent: use ioremap_wc() for DMA_MEMORY_MAP |
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On Fri, Dec 04, 2015 at 08:59:10AM -0800, Dan Williams wrote: > On Fri, Dec 4, 2015 at 2:50 AM, Catalin Marinas <catalin.marinas@arm.com> wrote: > > On Fri, Nov 20, 2015 at 02:20:26PM +0000, Brian Starkey wrote: > >> When the DMA_MEMORY_MAP flag is used, memory which can be accessed > >> directly should be returned, so use ioremap_wc() instead of ioremap(). > >> Also, ensure that the correct memset operation is used in > >> dma_alloc_from_coherent() with respect to the region's flags. > >> > >> This fixes the below alignment fault on arm64, caused by invalid use > >> of memset() on Device memory. > > > > This is indeed affecting both arm32 and arm64 systems. > > > >> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c > >> index 55b8398..45358d0 100644 > >> --- a/drivers/base/dma-coherent.c > >> +++ b/drivers/base/dma-coherent.c > >> @@ -31,7 +31,10 @@ static int dma_init_coherent_memory(phys_addr_t phys_addr, dma_addr_t device_add > >> if (!size) > >> goto out; > >> > >> - mem_base = ioremap(phys_addr, size); > >> + if (flags & DMA_MEMORY_MAP) > >> + mem_base = ioremap_wc(phys_addr, size); > >> + else > >> + mem_base = ioremap(phys_addr, size); > > > > I wonder whether a memremap() approach for the DMA_MEMORY_MAP case would > > be better. This API was added recently by commit 92281dee825f ("arch: > > introduce memremap()"). It only supports write-back and write-through > > but we could add a MEMREMAP_WC flag for this case. > > I originally included both MEMREMAP_WC and MEMREAMP_UC as potential > flags to this api, but ultimately decided against it. The memremap() > api is meant for memory that is known to have no i/o side effects. As > far as I can see WC and UC usages are a muddy mix of "sometimes > there's I/O side effects, but it depends by arch and driver". In > other words we can't drop the "__iomem" annotation from WC and UC > mappings by default.
In this context, the dma_declare_coherent_memory(DMA_MEMORY_MAP) implementation is aimed at normal RAM with no side effects as later returned by dma_alloc_coherent(). To me it looks like memremap is better suited here for the DMA_MEMORY_MAP case. As per the Documentation/DMA-API.txt:
DMA_MEMORY_MAP - request that the memory returned from dma_alloc_coherent() be directly writable.
which means no __iomem. Of course, we still need ioremap for DMA_MEMORY_IO which is supposed to be written with memcpy_toio() etc.
Which memory type should be used is left to the driver and it should pass the corresponding DMA_MEMORY_* flag.
-- Catalin
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