DMA-BUF

dma-buf.c

数据结构

struct dma_buf_list

struct dma_buf_list {
	struct list_head head;
	struct mutex lock;
};

struct dma_buf_ops

用户自定义ops

/**
 * struct dma_buf_ops - operations possible on struct dma_buf
 * @vmap: [optional] creates a virtual mapping for the buffer into kernel
 *	  address space. Same restrictions as for vmap and friends apply.
 * @vunmap: [optional] unmaps a vmap from the buffer
 */
struct dma_buf_ops {
	/**
	  * @cache_sgt_mapping:
	  *
	  * If true the framework will cache the first mapping made for each
	  * attachment. This avoids creating mappings for attachments multiple
	  * times.
	  */
	bool cache_sgt_mapping;

	/**
	 * @attach:
	 *
	 * This is called from dma_buf_attach() to make sure that a given
	 * &dma_buf_attachment.dev 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 fulfills 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
	 * &dma_buf_attachment.priv pointer.
	 *
	 * 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 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 *);

	/**
	 * @pin:
	 *
	 * This is called by dma_buf_pin() and lets the exporter know that the
	 * DMA-buf can't be moved any more. Ideally, the exporter should
	 * pin the buffer so that it is generally accessible by all
	 * devices.
	 *
	 * This is called with the &dmabuf.resv object locked and is mutual
	 * exclusive with @cache_sgt_mapping.
	 *
	 * This is called automatically for non-dynamic importers from
	 * dma_buf_attach().
	 *
	 * Note that similar to non-dynamic exporters in their @map_dma_buf
	 * callback the driver must guarantee that the memory is available for
	 * use and cleared of any old data by the time this function returns.
	 * Drivers which pipeline their buffer moves internally must wait for
	 * all moves and clears to complete.
	 *
	 * Returns:
	 *
	 * 0 on success, negative error code on failure.
	 */
	int (*pin)(struct dma_buf_attachment *attach);

	/**
	 * @unpin:
	 *
	 * This is called by dma_buf_unpin() and lets the exporter know that the
	 * DMA-buf can be moved again.
	 *
	 * This is called with the dmabuf->resv object locked and is mutual
	 * exclusive with @cache_sgt_mapping.
	 *
	 * This callback is optional.
	 */
	void (*unpin)(struct dma_buf_attachment *attach);

	/**
	 * @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 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.
	 *
	 * This is always called with the dmabuf->resv object locked when
	 * the dynamic_mapping flag is true.
	 *
	 * Note that for non-dynamic exporters the driver must guarantee that
	 * that the memory is available for use and cleared of any old data by
	 * the time this function returns.  Drivers which pipeline their buffer
	 * moves internally must wait for all moves and clears to complete.
	 * Dynamic exporters do not need to follow this rule: For non-dynamic
	 * importers the buffer is already pinned through @pin, which has the
	 * same requirements. Dynamic importers otoh are required to obey the
	 * dma_resv fences.
	 *
	 * Returns:
	 *
	 * A &sg_table scatter list of the backing storage of the DMA buffer,
	 * already mapped into the device address space of the &device attached
	 * with the provided &dma_buf_attachment. The addresses and lengths in
	 * the scatter list are PAGE_SIZE aligned.
	 *
	 * On failure, returns a negative error value wrapped into a pointer.
	 * May also return -EINTR when a signal was received while being
	 * blocked.
	 *
	 * Note that exporters should not try to cache the scatter list, or
	 * return the same one for multiple calls. Caching is done either by the
	 * DMA-BUF code (for non-dynamic importers) or the importer. Ownership
	 * of the scatter list is transferred to the caller, and returned by
	 * @unmap_dma_buf.
	 */
	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.
	 * For static dma_buf handling this might also unpin the backing
	 * storage if this is the last mapping of the DMA buffer.
	 */
	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.
	 */

