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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.
-
-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
- requred. 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 explictly
- 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/