dma-buf: Reorganize device dma access docs

                <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. All these fields are filled in struct

   dma_buf_export_info, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.

   Interface:

      DEFINE_DMA_BUF_EXPORT_INFO(exp_info)

      struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)

   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.

   'exp_name' in struct dma_buf_export_info is the name of exporter - to

   facilitate information while debugging. It is set to KBUILD_MODNAME by

   default, so exporters don't have to provide a specific name, if they don't

   wish to.

   DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,

   zeroes it out and pre-populates exp_name in it.

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, int flags)

   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 *,

                            enum dma_data_direction);

   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 fulfill 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

--------------------------------------------

Shared DMA Buffers

------------------

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' or 'importer' 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/importer.

The exporter

 - implements and manages operations in :c:type:`struct dma_buf_ops

   <dma_buf_ops>` for the buffer,

 - allows other users to share the buffer by using dma_buf sharing APIs,

 - manages the details of buffer allocation, wrapped int a :c:type:`struct

   dma_buf <dma_buf>`,

 - decides about the actual backing storage where this allocation happens,

 - and 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.

 - and 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. This interface is provided by :c:type:`struct

   dma_buf_attachment <dma_buf_attachment>`.

Basic Operation and Device DMA Access

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c

   :doc: dma buf device access

Kernel Functions and Structures Reference

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c

   :export:

	return file->f_op == &dma_buf_fops;

}

/**

 * DOC: dma buf device access

 *

 * For device DMA access to a shared DMA buffer the usual sequence of operations

 * is fairly simple:

 *

 * 1. The exporter defines his exporter instance using

 *    DEFINE_DMA_BUF_EXPORT_INFO() and calls dma_buf_export() to wrap a private

 *    buffer object into a &dma_buf. It then exports that &dma_buf to userspace

 *    as a file descriptor by calling dma_buf_fd().

 *

 * 2. Userspace passes this file-descriptors to all drivers it wants this buffer

 *    to share with: First the filedescriptor is converted to a &dma_buf using

 *    dma_buf_get(). The the buffer is attached to the device using

 *    dma_buf_attach().

 *

 *    Up to this stage the exporter is still free to migrate or reallocate the

 *    backing storage.

 *

 * 3. Once the buffer is attached to all devices userspace can inniate DMA

 *    access to the shared buffer. In the kernel this is done by calling

 *    dma_buf_map_attachment() and dma_buf_unmap_attachment().

 *

 * 4. Once a driver is done with a shared buffer it needs to call

 *    dma_buf_detach() (after cleaning up any mappings) and then release the

 *    reference acquired with dma_buf_get by calling dma_buf_put().

 *

 * For the detailed semantics exporters are expected to implement see

 * &dma_buf_ops.

 */

/**

 * dma_buf_export - Creates a new dma_buf, and associates an anon file

 * with this buffer, so it can be exported.

 * Additionally, provide a name string for exporter; useful in debugging.

 *

 * @exp_info:	[in]	holds all the export related information provided

 *			by the exporter. see struct dma_buf_export_info

 *			by the exporter. see struct &dma_buf_export_info

 *			for further details.

 *

 * Returns, on success, a newly created dma_buf object, which wraps the

 * supplied private data and operations for dma_buf_ops. On either missing

 * ops, or error in allocating struct dma_buf, will return negative error.

 *

 * For most cases the easiest way to create @exp_info is through the

 * %DEFINE_DMA_BUF_EXPORT_INFO macro.

 */

struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)

{

 * dma_buf_put - decreases refcount of the buffer

 * @dmabuf:	[in]	buffer to reduce refcount of

 *

 * Uses file's refcounting done implicitly by fput()

 * Uses file's refcounting done implicitly by fput().

 *

 * If, as a result of this call, the refcount becomes 0, the 'release' file

 * operation related to this fd is called. It calls the release operation of

 * struct &dma_buf_ops in turn, and frees the memory allocated for dmabuf when

 * exported.

