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