Subject: [RFC 2/2] dma-buf: Documentation for buffer sharing framework

From: [email-masked] (Randy Dunlap)
Date: Wed, 12 Oct 2011 15:30:18 -0700

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 <[email-masked]>
> Signed-off-by: Sumit Semwal <[email-masked]>
> ---
>  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@linaro dot org>
> +                 <sumit dot semwal@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

-- 
~Randy
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