 */

void dma_buf_put(struct dma_buf *dmabuf)

{

 * @dmabuf:	[in]	buffer to attach device to.

 * @dev:	[in]	device to be attached.

 *

 * Returns struct dma_buf_attachment * for this attachment; returns ERR_PTR on

 * error.

 * Returns struct dma_buf_attachment pointer for this attachment. Attachments

 * must be cleaned up by calling dma_buf_detach().

 *

 * Returns:

 *

 * A pointer to newly created &dma_buf_attachment on success, or a negative

 * error code wrapped into a pointer on failure.

 *

 * Note that this can fail if the backing storage of @dmabuf is in a place not

 * accessible to @dev, and cannot be moved to a more suitable place. This is

 * indicated with the error code -EBUSY.

 */

struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,

					  struct device *dev)

 * @dmabuf:	[in]	buffer to detach from.

 * @attach:	[in]	attachment to be detached; is free'd after this call.

 *

 * Clean up a device attachment obtained by calling dma_buf_attach().

 */

void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)

{

 * @direction:	[in]	direction of DMA transfer

 *

 * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR

 * on error.

 * on error. May return -EINTR if it is interrupted by a signal.

 *

 * A mapping must be unmapped again using dma_buf_map_attachment(). Note that

 * the underlying backing storage is pinned for as long as a mapping exists,

 * therefore users/importers should not hold onto a mapping for undue amounts of

 * time.

 */

struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,

					enum dma_data_direction direction)

 * @sg_table:	[in]	scatterlist info of the buffer to unmap

 * @direction:  [in]    direction of DMA transfer

 *

 * This unmaps a DMA mapping for @attached obtained by dma_buf_map_attachment().

 */

void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,

				struct sg_table *sg_table,

	.unlocked_ioctl = sync_file_ioctl,

	.compat_ioctl = sync_file_ioctl,

};

/**

 * struct dma_buf_ops - operations possible on struct dma_buf

 * @attach: [optional] allows different devices to 'attach' themselves to the

 *	    given buffer. It might return -EBUSY to signal that backing storage

 *	    is already allocated and incompatible with the requirements

 *	    of requesting device.

 * @detach: [optional] detach a given device from this buffer.

 * @map_dma_buf: returns list of scatter pages allocated, increases usecount

 *		 of the buffer. Requires atleast one attach to be called

 *		 before. Returned sg list should already be mapped into

 *		 _device_ address space. This call may sleep. May also return

 *		 -EINTR. Should return -EINVAL if attach hasn't been called yet.

 * @unmap_dma_buf: decreases usecount of buffer, might deallocate scatter

 *		   pages.

 * @release: release this buffer; to be called after the last dma_buf_put.

 * @begin_cpu_access: [optional] called before cpu access to invalidate cpu

 * 		      caches and allocate backing storage (if not yet done)

 * 		      respectively pin the object into memory.

 * @vunmap: [optional] unmaps a vmap from the buffer

 */

struct dma_buf_ops {

	/**

	 * @attach:

	 *

	 * This is called from dma_buf_attach() to make sure that a given

	 * &device can access the provided &dma_buf. Exporters which support

	 * buffer objects in special locations like VRAM or device-specific

	 * carveout areas should check whether the buffer could be move to

	 * system memory (or directly accessed by the provided device), and

	 * otherwise need to fail the attach operation.

	 *

	 * The exporter should also in general check whether the current

	 * allocation fullfills the DMA constraints of the new device. If this

	 * is not the case, and the allocation cannot be moved, it should also

	 * fail the attach operation.

	 *

	 * Any exporter-private housekeeping data can be stored in the priv

	 * pointer of &dma_buf_attachment structure.

	 *

	 * This callback is optional.

	 *

	 * Returns:

	 *

	 * 0 on success, negative error code on failure. It might return -EBUSY

	 * to signal that backing storage is already allocated and incompatible

	 * with the requirements of requesting device.

	 */

	int (*attach)(struct dma_buf *, struct device *,

		      struct dma_buf_attachment *);

	/**

	 * @detach:

	 *

	 * This is called by dma_buf_detach() to release a &dma_buf_attachment.

