<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:
}
/**
+ * 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.
* Also connect the allocator specific data and ops to the buffer.
* 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 *);
+ 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);
+ 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);
+ 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, \
projects / platform / kernel / linux-rpi3.git / commitdiff