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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
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treeb9996006addfd7ae70a39672b76843b49aebc189 /Documentation/dma-buf-sharing.txt
downloadlinux-linaro-highbank-3b4bd47f8f4ed3aaf7c81c9b5d2d37ad79fadf4a.tar.gz
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+ 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 A 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,
+
+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.
+
+dma-buf operations for device dma only
+--------------------------------------
+
+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,
+ size_t size, int flags)
+
+ If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
+ pointer to the same. It also associates an anonymous 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
+ anonymous 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 anonymous 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 bookkeeping bits related to managing
+ the list of all attachments to a buffer.
+
+Until 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_map_attachment API. At least one attach to
+ the buffer must have happened before map_dma_buf can be called.
+
+ Interface:
+ struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
+ enum dma_data_direction);
+
+ This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
+ "dma_buf->ops->" indirection from the users of this interface.
+
+ In struct dma_buf_ops, map_dma_buf is defined as
+ struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
+ enum dma_data_direction);
+
+ It is one of the buffer operations that must be implemented by the exporter.
+ It should return the sg_table containing 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.
+
+ map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
+
+
+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_unmap_attachment API.
+
+ Interface:
+ void dma_buf_unmap_attachment(struct dma_buf_attachment *,
+ struct sg_table *);
+
+ This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
+ "dma_buf->ops->" indirection from the users of this interface.
+
+ In struct dma_buf_ops, unmap_dma_buf is defined as
+ void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
+
+ unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
+ map_dma_buf, this API also must be implemented by the exporter.
+
+
+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 {map,unmap}_dma_buf 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 {map,unmap}_dma_buf 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
+ If after
+ - at least one map_dma_buf has happened,
+ - and the backing storage has been allocated for this buffer,
+ 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
+ map_dma_buf until all outstanding access is completed (as signalled by
+ unmap_dma_buf).
+ Once all users have finished accessing and have unmapped this buffer, the
+ exporter could potentially move the buffer to the stricter backing-storage,
+ and then allow further {map,unmap}_dma_buf operations from any buffer-user
+ from the migrated backing-storage.
+
+ If the exporter cannot fulfil 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
+ map_dma_buf() API has been called, it simply returns an error.
+
+Kernel cpu access to a dma-buf buffer object
+--------------------------------------------
+
+The motivation to allow cpu access from the kernel to a dma-buf object from the
+importers side are:
+- fallback operations, e.g. if the devices is connected to a usb bus and the
+ kernel needs to shuffle the data around first before sending it away.
+- full transparency for existing users on the importer side, i.e. userspace
+ should not notice the difference between a normal object from that subsystem
+ and an imported one backed by a dma-buf. This is really important for drm
+ opengl drivers that expect to still use all the existing upload/download
+ paths.
+
+Access to a dma_buf from the kernel context involves three steps:
+
+1. Prepare access, which invalidate any necessary caches and make the object
+ available for cpu access.
+2. Access the object page-by-page with the dma_buf map apis
+3. Finish access, which will flush any necessary cpu caches and free reserved
+ resources.
+
+1. Prepare access
+
+ Before an importer can access a dma_buf object with the cpu from the kernel
+ context, it needs to notify the exporter of the access that is about to
+ happen.
+
+ Interface:
+ int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
+ size_t start, size_t len,
+ enum dma_data_direction direction)
+
+ This allows the exporter to ensure that the memory is actually available for
+ cpu access - the exporter might need to allocate or swap-in and pin the
+ backing storage. The exporter also needs to ensure that cpu access is
+ coherent for the given range and access direction. The range and access
+ direction can be used by the exporter to optimize the cache flushing, i.e.
+ access outside of the range or with a different direction (read instead of
+ write) might return stale or even bogus data (e.g. when the exporter needs to
+ copy the data to temporary storage).
+
+ This step might fail, e.g. in oom conditions.
+
+2. Accessing the buffer
+
+ To support dma_buf objects residing in highmem cpu access is page-based using
+ an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
+ PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
+ a pointer in kernel virtual address space. Afterwards the chunk needs to be
+ unmapped again. There is no limit on how often a given chunk can be mapped
+ and unmapped, i.e. the importer does not need to call begin_cpu_access again
+ before mapping the same chunk again.
+
+ Interfaces:
+ void *dma_buf_kmap(struct dma_buf *, unsigned long);
+ void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
+
+ There are also atomic variants of these interfaces. Like for kmap they
+ facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
+ the callback) is allowed to block when using these.
+
+ Interfaces:
+ void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
+ void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
+
+ For importers all the restrictions of using kmap apply, like the limited
+ supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
+ atomic dma_buf kmaps at the same time (in any given process context).
+
+ dma_buf kmap calls outside of the range specified in begin_cpu_access are
+ undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
+ the partial chunks at the beginning and end but may return stale or bogus
+ data outside of the range (in these partial chunks).
+
+ Note that these calls need to always succeed. The exporter needs to complete
+ any preparations that might fail in begin_cpu_access.
+
+ For some cases the overhead of kmap can be too high, a vmap interface
+ is introduced. This interface should be used very carefully, as vmalloc
+ space is a limited resources on many architectures.
+
+ Interfaces:
+ void *dma_buf_vmap(struct dma_buf *dmabuf)
+ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
+
+ The vmap call can fail if there is no vmap support in the exporter, or if it
+ runs out of vmalloc space. Fallback to kmap should be implemented. Note that
+ the dma-buf layer keeps a reference count for all vmap access and calls down
+ into the exporter's vmap function only when no vmapping exists, and only
+ unmaps it once. Protection against concurrent vmap/vunmap calls is provided
+ by taking the dma_buf->lock mutex.