	 * Provided so that exporters can clean up any housekeeping for an

	 * &dma_buf_attachment.

	 *

	 * This callback is optional.

	 */

	void (*detach)(struct dma_buf *, struct dma_buf_attachment *);

	/* For {map,unmap}_dma_buf below, any specific buffer attributes

	 * required should get added to device_dma_parameters accessible

	 * via dev->dma_params.

	/**

	 * @map_dma_buf:

	 *

	 * This is called by dma_buf_map_attachment() and is used to map a

	 * shared &dma_buf into device address space, and it is mandatory. It

	 * can only be called if @attach has been called successfully. This

	 * essentially pins the DMA buffer into place, and it cannot be moved

	 * any more

	 *

	 * This call may sleep, e.g. when the backing storage first needs to be

	 * allocated, or moved to a location suitable for all currently attached

	 * devices.

	 *

	 * Note that any specific buffer attributes required for this function

	 * should get added to device_dma_parameters accessible via

	 * device->dma_params from the &dma_buf_attachment. The @attach callback

	 * should also check these constraints.

	 *

	 * 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.

	 *

	 * Returns:

	 *

	 * A &sg_table scatter list of or the backing storage of the DMA buffer,

	 * already mapped into the device address space of the &device attached

	 * with the provided &dma_buf_attachment.

	 *

	 * On failure, returns a negative error value wrapped into a pointer.

	 * May also return -EINTR when a signal was received while being

	 * blocked.

	 */

	struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,

					 enum dma_data_direction);

	/**

	 * @unmap_dma_buf:

	 *

	 * This is called by dma_buf_unmap_attachment() and should unmap and

	 * release the &sg_table allocated in @map_dma_buf, and it is mandatory.

	 * It should also unpin the backing storage if this is the last mapping

	 * of the DMA buffer, it the exporter supports backing storage

	 * migration.

	 */

	void (*unmap_dma_buf)(struct dma_buf_attachment *,

			      struct sg_table *,

			      enum dma_data_direction);

	/* TODO: Add try_map_dma_buf version, to return immed with -EBUSY

	 * if the call would block.

	 */

	/* after final dma_buf_put() */

	/**

	 * @release:

	 *

	 * Called after the last dma_buf_put to release the &dma_buf, and

	 * mandatory.

	 */

	void (*release)(struct dma_buf *);

	int (*begin_cpu_access)(struct dma_buf *, enum dma_data_direction);

 * @poll: for userspace poll support

 * @cb_excl: for userspace poll support

 * @cb_shared: for userspace poll support

 *

 * This represents a shared buffer, created by calling dma_buf_export(). The

 * userspace representation is a normal file descriptor, which can be created by

 * calling dma_buf_fd().

 *

 * Shared dma buffers are reference counted using dma_buf_put() and

 * get_dma_buf().

 *

 * Device DMA access is handled by the separate struct &dma_buf_attachment.

 */

struct dma_buf {

	size_t size;

 * This structure holds the attachment information between the dma_buf buffer

 * and its user device(s). The list contains one attachment struct per device

 * attached to the buffer.

 *

 * An attachment is created by calling dma_buf_attach(), and released again by

 * calling dma_buf_detach(). The DMA mapping itself needed to initiate a

 * transfer is created by dma_buf_map_attachment() and freed again by calling

 * dma_buf_unmap_attachment().

 */

struct dma_buf_attachment {

	struct dma_buf *dmabuf;

};

/**

 * helper macro for exporters; zeros and fills in most common values

 *

 * DEFINE_DMA_BUF_EXPORT_INFO - helper macro for exporters

 * @name: export-info name

 *

 * DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct &dma_buf_export_info,

 * zeroes it out and pre-populates exp_name in it.

 */

#define DEFINE_DMA_BUF_EXPORT_INFO(name)	\

	struct dma_buf_export_info name = { .exp_name = KBUILD_MODNAME, \