On 10/11/2011 02:23 AM, Sumit Semwal wrote:
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
Documentation/dma-buf-sharing.txt | 210 +++++++++++++++++++++++++++++++++++++ 1 files changed, 210 insertions(+), 0 deletions(-) create mode 100644 Documentation/dma-buf-sharing.txt
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt new file mode 100644 index 0000000..4da6644 --- /dev/null +++ b/Documentation/dma-buf-sharing.txt @@ -0,0 +1,210 @@
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
Please use DMA instead of dma (except combinations like dma-buf are OK). [multiple]
+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 B as buffer-user.
and A
+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,
+- optionally, provides mmap capability for drivers that need it.
+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.
+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,int flags)- If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
- pointer to the same. It also associates an anon file with this buffer, so it
s/anon/anonymous/ (multiple)
- 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
- anon 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 anon 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 book-keeping bits related to keeping
bookkeeping
- the list of all attachments to a buffer.
+Till this stage, the buffer-exporter has the option to choose not to actually
Until
+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->ops->get_scatterlist operation. Atleast
At least
- one attach to the buffer should have happened before get_scatterlist can be
- called.
- Interface: [member of struct dma_buf_ops]
struct scatterlist * (*get_scatterlist)(struct dma_buf_attachment *,enum dma_data_direction,int* nents);- It is one of the buffer operations that must be implemented by the exporter.
- It should return the 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.
+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->ops->put_scatterlist() operation.
- Interface:
void (*put_scatterlist)(struct dma_buf_attachment *, struct scatterlist *,int nents);- put_scatterlist signifies the end-of-dma for the attachment provided.
+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 {get,put}_scatterlist 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 {get,put}_scatterlist 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
- After
- atleast one get_scatterlist has happened,
at least
- and the backing storage has been allocated for this buffer,
- If 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
- get_scatterlist till all outstanding access is completed (as signalled by
until
- put_scatterlist).
- Once all ongoing access is completed, the exporter could potentially move
- the buffer to the stricter backing-storage, and then allow further
- {get,put}_scatterlist operations from any buffer-user from the migrated
- backing-storage.
- If the exporter cannot fulfill 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
- get_scatterlist has been called, it simply returns an error.
+- mmap file operation
- An mmap() file operation is provided for the fd associated with the buffer.
- If the exporter defines an mmap operation, the mmap() fop calls this to allow
- mmap for devices that might need it; if not, it 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