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Subject[RFC 2/2] dma-buf: Documentation for buffer sharing framework
Date
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
+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.
+
+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
+ 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
+ the list of all attachments to a buffer.
+
+Till 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->ops->get_scatterlist operation. Atleast
+ 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,
+ - 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
+ 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
--
1.7.4.1


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