2016-12-10 02:53:05 +08:00
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Buffer Sharing and Synchronization
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==================================
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The dma-buf subsystem provides the framework for sharing buffers for
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hardware (DMA) access across multiple device drivers and subsystems, and
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for synchronizing asynchronous hardware access.
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This is used, for example, by drm "prime" multi-GPU support, but is of
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course not limited to GPU use cases.
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The three main components of this are: (1) dma-buf, representing a
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sg_table and exposed to userspace as a file descriptor to allow passing
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between devices, (2) fence, which provides a mechanism to signal when
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one device has finished access, and (3) reservation, which manages the
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2016-12-10 02:53:05 +08:00
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shared or exclusive fence(s) associated with the buffer.
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Shared DMA Buffers
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------------------
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2016-12-10 02:53:07 +08:00
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This document serves as a guide to device-driver writers on what is the dma-buf
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buffer sharing API, how to use it for exporting and using shared buffers.
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Any device driver which wishes to be a part of DMA buffer sharing, can do so as
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either the 'exporter' of buffers, or the 'user' or 'importer' of buffers.
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Say a driver A wants to use buffers created by driver B, then we call B as the
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exporter, and A as buffer-user/importer.
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The exporter
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- implements and manages operations in :c:type:`struct dma_buf_ops
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<dma_buf_ops>` for the buffer,
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- allows other users to share the buffer by using dma_buf sharing APIs,
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- manages the details of buffer allocation, wrapped in a :c:type:`struct
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dma_buf <dma_buf>`,
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- decides about the actual backing storage where this allocation happens,
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- and takes care of any migration of scatterlist - for all (shared) users of
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this buffer.
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The buffer-user
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- is one of (many) sharing users of the buffer.
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- doesn't need to worry about how the buffer is allocated, or where.
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- and needs a mechanism to get access to the scatterlist that makes up this
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buffer in memory, mapped into its own address space, so it can access the
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same area of memory. This interface is provided by :c:type:`struct
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dma_buf_attachment <dma_buf_attachment>`.
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2016-12-10 05:50:55 +08:00
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Any exporters or users of the dma-buf buffer sharing framework must have a
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'select DMA_SHARED_BUFFER' in their respective Kconfigs.
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Userspace Interface Notes
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
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and hence the generic interface exposed is very minimal. There's a few things to
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consider though:
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- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
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with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
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the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
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llseek operation will report -EINVAL.
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If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
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cases. Userspace can use this to detect support for discovering the dma-buf
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size using llseek.
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- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
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on the file descriptor. This is not just a resource leak, but a
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potential security hole. It could give the newly exec'd application
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access to buffers, via the leaked fd, to which it should otherwise
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not be permitted access.
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The problem with doing this via a separate fcntl() call, versus doing it
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atomically when the fd is created, is that this is inherently racy in a
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multi-threaded app[3]. The issue is made worse when it is library code
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opening/creating the file descriptor, as the application may not even be
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aware of the fd's.
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To avoid this problem, userspace must have a way to request O_CLOEXEC
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flag be set when the dma-buf fd is created. So any API provided by
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the exporting driver to create a dmabuf fd must provide a way to let
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userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
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- Memory mapping the contents of the DMA buffer is also supported. See the
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discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
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- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
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details.
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2016-12-10 02:53:07 +08:00
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Basic Operation and Device DMA Access
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: dma buf device access
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2016-12-10 02:53:08 +08:00
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CPU Access to DMA Buffer Objects
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: cpu access
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2020-06-12 15:05:35 +08:00
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Implicit Fence Poll Support
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: implicit fence polling
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2016-12-10 02:53:07 +08:00
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Kernel Functions and Structures Reference
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2016-12-10 02:53:05 +08:00
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:export:
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.. kernel-doc:: include/linux/dma-buf.h
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:internal:
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Reservation Objects
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-------------------
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2019-09-27 19:15:04 +08:00
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.. kernel-doc:: drivers/dma-buf/dma-resv.c
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:doc: Reservation Object Overview
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2019-09-27 19:15:04 +08:00
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.. kernel-doc:: drivers/dma-buf/dma-resv.c
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:export:
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2019-09-27 19:15:04 +08:00
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.. kernel-doc:: include/linux/dma-resv.h
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:internal:
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DMA Fences
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----------
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2018-07-04 17:29:09 +08:00
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.. kernel-doc:: drivers/dma-buf/dma-fence.c
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:doc: DMA fences overview
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dma-fence: basic lockdep annotations
Design is similar to the lockdep annotations for workers, but with
some twists:
- We use a read-lock for the execution/worker/completion side, so that
this explicit annotation can be more liberally sprinkled around.
With read locks lockdep isn't going to complain if the read-side
isn't nested the same way under all circumstances, so ABBA deadlocks
are ok. Which they are, since this is an annotation only.
- We're using non-recursive lockdep read lock mode, since in recursive
read lock mode lockdep does not catch read side hazards. And we
_very_ much want read side hazards to be caught. For full details of
this limitation see
commit e91498589746065e3ae95d9a00b068e525eec34f
Author: Peter Zijlstra <peterz@infradead.org>
Date: Wed Aug 23 13:13:11 2017 +0200
locking/lockdep/selftests: Add mixed read-write ABBA tests
- To allow nesting of the read-side explicit annotations we explicitly
keep track of the nesting. lock_is_held() allows us to do that.