	/**
	 * @release:
	 *
	 * Called after the last dma_buf_put to release the &dma_buf, and
	 * mandatory.
	 */
	void (*release)(struct dma_buf *);

	/**
	 * @begin_cpu_access:
	 *
	 * This is called from dma_buf_begin_cpu_access() and allows the
	 * exporter to ensure that the memory is actually coherent for cpu
	 * access. The exporter also needs to ensure that cpu access is coherent
	 * for the access direction. The direction can be used by the exporter
	 * to optimize the cache flushing, i.e. access 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).
	 *
	 * Note that this is both called through the DMA_BUF_IOCTL_SYNC IOCTL
	 * command for userspace mappings established through @mmap, and also
	 * for kernel mappings established with @vmap.
	 *
	 * This callback is optional.
	 *
	 * Returns:
	 *
	 * 0 on success or a negative error code on failure. This can for
	 * example fail when the backing storage can't be allocated. Can also
	 * return -ERESTARTSYS or -EINTR when the call has been interrupted and
	 * needs to be restarted.
	 */
	int (*begin_cpu_access)(struct dma_buf *, enum dma_data_direction);

	/**
	 * @end_cpu_access:
	 *
	 * This is called from dma_buf_end_cpu_access() when the importer is
	 * done accessing the CPU. The exporter can use this to flush caches and
	 * undo anything else done in @begin_cpu_access.
	 *
	 * This callback is optional.
	 *
	 * Returns:
	 *
	 * 0 on success or a negative error code on failure. Can return
	 * -ERESTARTSYS or -EINTR when the call has been interrupted and needs
	 * to be restarted.
	 */
	int (*end_cpu_access)(struct dma_buf *, enum dma_data_direction);

	/**
	 * @mmap:
	 *
	 * This callback is used by the dma_buf_mmap() function
	 *
	 * Note that the mapping needs to be incoherent, userspace is expected
	 * to bracket CPU access using the DMA_BUF_IOCTL_SYNC interface.
	 *
	 * 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.
	 * Drivers do not need to check this themselves.
	 *
	 * 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.
	 *
	 * This callback is optional.
	 *
	 * Returns:
	 *
	 * 0 on success or a negative error code on failure.
	 */
	int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);

	int (*vmap)(struct dma_buf *dmabuf, struct iosys_map *map);
	void (*vunmap)(struct dma_buf *dmabuf, struct iosys_map *map);
};

struct dma_buf

dmabuf结构体

/**
 * struct dma_buf - shared buffer object
 *
 * 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:
	 *
	 * Size of the buffer; invariant over the lifetime of the buffer.
	 */
	size_t size;

	/**
	 * @file:
	 *
	 * File pointer used for sharing buffers across, and for refcounting.
	 * See dma_buf_get() and dma_buf_put().
	 */
	struct file *file;

	/**
	 * @attachments:
	 *
	 * List of dma_buf_attachment that denotes all devices attached,
	 * protected by &dma_resv lock @resv.
	 */
	struct list_head attachments;

	/** @ops: dma_buf_ops associated with this buffer object. */
	const struct dma_buf_ops *ops;

	/**
	 * @lock:
	 *
	 * Used internally to serialize list manipulation, attach/detach and
	 * vmap/unmap. Note that in many cases this is superseeded by
	 * dma_resv_lock() on @resv.
	 */
	struct mutex lock;

	/**
	 * @vmapping_counter:
	 *
	 * Used internally to refcnt the vmaps returned by dma_buf_vmap().
	 * Protected by @lock.
	 */
	unsigned vmapping_counter;

	/**
	 * @vmap_ptr:
	 * The current vmap ptr if @vmapping_counter > 0. Protected by @lock.
	 */
	struct iosys_map vmap_ptr;

	/**
	 * @exp_name:
	 *
	 * Name of the exporter; useful for debugging. See the
	 * DMA_BUF_SET_NAME IOCTL.
	 */
	const char *exp_name;

