FreeBSD/Linux Kernel Cross Reference
sys/Documentation/DMA-attributes.txt

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

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