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Buffer Sharing and Synchronization
The dma-buf subsystem provides the framework for sharing buffers for
hardware (DMA) access across multiple device drivers and subsystems, and
for synchronizing asynchronous hardware access.
This is used, for example, by drm "prime" multi-GPU support, but is of
course not limited to GPU use cases.
The three main components of this are: (1) dma-buf, representing a
sg_table and exposed to userspace as a file descriptor to allow passing
between devices, (2) fence, which provides a mechanism to signal when
one device as finished access, and (3) reservation, which manages the
shared or exclusive fence(s) associated with the buffer.
Shared DMA Buffers
This document serves as a guide to device-driver writers on what is the dma-buf
buffer sharing API, how to use it for exporting and using shared buffers.
Any device driver which wishes to be a part of DMA buffer sharing, can do so as
either the 'exporter' of buffers, or the 'user' or 'importer' of buffers.
Say a driver A wants to use buffers created by driver B, then we call B as the
exporter, and A as buffer-user/importer.
The exporter
- implements and manages operations in :c:type:`struct dma_buf_ops
<dma_buf_ops>` for the buffer,
- allows other users to share the buffer by using dma_buf sharing APIs,
- manages the details of buffer allocation, wrapped int a :c:type:`struct
dma_buf <dma_buf>`,
- decides about the actual backing storage where this allocation happens,
- and takes care of any migration of scatterlist - for all (shared) users of
this buffer.
The buffer-user
- is one of (many) sharing users of the buffer.
- doesn't need to worry about how the buffer is allocated, or where.
- and needs a mechanism to get access to the scatterlist that makes up this
buffer in memory, mapped into its own address space, so it can access the
same area of memory. This interface is provided by :c:type:`struct
dma_buf_attachment <dma_buf_attachment>`.
Any exporters or users of the dma-buf buffer sharing framework must have a
'select DMA_SHARED_BUFFER' in their respective Kconfigs.
Userspace Interface Notes
Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
and hence the generic interface exposed is very minimal. There's a few things to
consider though:
- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
llseek operation will report -EINVAL.
If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
cases. Userspace can use this to detect support for discovering the dma-buf
size using llseek.
- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
on the file descriptor. This is not just a resource leak, but a
potential security hole. It could give the newly exec'd application
access to buffers, via the leaked fd, to which it should otherwise
not be permitted access.
The problem with doing this via a separate fcntl() call, versus doing it
atomically when the fd is created, is that this is inherently racy in a
multi-threaded app[3]. The issue is made worse when it is library code
opening/creating the file descriptor, as the application may not even be
aware of the fd's.
To avoid this problem, userspace must have a way to request O_CLOEXEC
flag be set when the dma-buf fd is created. So any API provided by
the exporting driver to create a dmabuf fd must provide a way to let
userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
- Memory mapping the contents of the DMA buffer is also supported. See the
discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
Basic Operation and Device DMA Access
.. kernel-doc:: drivers/dma-buf/dma-buf.c
:doc: dma buf device access
CPU Access to DMA Buffer Objects
.. kernel-doc:: drivers/dma-buf/dma-buf.c
:doc: cpu access
Fence Poll Support
.. kernel-doc:: drivers/dma-buf/dma-buf.c
:doc: fence polling
Kernel Functions and Structures Reference
.. kernel-doc:: drivers/dma-buf/dma-buf.c
.. kernel-doc:: include/linux/dma-buf.h
Reservation Objects
.. kernel-doc:: drivers/dma-buf/reservation.c
:doc: Reservation Object Overview
.. kernel-doc:: drivers/dma-buf/reservation.c
.. kernel-doc:: include/linux/reservation.h
DMA Fences
.. kernel-doc:: drivers/dma-buf/dma-fence.c
:doc: DMA fences overview
DMA Fences Functions Reference
.. kernel-doc:: drivers/dma-buf/dma-fence.c
.. kernel-doc:: include/linux/dma-fence.h
Seqno Hardware Fences
.. kernel-doc:: include/linux/seqno-fence.h
DMA Fence Array
.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
.. kernel-doc:: include/linux/dma-fence-array.h
DMA Fence uABI/Sync File
.. kernel-doc:: drivers/dma-buf/sync_file.c
.. kernel-doc:: include/linux/sync_file.h