mirror of
https://github.com/torvalds/linux.git
synced 2024-11-27 14:41:39 +00:00
7283fff8b5
This flag is not implemented by any backend and only set by the ib_umem module in a single instance. Link: https://lore.kernel.org/r/20191113073214.9514-2-hch@lst.de Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Jason Gunthorpe <jgg@mellanox.com> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
141 lines
6.0 KiB
Plaintext
141 lines
6.0 KiB
Plaintext
==============
|
|
DMA attributes
|
|
==============
|
|
|
|
This document describes the semantics of the DMA attributes that are
|
|
defined in linux/dma-mapping.h.
|
|
|
|
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_ALLOC_SINGLE_PAGES
|
|
---------------------------
|
|
|
|
This is a hint to the DMA-mapping subsystem that it's probably not worth
|
|
the time to try to allocate memory to in a way that gives better TLB
|
|
efficiency (AKA it's not worth trying to build the mapping out of larger
|
|
pages). You might want to specify this if:
|
|
|
|
- You know that the accesses to this memory won't thrash the TLB.
|
|
You might know that the accesses are likely to be sequential or
|
|
that they aren't sequential but it's unlikely you'll ping-pong
|
|
between many addresses that are likely to be in different physical
|
|
pages.
|
|
- You know that the penalty of TLB misses while accessing the
|
|
memory will be small enough to be inconsequential. If you are
|
|
doing a heavy operation like decryption or decompression this
|
|
might be the case.
|
|
- You know that the DMA mapping is fairly transitory. If you expect
|
|
the mapping to have a short lifetime then it may be worth it to
|
|
optimize allocation (avoid coming up with large pages) instead of
|
|
getting the slight performance win of larger pages.
|
|
|
|
Setting this hint doesn't guarantee that you won't get huge pages, but it
|
|
means that we won't try quite as hard to get them.
|
|
|
|
.. note:: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
|
|
though ARM64 patches will likely be posted soon.
|
|
|
|
DMA_ATTR_NO_WARN
|
|
----------------
|
|
|
|
This tells the DMA-mapping subsystem to suppress allocation failure reports
|
|
(similarly to __GFP_NOWARN).
|
|
|
|
On some architectures allocation failures are reported with error messages
|
|
to the system logs. Although this can help to identify and debug problems,
|
|
drivers which handle failures (eg, retry later) have no problems with them,
|
|
and can actually flood the system logs with error messages that aren't any
|
|
problem at all, depending on the implementation of the retry mechanism.
|
|
|
|
So, this provides a way for drivers to avoid those error messages on calls
|
|
where allocation failures are not a problem, and shouldn't bother the logs.
|
|
|
|
.. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
|
|
|
|
DMA_ATTR_PRIVILEGED
|
|
-------------------
|
|
|
|
Some advanced peripherals such as remote processors and GPUs perform
|
|
accesses to DMA buffers in both privileged "supervisor" and unprivileged
|
|
"user" modes. This attribute is used to indicate to the DMA-mapping
|
|
subsystem that the buffer is fully accessible at the elevated privilege
|
|
level (and ideally inaccessible or at least read-only at the
|
|
lesser-privileged levels).
|