	/**
	 * @name:
	 *
	 * Userspace-provided name; useful for accounting and debugging,
	 * protected by dma_resv_lock() on @resv and @name_lock for read access.
	 */
	const char *name;

	/** @name_lock: Spinlock to protect name acces for read access. */
	spinlock_t name_lock;

	/**
	 * @owner:
	 *
	 * Pointer to exporter module; used for refcounting when exporter is a
	 * kernel module.
	 */
	struct module *owner;

	/** @list_node: node for dma_buf accounting and debugging. */
	struct list_head list_node;

	/** @priv: exporter specific private data for this buffer object. */
	void *priv;

	/**
	 * @resv:
	 *
	 * Reservation object linked to this dma-buf.
	 *
	 * IMPLICIT SYNCHRONIZATION RULES:
	 *
	 * Drivers which support implicit synchronization of buffer access as
	 * e.g. exposed in `Implicit Fence Poll Support`_ must follow the
	 * below rules.
	 *
	 * - Drivers must add a read fence through dma_resv_add_fence() with the
	 *   DMA_RESV_USAGE_READ flag for anything the userspace API considers a
	 *   read access. This highly depends upon the API and window system.
	 *
	 * - Similarly drivers must add a write fence through
	 *   dma_resv_add_fence() with the DMA_RESV_USAGE_WRITE flag for
	 *   anything the userspace API considers write access.
	 *
	 * - Drivers may just always add a write fence, since that only
	 *   causes unecessarily synchronization, but no correctness issues.
	 *
	 * - Some drivers only expose a synchronous userspace API with no
	 *   pipelining across drivers. These do not set any fences for their
	 *   access. An example here is v4l.
	 *
	 * - Driver should use dma_resv_usage_rw() when retrieving fences as
	 *   dependency for implicit synchronization.
	 *
	 * DYNAMIC IMPORTER RULES:
	 *
	 * Dynamic importers, see dma_buf_attachment_is_dynamic(), have
	 * additional constraints on how they set up fences:
	 *
	 * - Dynamic importers must obey the write fences and wait for them to
	 *   signal before allowing access to the buffer's underlying storage
	 *   through the device.
	 *
	 * - Dynamic importers should set fences for any access that they can't
	 *   disable immediately from their &dma_buf_attach_ops.move_notify
	 *   callback.
	 *
	 * IMPORTANT:
	 *
	 * All drivers and memory management related functions must obey the
	 * struct dma_resv rules, specifically the rules for updating and
	 * obeying fences. See enum dma_resv_usage for further descriptions.
	 */
	struct dma_resv *resv;

	/** @poll: for userspace poll support */
	wait_queue_head_t poll;

	/** @cb_in: for userspace poll support */
	/** @cb_out: for userspace poll support */
	struct dma_buf_poll_cb_t {
		struct dma_fence_cb cb;
		wait_queue_head_t *poll;

		__poll_t active;
	} cb_in, cb_out;
#ifdef CONFIG_DMABUF_SYSFS_STATS
	/**
	 * @sysfs_entry:
	 *
	 * For exposing information about this buffer in sysfs. See also
	 * `DMA-BUF statistics`_ for the uapi this enables.
	 */
	struct dma_buf_sysfs_entry {
		struct kobject kobj;
		struct dma_buf *dmabuf;
	} *sysfs_entry;
#endif
};

struct dma_buf_export_info

创建dmabuf时的配置信息。
const char *exp_name：调试用
const struct dma_buf_ops *ops：用户定义的额外的dmabuf操作函数
size_t size：dmabuf大小
void *priv：用户分配这块dmabuf时的额外私有数据

