mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-29 07:04:10 +08:00
dma-buf: Documentation for buffer sharing framework
Add documentation for dma buffer sharing framework, explaining the various operations, members and API of the dma buffer sharing framework. Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org> Signed-off-by: Sumit Semwal <sumit.semwal@ti.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Dave Airlie <airlied@redhat.com>
This commit is contained in:
parent
d15bd7ee44
commit
a7df4719a3
224
Documentation/dma-buf-sharing.txt
Normal file
224
Documentation/dma-buf-sharing.txt
Normal file
@ -0,0 +1,224 @@
|
||||
DMA Buffer Sharing API Guide
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Sumit Semwal
|
||||
<sumit dot semwal at linaro dot org>
|
||||
<sumit dot semwal at ti dot com>
|
||||
|
||||
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' 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.
|
||||
|
||||
The exporter
|
||||
- implements and manages operations[1] for the buffer
|
||||
- allows other users to share the buffer by using dma_buf sharing APIs,
|
||||
- manages the details of buffer allocation,
|
||||
- decides about the actual backing storage where this allocation happens,
|
||||
- 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.
|
||||
- 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.
|
||||
|
||||
*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
|
||||
For this first version, A buffer shared using the dma_buf sharing API:
|
||||
- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
|
||||
this framework.
|
||||
- may be used *ONLY* by importers that do not need CPU access to the buffer.
|
||||
|
||||
The dma_buf buffer sharing API usage contains the following steps:
|
||||
|
||||
1. Exporter announces that it wishes to export a buffer
|
||||
2. Userspace gets the file descriptor associated with the exported buffer, and
|
||||
passes it around to potential buffer-users based on use case
|
||||
3. Each buffer-user 'connects' itself to the buffer
|
||||
4. When needed, buffer-user requests access to the buffer from exporter
|
||||
5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
|
||||
6. when buffer-user is done using this buffer completely, it 'disconnects'
|
||||
itself from the buffer.
|
||||
|
||||
|
||||
1. Exporter's announcement of buffer export
|
||||
|
||||
The buffer exporter announces its wish to export a buffer. In this, it
|
||||
connects its own private buffer data, provides implementation for operations
|
||||
that can be performed on the exported dma_buf, and flags for the file
|
||||
associated with this buffer.
|
||||
|
||||
Interface:
|
||||
struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
|
||||
size_t size, int flags)
|
||||
|
||||
If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
|
||||
pointer to the same. It also associates an anonymous file with this buffer,
|
||||
so it can be exported. On failure to allocate the dma_buf object, it returns
|
||||
NULL.
|
||||
|
||||
2. Userspace gets a handle to pass around to potential buffer-users
|
||||
|
||||
Userspace entity requests for a file-descriptor (fd) which is a handle to the
|
||||
anonymous file associated with the buffer. It can then share the fd with other
|
||||
drivers and/or processes.
|
||||
|
||||
Interface:
|
||||
int dma_buf_fd(struct dma_buf *dmabuf)
|
||||
|
||||
This API installs an fd for the anonymous file associated with this buffer;
|
||||
returns either 'fd', or error.
|
||||
|
||||
3. Each buffer-user 'connects' itself to the buffer
|
||||
|
||||
Each buffer-user now gets a reference to the buffer, using the fd passed to
|
||||
it.
|
||||
|
||||
Interface:
|
||||
struct dma_buf *dma_buf_get(int fd)
|
||||
|
||||
This API will return a reference to the dma_buf, and increment refcount for
|
||||
it.
|
||||
|
||||
After this, the buffer-user needs to attach its device with the buffer, which
|
||||
helps the exporter to know of device buffer constraints.
|
||||
|
||||
Interface:
|
||||
struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
|
||||
struct device *dev)
|
||||
|
||||
This API returns reference to an attachment structure, which is then used
|
||||
for scatterlist operations. It will optionally call the 'attach' dma_buf
|
||||
operation, if provided by the exporter.
|
||||
|
||||
The dma-buf sharing framework does the bookkeeping bits related to managing
|
||||
the list of all attachments to a buffer.
|
||||
|
||||
Until this stage, the buffer-exporter has the option to choose not to actually
|
||||
allocate the backing storage for this buffer, but wait for the first buffer-user
|
||||
to request use of buffer for allocation.
|
||||
|
||||
|
||||
4. When needed, buffer-user requests access to the buffer
|
||||
|
||||
Whenever a buffer-user wants to use the buffer for any DMA, it asks for
|
||||
access to the buffer using dma_buf_map_attachment API. At least one attach to
|
||||
the buffer must have happened before map_dma_buf can be called.