+
+3. Finish access
+
+ When the importer is done accessing the range specified in begin_cpu_access,
+ it needs to announce this to the exporter (to facilitate cache flushing and
+ unpinning of any pinned resources). The result of of any dma_buf kmap calls
+ after end_cpu_access is undefined.
+
+ Interface:
+ void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
+ size_t start, size_t len,
+ enum dma_data_direction dir);
+
+
+Direct Userspace Access/mmap Support
+------------------------------------
+
+Being able to mmap an export dma-buf buffer object has 2 main use-cases:
+- CPU fallback processing in a pipeline and
+- supporting existing mmap interfaces in importers.
+
+1. CPU fallback processing in a pipeline
+
+ In many processing pipelines it is sometimes required that the cpu can access
+ the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
+ the need to handle this specially in userspace frameworks for buffer sharing
+ it's ideal if the dma_buf fd itself can be used to access the backing storage
+ from userspace using mmap.
+
+ Furthermore Android's ION framework already supports this (and is otherwise
+ rather similar to dma-buf from a userspace consumer side with using fds as
+ handles, too). So it's beneficial to support this in a similar fashion on
+ dma-buf to have a good transition path for existing Android userspace.
+
+ No special interfaces, userspace simply calls mmap on the dma-buf fd.
+
+2. Supporting existing mmap interfaces in exporters
+
+ Similar to the motivation for kernel cpu access it is again important that
+ the userspace code of a given importing subsystem can use the same interfaces
+ with a imported dma-buf buffer object as with a native buffer object. This is
+ especially important for drm where the userspace part of contemporary OpenGL,
+ X, and other drivers is huge, and reworking them to use a different way to
+ mmap a buffer rather invasive.
+
+ The assumption in the current dma-buf interfaces is that redirecting the
+ initial mmap is all that's needed. A survey of some of the existing
+ subsystems shows that no driver seems to do any nefarious thing like syncing
+ up with outstanding asynchronous processing on the device or allocating
+ special resources at fault time. So hopefully this is good enough, since
+ adding interfaces to intercept pagefaults and allow pte shootdowns would
+ increase the complexity quite a bit.
+
+ Interface:
+ int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
+ unsigned long);
+
+ If the importing subsystem simply provides a special-purpose mmap call to set
+ up a mapping in userspace, calling do_mmap with dma_buf->file will equally
+ achieve that for a dma-buf object.
+
+3. Implementation notes for exporters
+
+ Because dma-buf buffers have invariant size over their lifetime, the dma-buf
+ core checks whether a vma is too large and rejects such mappings. The
+ exporter hence does not need to duplicate this check.
+
+ Because existing importing subsystems might presume coherent mappings for
+ userspace, the exporter needs to set up a coherent mapping. If that's not
+ possible, it needs to fake coherency by manually shooting down ptes when
+ leaving the cpu domain and flushing caches at fault time. Note that all the
+ dma_buf files share the same anon inode, hence the exporter needs to replace
+ the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
+ required. This is because the kernel uses the underlying inode's address_space
+ for vma tracking (and hence pte tracking at shootdown time with
+ unmap_mapping_range).
+
+ If the above shootdown dance turns out to be too expensive in certain
+ scenarios, we can extend dma-buf with a more explicit cache tracking scheme
+ for userspace mappings. But the current assumption is that using mmap is
+ always a slower path, so some inefficiencies should be acceptable.
+
+ Exporters that shoot down mappings (for any reasons) shall not do any
+ synchronization at fault time with outstanding device operations.
+ Synchronization is an orthogonal issue to sharing the backing storage of a
+ buffer and hence should not be handled by dma-buf itself. This is explicitly
+ mentioned here because many people seem to want something like this, but if
+ different exporters handle this differently, buffer sharing can fail in
+ interesting ways depending upong the exporter (if userspace starts depending
+ upon this implicit synchronization).
+
+Miscellaneous notes
+-------------------
+
+- Any exporters or users of the dma-buf buffer sharing framework must have
+ a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+ on the file descriptor. This is not just a resource leak, but a
+ potential security hole. It could give the newly exec'd application
+ access to buffers, via the leaked fd, to which it should otherwise
+ not be permitted access.
+
+ The problem with doing this via a separate fcntl() call, versus doing it
+ atomically when the fd is created, is that this is inherently racy in a
+ multi-threaded app[3]. The issue is made worse when it is library code
+ opening/creating the file descriptor, as the application may not even be
+ aware of the fd's.
+
+ To avoid this problem, userspace must have a way to request O_CLOEXEC
+ flag be set when the dma-buf fd is created. So any API provided by
+ the exporting driver to create a dmabuf fd must provide a way to let
+ userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
+
+- If an exporter needs to manually flush caches and hence needs to fake
+ coherency for mmap support, it needs to be able to zap all the ptes pointing
+ at the backing storage. Now linux mm needs a struct address_space associated
+ with the struct file stored in vma->vm_file to do that with the function
+ unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
+ with the anon_file struct file, i.e. all dma_bufs share the same file.
+
+ Hence exporters need to setup their own file (and address_space) association
+ by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
+ callback. In the specific case of a gem driver the exporter could use the
+ shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
+ zap ptes by unmapping the corresponding range of the struct address_space
+ associated with their own file.
+
+References:
+[1] struct dma_buf_ops in include/linux/dma-buf.h
+[2] All interfaces mentioned above defined in include/linux/dma-buf.h
+[3] https://lwn.net/Articles/236486/