/**
 * struct dma_buf_export_info - holds information needed to export a dma_buf
 * @exp_name:	name of the exporter - useful for debugging.
 * @owner:	pointer to exporter module - used for refcounting kernel module
 * @ops:	Attach allocator-defined dma buf ops to the new buffer
 * @size:	Size of the buffer - invariant over the lifetime of the buffer
 * @flags:	mode flags for the file
 * @resv:	reservation-object, NULL to allocate default one
 * @priv:	Attach private data of allocator to this buffer
 *
 * This structure holds the information required to export the buffer. Used
 * with dma_buf_export() only.
 */
struct dma_buf_export_info {
	const char *exp_name;
	struct module *owner;
	const struct dma_buf_ops *ops;
	size_t size;
	int flags;
	struct dma_resv *resv;
	void *priv;
};

struct dma_buf_attachment

/**
 * struct dma_buf_attachment - holds device-buffer attachment data
 * @dmabuf: buffer for this attachment.
 * @dev: device attached to the buffer.
 * @node: list of dma_buf_attachment, protected by dma_resv lock of the dmabuf.
 * @sgt: cached mapping.
 * @dir: direction of cached mapping.
 * @peer2peer: true if the importer can handle peer resources without pages.
 * @priv: exporter specific attachment data.
 * @importer_ops: importer operations for this attachment, if provided
 * dma_buf_map/unmap_attachment() must be called with the dma_resv lock held.
 * @importer_priv: importer specific attachment data.
 *
 * 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;
	struct device *dev;
	struct list_head node;
	struct sg_table *sgt;
	enum dma_data_direction dir;
	bool peer2peer;
	const struct dma_buf_attach_ops *importer_ops;
	void *importer_priv;
	void *priv;
};

dmabuf描述：

描述原文：

/**
 * 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 file descriptor is converted to a &dma_buf using
 *    dma_buf_get(). Then 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 initiate 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.
 */

解析：

1. 用户先使用DEFINE_DMA_BUF_EXPORT_INFO()来定义dmabuf info实例，然后调用函数dma_buf_export()将这块私有buffer转换为&dma_buf实例。然后通过函数dma_buf_fd()把&dma_buf以文件描述符的形式暴露到用户态。
2. 用户态通过文件描述符把这块内存share给用户想要共享的驱动。首先把文件描述符通过函数dma_buf_get()转换为&dma_buf，然后通过函数dma_buf_attach()绑定到对应设备。
3. 注意：到目前为止，用户依然可以自由迁移或者分配 the backing storage
4. 一旦buffer绑定到了所有的设备上之后，用户态就可以对这些共享内存发起DMA访问。在内核态，这些操作是通过dma_buf_map_attachment()函数和dma_buf_unmap_attachment()函数实现的。
5. 一旦某一个驱动用完了这块共享内存，它需要调用函数dma_buf_detach()来释放内存，然后调用dma_buf_put()来减去引用计数。

函数解析

dma_buf_init

函数原型：
static int __init dma_buf_init(void)
模块初始化函数，注册dma_buf_fs，初始化db_list，初始化debugfs。
dma_buf_init调用流程：

dma_buf_export

函数原型：
struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
创建一个dmabuf，并把它关联到一个anon file上，以便暴露这块内存。
成功会返回&dma_buf的指针，失败会返回一个负数（通过ERR_PTR包装）。
定义exp_info最简便的方法是通过宏定义：DEFINE_DMA_BUF_EXPORT_INFO
核心操作：

1. 将exp_info里面的内容放到dmabuf指针中(指针的形式)。
2. 创建一个struct file的文件描述符，并将文件描述符与dmabuf绑定。
3. 把dma_buf_fops指定给文件描述符。

dma_buf_fops

static const struct file_operations dma_buf_fops = {
	.release	= dma_buf_file_release,
	.mmap		= dma_buf_mmap_internal,
	.llseek		= dma_buf_llseek,
	.poll		= dma_buf_poll,
	.unlocked_ioctl	= dma_buf_ioctl,
	.compat_ioctl	= compat_ptr_ioctl,
	.show_fdinfo	= dma_buf_show_fdinfo,
};

dma_buf_show_fdinfo

函数原型：
static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
显示这个文件的描述信息：size/count/exp_name/name

dma_buf_ioctl

函数原型：
static long dma_buf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
dmabuf的ioctl函数，支持操作：