|
||||
|
||||
Interface:
|
||||
struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
|
||||
enum dma_data_direction);
|
||||
|
||||
This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
|
||||
"dma_buf->ops->" indirection from the users of this interface.
|
||||
|
||||
In struct dma_buf_ops, map_dma_buf is defined as
|
||||
struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
|
||||
enum dma_data_direction);
|
||||
|
||||
It is one of the buffer operations that must be implemented by the exporter.
|
||||
It should return the sg_table containing scatterlist for this buffer, mapped
|
||||
into caller's address space.
|
||||
|
||||
If this is being called for the first time, the exporter can now choose to
|
||||
scan through the list of attachments for this buffer, collate the requirements
|
||||
of the attached devices, and choose an appropriate backing storage for the
|
||||
buffer.
|
||||
|
||||
Based on enum dma_data_direction, it might be possible to have multiple users
|
||||
accessing at the same time (for reading, maybe), or any other kind of sharing
|
||||
that the exporter might wish to make available to buffer-users.
|
||||
|
||||
map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
|
||||
|
||||
|
||||
5. When finished, the buffer-user notifies end-of-DMA to exporter
|
||||
|
||||
Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
|
||||
the exporter using the dma_buf_unmap_attachment API.
|
||||
|
||||
Interface:
|
||||
void dma_buf_unmap_attachment(struct dma_buf_attachment *,
|
||||
struct sg_table *);
|
||||
|
||||
This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
|
||||
"dma_buf->ops->" indirection from the users of this interface.
|
||||
|
||||
In struct dma_buf_ops, unmap_dma_buf is defined as
|
||||
void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
|
||||
|
||||
unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
|
||||
map_dma_buf, this API also must be implemented by the exporter.
|
||||
|
||||
|
||||
6. when buffer-user is done using this buffer, it 'disconnects' itself from the
|
||||
buffer.
|
||||
|
||||
After the buffer-user has no more interest in using this buffer, it should
|
||||
disconnect itself from the buffer:
|
||||
|
||||
- it first detaches itself from the buffer.
|
||||
|
||||
Interface:
|
||||
void dma_buf_detach(struct dma_buf *dmabuf,
|
||||
struct dma_buf_attachment *dmabuf_attach);
|
||||
|
||||
This API removes the attachment from the list in dmabuf, and optionally calls
|
||||
dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
|
||||
|
||||
- Then, the buffer-user returns the buffer reference to exporter.
|
||||
|
||||
Interface:
|
||||
void dma_buf_put(struct dma_buf *dmabuf);
|
||||
|
||||
This API then reduces the refcount for this buffer.
|
||||
|
||||
If, as a result of this call, the refcount becomes 0, the 'release' file
|
||||
operation related to this fd is called. It calls the dmabuf->ops->release()
|
||||
operation in turn, and frees the memory allocated for dmabuf when exported.
|
||||
|
||||
NOTES:
|
||||
- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
|
||||
The attach-detach calls allow the exporter to figure out backing-storage
|
||||
constraints for the currently-interested devices. This allows preferential
|
||||
allocation, and/or migration of pages across different types of storage
|
||||
available, if possible.
|
||||
|
||||
Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
|
||||
to allow just-in-time backing of storage, and migration mid-way through a
|
||||
use-case.
|
||||
|
||||
- Migration of backing storage if needed
|
||||
If after
|
||||
- at least one map_dma_buf has happened,
|
||||
- and the backing storage has been allocated for this buffer,
|
||||
another new buffer-user intends to attach itself to this buffer, it might
|
||||
be allowed, if possible for the exporter.
|
||||
|
||||
In case it is allowed by the exporter:
|
||||
if the new buffer-user has stricter 'backing-storage constraints', and the
|
||||
exporter can handle these constraints, the exporter can just stall on the
|
||||
map_dma_buf until all outstanding access is completed (as signalled by
|
||||
unmap_dma_buf).
|
||||
Once all users have finished accessing and have unmapped this buffer, the
|
||||
exporter could potentially move the buffer to the stricter backing-storage,
|
||||
and then allow further {map,unmap}_dma_buf operations from any buffer-user
|
||||
from the migrated backing-storage.
|
||||
|
||||
If the exporter cannot fulfil the backing-storage constraints of the new
|
||||
buffer-user device as requested, dma_buf_attach() would return an error to
|
||||
denote non-compatibility of the new buffer-sharing request with the current
|
||||
buffer.
|
||||
|
||||
If the exporter chooses not to allow an attach() operation once a
|
||||
map_dma_buf() API has been called, it simply returns an error.
|
||||
|
||||
References:
|
||||
[1] struct dma_buf_ops in include/linux/dma-buf.h
|
||||
[2] All interfaces mentioned above defined in include/linux/dma-buf.h
|
Loading…
Reference in New Issue
Block a user