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diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt new file mode 100644 index 000000000000..225f96d88f55 --- /dev/null +++ b/Documentation/dma-buf-sharing.txt @@ -0,0 +1,228 @@ + 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. + +*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] +For this first version, A buffer shared using the dma_buf sharing API: +- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of + this framework. +- may be used *ONLY* by importers that do not need CPU access to the buffer. + +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. + +Miscellaneous notes: +- Any exporters or users of the dma-buf buffer sharing framework must have + a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. + +References: +[1] struct dma_buf_ops in include/linux/dma-buf.h +[2] All interfaces mentioned above defined in include/linux/dma-buf.h |