1. DMA_BUF_IOCTL_SYNC: dmabuf同步，读、写、读写三种情况。
2. DMA_BUF_SET_NAME_A/DMA_BUF_SET_NAME_B: 配置名字。

dma_buf_begin_cpu_access

函数原型：
int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction)
如果用户定义了begin_cpu_access调用用户的begin_cpu_access接口然后调用内部函数__dma_buf_begin_cpu_access，等待dmabuf被访问。

dma_buf_set_name

函数原型：
static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
重新配置dmabuf的名字。

dma_buf_poll

函数原型：
static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
文件描述符poll函数的内核实现。
先调用poll_wait把进程挂到等待队列。然后判断等待的poll事件是不是POLLIN | POLLOUT
然后从resv获取fence，把dmabuf的poll callback添加进去。

dma_buf_poll_cb

函数原型：
static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
在pool时add的callback，主要作用是唤醒poll等待进程。

dma_buf_llseek

函数原型：
static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
返回base + offset。

dma_buf_mmap_internal

函数原型：
static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
调用用户实现的mmap接口实现内存映射。

dma_buf_file_release

函数原型：
static int dma_buf_file_release(struct inode *inode, struct file *file)
做file close之后的清理工作。

dma_buf_fd

函数原型：
int dma_buf_fd(struct dma_buf *dmabuf, int flags)
从系统中获取一个可用的fd，并把它跟dmabuf->file绑定起来。
调用函数：get_unused_fd_flags --> fd_install

dma_buf_get

函数原型：
struct dma_buf *dma_buf_get(int fd)
通过fd获取dmabuf的句柄指针。
调用函数：fget --> file->private_data

dma_buf_put

函数原型：
void dma_buf_put(struct dma_buf *dmabuf)
把dmabuf的文件句柄引用计数减1

dma_buf_dynamic_attach

函数原型：
struct dma_buf_attachment * dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev, const struct dma_buf_attach_ops *importer_ops, void *importer_priv)
把device设备绑定到dmabuf的attachment list里面。
初始化一个attach节点，并把它加入到dmabuf的attachments列表中。

dma_buf_attach

函数原型：
struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev)
把device设备绑定到dmabuf的attachment list里面。
没有importer_ops和importer_priv入参。

dma_buf_detach

函数原型：
void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)

dma_buf_pin

函数原型：
int dma_buf_pin(struct dma_buf_attachment *attach)
锁住一块dmabuf。但是不允许用户用这个接口锁住任意数量的dmabuf。

dma_buf_unpin

void dma_buf_unpin(struct dma_buf_attachment *attach)
将锁住的dmabuf解锁。

dma_buf_map_attachment

struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach, enum dma_data_direction direction)
调用用户定义的map_dma_buf回调。

dma_buf_unmap_attachment

struct sg_table *dma_buf_unmap_attachment(struct dma_buf_attachment *attach, enum dma_data_direction direction)
调用用户定义的unmap_dma_buf回调。

dma_buf_move_notify

void dma_buf_move_notify(struct dma_buf *dmabuf)
通知所有attachments，dmabuf已经被移动，需要销毁或者重新创建映射。

dma_buf_begin_cpu_access

int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction)
开始对dmabuf发起CPU访问，会等待fence被生产者唤醒。

dma_buf_end_cpu_access

int dma_buf_end_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction)
结束对dmabuf的访问。

dma_buf_mmap

int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma, unsigned long pgoff)
对dmabuf内存地址进行映射

dma_buf_vmap

int dma_buf_vmap(struct dma_buf *dmabuf, struct iosys_map *map)

dma_buf_vunmap

int dma_buf_vunmap(struct dma_buf *dmabuf, struct iosys_map *map)

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