251 строка
8.9 KiB
ReStructuredText
251 строка
8.9 KiB
ReStructuredText
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 has 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 in 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 `Implicit Fence Poll Support`_ below for
|
|
details.
|
|
|
|
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
|
|
|
|
Implicit Fence Poll Support
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-buf.c
|
|
:doc: implicit fence polling
|
|
|
|
Kernel Functions and Structures Reference
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-buf.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/dma-buf.h
|
|
:internal:
|
|
|
|
Reservation Objects
|
|
-------------------
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-resv.c
|
|
:doc: Reservation Object Overview
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-resv.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/dma-resv.h
|
|
:internal:
|
|
|
|
DMA Fences
|
|
----------
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-fence.c
|
|
:doc: DMA fences overview
|
|
|
|
DMA Fence Cross-Driver Contract
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-fence.c
|
|
:doc: fence cross-driver contract
|
|
|
|
DMA Fence Signalling Annotations
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-fence.c
|
|
:doc: fence signalling annotation
|
|
|
|
DMA Fences Functions Reference
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-fence.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/dma-fence.h
|
|
:internal:
|
|
|
|
Seqno Hardware Fences
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: include/linux/seqno-fence.h
|
|
:internal:
|
|
|
|
DMA Fence Array
|
|
~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/dma-fence-array.h
|
|
:internal:
|
|
|
|
DMA Fence uABI/Sync File
|
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: drivers/dma-buf/sync_file.c
|
|
:export:
|
|
|
|
.. kernel-doc:: include/linux/sync_file.h
|
|
:internal:
|
|
|
|
Indefinite DMA Fences
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
At various times &dma_fence with an indefinite time until dma_fence_wait()
|
|
finishes have been proposed. Examples include:
|
|
|
|
* Future fences, used in HWC1 to signal when a buffer isn't used by the display
|
|
any longer, and created with the screen update that makes the buffer visible.
|
|
The time this fence completes is entirely under userspace's control.
|
|
|
|
* Proxy fences, proposed to handle &drm_syncobj for which the fence has not yet
|
|
been set. Used to asynchronously delay command submission.
|
|
|
|
* Userspace fences or gpu futexes, fine-grained locking within a command buffer
|
|
that userspace uses for synchronization across engines or with the CPU, which
|
|
are then imported as a DMA fence for integration into existing winsys
|
|
protocols.
|
|
|
|
* Long-running compute command buffers, while still using traditional end of
|
|
batch DMA fences for memory management instead of context preemption DMA
|
|
fences which get reattached when the compute job is rescheduled.
|
|
|
|
Common to all these schemes is that userspace controls the dependencies of these
|
|
fences and controls when they fire. Mixing indefinite fences with normal
|
|
in-kernel DMA fences does not work, even when a fallback timeout is included to
|
|
protect against malicious userspace:
|
|
|
|
* Only the kernel knows about all DMA fence dependencies, userspace is not aware
|
|
of dependencies injected due to memory management or scheduler decisions.
|
|
|
|
* Only userspace knows about all dependencies in indefinite fences and when
|
|
exactly they will complete, the kernel has no visibility.
|
|
|
|
Furthermore the kernel has to be able to hold up userspace command submission
|
|
for memory management needs, which means we must support indefinite fences being
|
|
dependent upon DMA fences. If the kernel also support indefinite fences in the
|
|
kernel like a DMA fence, like any of the above proposal would, there is the
|
|
potential for deadlocks.
|
|
|
|
.. kernel-render:: DOT
|
|
:alt: Indefinite Fencing Dependency Cycle
|
|
:caption: Indefinite Fencing Dependency Cycle
|
|
|
|
digraph "Fencing Cycle" {
|
|
node [shape=box bgcolor=grey style=filled]
|
|
kernel [label="Kernel DMA Fences"]
|
|
userspace [label="userspace controlled fences"]
|
|
kernel -> userspace [label="memory management"]
|
|
userspace -> kernel [label="Future fence, fence proxy, ..."]
|
|
|
|
{ rank=same; kernel userspace }
|
|
}
|
|
|
|
This means that the kernel might accidentally create deadlocks
|
|
through memory management dependencies which userspace is unaware of, which
|
|
randomly hangs workloads until the timeout kicks in. Workloads, which from
|
|
userspace's perspective, do not contain a deadlock. In such a mixed fencing
|
|
architecture there is no single entity with knowledge of all dependencies.
|
|
Thefore preventing such deadlocks from within the kernel is not possible.
|
|
|
|
The only solution to avoid dependencies loops is by not allowing indefinite
|
|
fences in the kernel. This means:
|
|
|
|
* No future fences, proxy fences or userspace fences imported as DMA fences,
|
|
with or without a timeout.
|
|
|
|
* No DMA fences that signal end of batchbuffer for command submission where
|
|
userspace is allowed to use userspace fencing or long running compute
|
|
workloads. This also means no implicit fencing for shared buffers in these
|
|
cases.
|