			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 idiosyncrasies 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 treat 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 synchronization operation on the CPU cache. CPU cache synchronization
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
specifying this attribute the allocated buffer is forced to be contiguous
also in physical memory.

DMA_ATTR_STRONGLY_ORDERED
-------------------------

DMA_ATTR_STRONGLY_ORDERED allocates memory with a very restrictive type
of mapping (no unaligned accesses, no re-ordering, no write merging, no
buffering, no pre-fetching). This has severe performance penalties and
should not be used for general purpose DMA allocations. It should only
be used if one of the restrictions on strongly ordered memory is required.

DMA_ATTR_NO_DELAYED_UNMAP
-------------------------

DMA_ATTR_NO_DELAYED_UNMAP is used only by the msm specific lazy mapping
feature. By default, the lazy mapping and unmapping holds an additional
reference so that when the buffer is freed, the mapping is not destroyed
and can be re-used. By specifying this attribute, an additional reference
will NOT be held by the lazy mapping code and it will be released as soon
as the buffer is freed.

DMA_ATTR_EXEC_MAPPING
---------------------

By default, the DMA mappings are non-executable. Some use cases might require
an executable mapping. This attribute can be used to indicate to the DMA
subsystem to create an executable mappings for the buffer.

DMA_ATTR_FORCE_COHERENT
-----------------------

When passed to a DMA map call the DMA_ATTR_FORCE_COHERENT DMA
attribute can be used to force a buffer to be mapped as IO coherent.

When the DMA_ATTR_FORCE_COHERENT attribute is set during a map call ensure
that it is also set during for the matching unmap call to ensure that the
correct cache maintenance is carried out.

This DMA attribute is only currently supported for arm64 stage 1 IOMMU
mappings.

DMA_ATTR_FORCE_NON_COHERENT
---------------------------
When passed to a DMA map call the DMA_ATTR_FORCE_NON_COHERENT DMA
attribute can be used to force a buffer to not be mapped as IO
coherent.
The DMA_ATTR_FORCE_NON_COHERENT DMA attribute overrides the buffer IO
coherency configuration set by making the device IO coherent.

When the DMA_ATTR_FORCE_NON_COHERENT attribute is set during a map call
ensure that it is also set during for the matching unmap call to ensure
that the correct cache maintenance is carried out.

This DMA attribute is only currently supported for arm64 stage 1 IOMMU
mappings.