4b9347dcbe
This patch adds DMA_ATTR_FORCE_CONTIGUOUS attribute to the DMA-mapping subsystem. By default DMA-mapping subsystem is allowed to assemble the buffer allocated by dma_alloc_attrs() function from individual pages if it can be mapped as contiguous chunk into device dma address space. By specifing this attribute the allocated buffer is forced to be contiguous also in physical memory. Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
103 lines
4.5 KiB
Plaintext
103 lines
4.5 KiB
Plaintext
DMA attributes
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==============
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This document describes the semantics of the DMA attributes that are
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defined in linux/dma-attrs.h.
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DMA_ATTR_WRITE_BARRIER
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----------------------
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DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA
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to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
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all pending DMA writes to complete, and thus provides a mechanism to
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strictly order DMA from a device across all intervening busses and
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bridges. This barrier is not specific to a particular type of
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interconnect, it applies to the system as a whole, and so its
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implementation must account for the idiosyncracies of the system all
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the way from the DMA device to memory.
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As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
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useful, suppose that a device does a DMA write to indicate that data is
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ready and available in memory. The DMA of the "completion indication"
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could race with data DMA. Mapping the memory used for completion
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indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
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DMA_ATTR_WEAK_ORDERING
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----------------------
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DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
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may be weakly ordered, that is that reads and writes may pass each other.
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Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
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those that do not will simply ignore the attribute and exhibit default
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behavior.
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DMA_ATTR_WRITE_COMBINE
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----------------------
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DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
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buffered to improve performance.
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Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
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those that do not will simply ignore the attribute and exhibit default
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behavior.
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DMA_ATTR_NON_CONSISTENT
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-----------------------
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DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
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consistent or non-consistent memory as it sees fit. By using this API,
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you are guaranteeing to the platform that you have all the correct and
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necessary sync points for this memory in the driver.
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DMA_ATTR_NO_KERNEL_MAPPING
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--------------------------
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DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
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virtual mapping for the allocated buffer. On some architectures creating
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such mapping is non-trivial task and consumes very limited resources
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(like kernel virtual address space or dma consistent address space).
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Buffers allocated with this attribute can be only passed to user space
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by calling dma_mmap_attrs(). By using this API, you are guaranteeing
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that you won't dereference the pointer returned by dma_alloc_attr(). You
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can threat it as a cookie that must be passed to dma_mmap_attrs() and
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dma_free_attrs(). Make sure that both of these also get this attribute
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set on each call.
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Since it is optional for platforms to implement
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DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
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attribute and exhibit default behavior.
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DMA_ATTR_SKIP_CPU_SYNC
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----------------------
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By default dma_map_{single,page,sg} functions family transfer a given
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buffer from CPU domain to device domain. Some advanced use cases might
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require sharing a buffer between more than one device. This requires
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having a mapping created separately for each device and is usually
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performed by calling dma_map_{single,page,sg} function more than once
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for the given buffer with device pointer to each device taking part in
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the buffer sharing. The first call transfers a buffer from 'CPU' domain
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to 'device' domain, what synchronizes CPU caches for the given region
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(usually it means that the cache has been flushed or invalidated
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depending on the dma direction). However, next calls to
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dma_map_{single,page,sg}() for other devices will perform exactly the
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same sychronization operation on the CPU cache. CPU cache sychronization
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might be a time consuming operation, especially if the buffers are
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large, so it is highly recommended to avoid it if possible.
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DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
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the CPU cache for the given buffer assuming that it has been already
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transferred to 'device' domain. This attribute can be also used for
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dma_unmap_{single,page,sg} functions family to force buffer to stay in
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device domain after releasing a mapping for it. Use this attribute with
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care!
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DMA_ATTR_FORCE_CONTIGUOUS
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-------------------------
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By default DMA-mapping subsystem is allowed to assemble the buffer
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allocated by dma_alloc_attrs() function from individual pages if it can
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be mapped as contiguous chunk into device dma address space. By
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specifing this attribute the allocated buffer is forced to be contiguous
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also in physical memory.
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