463 lines
21 KiB
Plaintext
463 lines
21 KiB
Plaintext
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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dma-buf operations for device dma only
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--------------------------------------
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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_named(void *priv, struct dma_buf_ops *ops,
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size_t size, int flags,
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const char *exp_name)
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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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'exp_name' is the name of exporter - to facilitate information while
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debugging.
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Exporting modules which do not wish to provide any specific name may use the
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helper define 'dma_buf_export()', with the same arguments as above, but
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without the last argument; a KBUILD_MODNAME pre-processor directive will be
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inserted in place of 'exp_name' instead.
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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 fulfill 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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Kernel cpu access to a dma-buf buffer object
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--------------------------------------------
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The motivation to allow cpu access from the kernel to a dma-buf object from the
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importers side are:
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- fallback operations, e.g. if the devices is connected to a usb bus and the
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kernel needs to shuffle the data around first before sending it away.
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- full transparency for existing users on the importer side, i.e. userspace
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should not notice the difference between a normal object from that subsystem
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and an imported one backed by a dma-buf. This is really important for drm
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opengl drivers that expect to still use all the existing upload/download
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paths.
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Access to a dma_buf from the kernel context involves three steps:
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1. Prepare access, which invalidate any necessary caches and make the object
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available for cpu access.
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2. Access the object page-by-page with the dma_buf map apis
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3. Finish access, which will flush any necessary cpu caches and free reserved
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resources.
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1. Prepare access
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Before an importer can access a dma_buf object with the cpu from the kernel
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context, it needs to notify the exporter of the access that is about to
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happen.
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Interface:
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int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
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size_t start, size_t len,
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enum dma_data_direction direction)
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This allows the exporter to ensure that the memory is actually available for
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cpu access - the exporter might need to allocate or swap-in and pin the
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backing storage. The exporter also needs to ensure that cpu access is
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coherent for the given range and access direction. The range and access
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direction can be used by the exporter to optimize the cache flushing, i.e.
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access outside of the range or with a different direction (read instead of
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write) might return stale or even bogus data (e.g. when the exporter needs to
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copy the data to temporary storage).
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This step might fail, e.g. in oom conditions.
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2. Accessing the buffer
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To support dma_buf objects residing in highmem cpu access is page-based using
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an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
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PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
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a pointer in kernel virtual address space. Afterwards the chunk needs to be
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unmapped again. There is no limit on how often a given chunk can be mapped
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and unmapped, i.e. the importer does not need to call begin_cpu_access again
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before mapping the same chunk again.
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Interfaces:
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void *dma_buf_kmap(struct dma_buf *, unsigned long);
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void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
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There are also atomic variants of these interfaces. Like for kmap they
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facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
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the callback) is allowed to block when using these.
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Interfaces:
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void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
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void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
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For importers all the restrictions of using kmap apply, like the limited
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supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
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atomic dma_buf kmaps at the same time (in any given process context).
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dma_buf kmap calls outside of the range specified in begin_cpu_access are
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undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
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the partial chunks at the beginning and end but may return stale or bogus
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data outside of the range (in these partial chunks).
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Note that these calls need to always succeed. The exporter needs to complete
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any preparations that might fail in begin_cpu_access.
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For some cases the overhead of kmap can be too high, a vmap interface
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is introduced. This interface should be used very carefully, as vmalloc
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space is a limited resources on many architectures.
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Interfaces:
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void *dma_buf_vmap(struct dma_buf *dmabuf)
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void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
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The vmap call can fail if there is no vmap support in the exporter, or if it
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runs out of vmalloc space. Fallback to kmap should be implemented. Note that
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the dma-buf layer keeps a reference count for all vmap access and calls down
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into the exporter's vmap function only when no vmapping exists, and only
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unmaps it once. Protection against concurrent vmap/vunmap calls is provided
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by taking the dma_buf->lock mutex.
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3. Finish access
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When the importer is done accessing the range specified in begin_cpu_access,
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it needs to announce this to the exporter (to facilitate cache flushing and
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unpinning of any pinned resources). The result of any dma_buf kmap calls
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after end_cpu_access is undefined.
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Interface:
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void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
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size_t start, size_t len,
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enum dma_data_direction dir);
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Direct Userspace Access/mmap Support
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------------------------------------
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Being able to mmap an export dma-buf buffer object has 2 main use-cases:
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- CPU fallback processing in a pipeline and
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- supporting existing mmap interfaces in importers.
