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dma-buf: Document DMA_BUF_IOCTL_SYNC (v3)

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This adds a new "DMA Buffer ioctls" section to the dma-buf docs and adds
documentation for DMA_BUF_IOCTL_SYNC.

v2 (Daniel Vetter):
 - Fix a couple typos
 - Add commentary about synchronization with other devices
 - Use item list format for describing flags

v3 (Pekka Paalanen):
 - Clarify stalling requirements.
 - Be more clear that that DMA_BUF_IOCTL_SYNC with SINC_END has to be
   called before more GPU work happens.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617194258.579011-1-jason@jlekstrand.net
This commit is contained in:
Jason Ekstrand 2021-06-17 14:42:58 -05:00 committed by Daniel Vetter
parent d35c97f606
commit 51f52547df
2 changed files with 57 additions and 1 deletions
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8
Documentation/driver-api/dma-buf.rst
View File

@ -88,6 +88,9 @@ consider though:
- The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for
details.
- The DMA buffer FD also supports a few dma-buf-specific ioctls, see
`DMA Buffer ioctls`_ below for details.
Basic Operation and Device DMA Access
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -111,6 +114,11 @@ DMA-BUF statistics
.. kernel-doc:: drivers/dma-buf/dma-buf-sysfs-stats.c
:doc: overview
DMA Buffer ioctls
~~~~~~~~~~~~~~~~~
.. kernel-doc:: include/uapi/linux/dma-buf.h
Kernel Functions and Structures Reference
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

50
include/uapi/linux/dma-buf.h
View File

@ -22,8 +22,56 @@
#include <linux/types.h>
/* begin/end dma-buf functions used for userspace mmap. */
/**
* struct dma_buf_sync - Synchronize with CPU access.
*
* When a DMA buffer is accessed from the CPU via mmap, it is not always
* possible to guarantee coherency between the CPU-visible map and underlying
* memory. To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket
* any CPU access to give the kernel the chance to shuffle memory around if
* needed.
*
* Prior to accessing the map, the client must call DMA_BUF_IOCTL_SYNC
* with DMA_BUF_SYNC_START and the appropriate read/write flags. Once the
* access is complete, the client should call DMA_BUF_IOCTL_SYNC with
* DMA_BUF_SYNC_END and the same read/write flags.
*
* The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache
* coherency. It does not prevent other processes or devices from
* accessing the memory at the same time. If synchronization with a GPU or
* other device driver is required, it is the client's responsibility to
* wait for buffer to be ready for reading or writing before calling this
* ioctl with DMA_BUF_SYNC_START. Likewise, the client must ensure that
* follow-up work is not submitted to GPU or other device driver until
* after this ioctl has been called with DMA_BUF_SYNC_END?
*
* If the driver or API with which the client is interacting uses implicit
* synchronization, waiting for prior work to complete can be done via
* poll() on the DMA buffer file descriptor. If the driver or API requires
* explicit synchronization, the client may have to wait on a sync_file or
* other synchronization primitive outside the scope of the DMA buffer API.
*/
struct dma_buf_sync {
/**
* @flags: Set of access flags
*
* DMA_BUF_SYNC_START:
* Indicates the start of a map access session.
*
* DMA_BUF_SYNC_END:
* Indicates the end of a map access session.
*
* DMA_BUF_SYNC_READ:
* Indicates that the mapped DMA buffer will be read by the
* client via the CPU map.
*
* DMA_BUF_SYNC_WRITE:
* Indicates that the mapped DMA buffer will be written by the
* client via the CPU map.
*
* DMA_BUF_SYNC_RW:
* An alias for DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE.
*/
__u64 flags;
};