- The wait-side annotation is a write lock, and entirely done within
dma_fence_wait() for everyone by default.
- To be able to freely annotate helper functions I want to make it ok
to call dma_fence_begin/end_signalling from soft/hardirq context.
First attempt was using the hardirq locking context for the write
side in lockdep, but this forces all normal spinlocks nested within
dma_fence_begin/end_signalling to be spinlocks. That bollocks.
The approach now is to simple check in_atomic(), and for these cases
entirely rely on the might_sleep() check in dma_fence_wait(). That
will catch any wrong nesting against spinlocks from soft/hardirq
contexts.
The idea here is that every code path that's critical for eventually
signalling a dma_fence should be annotated with
dma_fence_begin/end_signalling. The annotation ideally starts right
after a dma_fence is published (added to a dma_resv, exposed as a
sync_file fd, attached to a drm_syncobj fd, or anything else that
makes the dma_fence visible to other kernel threads), up to and
including the dma_fence_wait(). Examples are irq handlers, the
scheduler rt threads, the tail of execbuf (after the corresponding
fences are visible), any workers that end up signalling dma_fences and
really anything else. Not annotated should be code paths that only
complete fences opportunistically as the gpu progresses, like e.g.
shrinker/eviction code.
The main class of deadlocks this is supposed to catch are:
Thread A:
mutex_lock(A);
mutex_unlock(A);
dma_fence_signal();
Thread B:
mutex_lock(A);
dma_fence_wait();
mutex_unlock(A);
Thread B is blocked on A signalling the fence, but A never gets around
to that because it cannot acquire the lock A.
Note that dma_fence_wait() is allowed to be nested within
dma_fence_begin/end_signalling sections. To allow this to happen the
read lock needs to be upgraded to a write lock, which means that any
other lock is acquired between the dma_fence_begin_signalling() call and
the call to dma_fence_wait(), and still held, this will result in an
immediate lockdep complaint. The only other option would be to not
annotate such calls, defeating the point. Therefore these annotations
cannot be sprinkled over the code entirely mindless to avoid false
positives.
Originally I hope that the cross-release lockdep extensions would
alleviate the need for explicit annotations:
https://lwn.net/Articles/709849/
But there's a few reasons why that's not an option:
- It's not happening in upstream, since it got reverted due to too
many false positives:
commit e966eaeeb623f09975ef362c2866fae6f86844f9
Author: Ingo Molnar <mingo@kernel.org>
Date: Tue Dec 12 12:31:16 2017 +0100
locking/lockdep: Remove the cross-release locking checks
This code (CONFIG_LOCKDEP_CROSSRELEASE=y and CONFIG_LOCKDEP_COMPLETIONS=y),
while it found a number of old bugs initially, was also causing too many
false positives that caused people to disable lockdep - which is arguably
a worse overall outcome.
- cross-release uses the complete() call to annotate the end of
critical sections, for dma_fence that would be dma_fence_signal().
But we do not want all dma_fence_signal() calls to be treated as
critical, since many are opportunistic cleanup of gpu requests. If
these get stuck there's still the main completion interrupt and
workers who can unblock everyone. Automatically annotating all
dma_fence_signal() calls would hence cause false positives.
- cross-release had some educated guesses for when a critical section
starts, like fresh syscall or fresh work callback. This would again
cause false positives without explicit annotations, since for
dma_fence the critical sections only starts when we publish a fence.
- Furthermore there can be cases where a thread never does a
dma_fence_signal, but is still critical for reaching completion of
fences. One example would be a scheduler kthread which picks up jobs
and pushes them into hardware, where the interrupt handler or
another completion thread calls dma_fence_signal(). But if the
scheduler thread hangs, then all the fences hang, hence we need to
manually annotate it. cross-release aimed to solve this by chaining
cross-release dependencies, but the dependency from scheduler thread
to the completion interrupt handler goes through hw where
cross-release code can't observe it.
In short, without manual annotations and careful review of the start
and end of critical sections, cross-relese dependency tracking doesn't
work. We need explicit annotations.
v2: handle soft/hardirq ctx better against write side and dont forget
EXPORT_SYMBOL, drivers can't use this otherwise.
v3: Kerneldoc.
v4: Some spelling fixes from Mika
v5: Amend commit message to explain in detail why cross-release isn't
the solution.
v6: Pull out misplaced .rst hunk.
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Thomas Hellstrom <thomas.hellstrom@intel.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-rdma@vger.kernel.org
Cc: amd-gfx@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707201229.472834-2-daniel.vetter@ffwll.ch
2020-07-08 04:12:05 +08:00
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DMA Fence Signalling Annotations
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-fence.c
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:doc: fence signalling annotation
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2018-07-04 17:29:09 +08:00
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DMA Fences Functions Reference
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2016-12-10 02:53:05 +08:00
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.. kernel-doc:: drivers/dma-buf/dma-fence.c
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:export:
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.. kernel-doc:: include/linux/dma-fence.h
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:internal:
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Seqno Hardware Fences
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~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/linux/seqno-fence.h
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:internal:
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DMA Fence Array
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~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
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:export:
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.. kernel-doc:: include/linux/dma-fence-array.h
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:internal:
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DMA Fence uABI/Sync File
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~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/sync_file.c
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:export:
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.. kernel-doc:: include/linux/sync_file.h
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:internal:
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