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1. CPU fallback processing in a pipeline
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In many processing pipelines it is sometimes required that the cpu can access
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the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
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the need to handle this specially in userspace frameworks for buffer sharing
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it's ideal if the dma_buf fd itself can be used to access the backing storage
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from userspace using mmap.
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Furthermore Android's ION framework already supports this (and is otherwise
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rather similar to dma-buf from a userspace consumer side with using fds as
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handles, too). So it's beneficial to support this in a similar fashion on
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dma-buf to have a good transition path for existing Android userspace.
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No special interfaces, userspace simply calls mmap on the dma-buf fd.
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2. Supporting existing mmap interfaces in importers
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Similar to the motivation for kernel cpu access it is again important that
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the userspace code of a given importing subsystem can use the same interfaces
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with a imported dma-buf buffer object as with a native buffer object. This is
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especially important for drm where the userspace part of contemporary OpenGL,
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X, and other drivers is huge, and reworking them to use a different way to
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mmap a buffer rather invasive.
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The assumption in the current dma-buf interfaces is that redirecting the
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initial mmap is all that's needed. A survey of some of the existing
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subsystems shows that no driver seems to do any nefarious thing like syncing
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up with outstanding asynchronous processing on the device or allocating
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special resources at fault time. So hopefully this is good enough, since
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adding interfaces to intercept pagefaults and allow pte shootdowns would
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increase the complexity quite a bit.
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Interface:
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int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
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unsigned long);
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If the importing subsystem simply provides a special-purpose mmap call to set
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up a mapping in userspace, calling do_mmap with dma_buf->file will equally
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achieve that for a dma-buf object.
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3. Implementation notes for exporters
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Because dma-buf buffers have invariant size over their lifetime, the dma-buf
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core checks whether a vma is too large and rejects such mappings. The
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exporter hence does not need to duplicate this check.
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Because existing importing subsystems might presume coherent mappings for
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userspace, the exporter needs to set up a coherent mapping. If that's not
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possible, it needs to fake coherency by manually shooting down ptes when
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leaving the cpu domain and flushing caches at fault time. Note that all the
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dma_buf files share the same anon inode, hence the exporter needs to replace
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the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
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required. This is because the kernel uses the underlying inode's address_space
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for vma tracking (and hence pte tracking at shootdown time with
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unmap_mapping_range).
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If the above shootdown dance turns out to be too expensive in certain
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scenarios, we can extend dma-buf with a more explicit cache tracking scheme
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for userspace mappings. But the current assumption is that using mmap is
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always a slower path, so some inefficiencies should be acceptable.
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Exporters that shoot down mappings (for any reasons) shall not do any
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synchronization at fault time with outstanding device operations.
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Synchronization is an orthogonal issue to sharing the backing storage of a
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buffer and hence should not be handled by dma-buf itself. This is explicitly
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mentioned here because many people seem to want something like this, but if
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different exporters handle this differently, buffer sharing can fail in
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interesting ways depending upong the exporter (if userspace starts depending
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upon this implicit synchronization).
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Other Interfaces Exposed to Userspace on the dma-buf FD
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------------------------------------------------------
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- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
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with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
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the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
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llseek operation will report -EINVAL.
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If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
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cases. Userspace can use this to detect support for discovering the dma-buf
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size using llseek.
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Miscellaneous notes
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-------------------
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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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- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
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on the file descriptor. This is not just a resource leak, but a
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potential security hole. It could give the newly exec'd application
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access to buffers, via the leaked fd, to which it should otherwise
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not be permitted access.
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The problem with doing this via a separate fcntl() call, versus doing it
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atomically when the fd is created, is that this is inherently racy in a
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multi-threaded app[3]. The issue is made worse when it is library code
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opening/creating the file descriptor, as the application may not even be
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aware of the fd's.
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To avoid this problem, userspace must have a way to request O_CLOEXEC
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flag be set when the dma-buf fd is created. So any API provided by
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the exporting driver to create a dmabuf fd must provide a way to let
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userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
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- If an exporter needs to manually flush caches and hence needs to fake
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coherency for mmap support, it needs to be able to zap all the ptes pointing
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at the backing storage. Now linux mm needs a struct address_space associated
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with the struct file stored in vma->vm_file to do that with the function
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unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
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with the anon_file struct file, i.e. all dma_bufs share the same file.
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Hence exporters need to setup their own file (and address_space) association
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by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
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callback. In the specific case of a gem driver the exporter could use the
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shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
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zap ptes by unmapping the corresponding range of the struct address_space
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associated with their own file.
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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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[3] https://lwn.net/Articles/236486/
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