- Put the initial overview for dma-buf into dma-buf.rst. - Put all the comments about detailed semantics into the right kernel-doc comment for functions or ops structure member. - To allow that detail, switch the reworked kerneldoc to inline style for dma_buf_ops. - Tie everything together into a much more streamlined overview comment, relying on the hyperlinks for all the details. - Also sprinkle some links into the kerneldoc for dma_buf and dma_buf_attachment to tie it all together.
Cc: linux-doc@vger.kernel.org Cc: Jonathan Corbet corbet@lwn.net Cc: Sumit Semwal sumit.semwal@linaro.org Signed-off-by: Daniel Vetter daniel.vetter@intel.com --- Documentation/dma-buf-sharing.txt | 222 ----------------------------------- Documentation/driver-api/dma-buf.rst | 38 ++++++ drivers/dma-buf/dma-buf.c | 64 +++++++++- drivers/dma-buf/sync_file.c | 1 - include/linux/dma-buf.h | 133 +++++++++++++++++---- 5 files changed, 207 insertions(+), 251 deletions(-)
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index ca44c5820585..dca2fb7ac3b4 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -5,228 +5,6 @@ <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 -------------------------------------------- diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index a9b457a4b949..906d1532efad 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -17,6 +17,44 @@ shared or exclusive fence(s) associated with the buffer. 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:
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index e72e64484131..09f948fd62ad 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -314,19 +314,52 @@ static inline int is_dma_buf_file(struct file *file) }
/** + * 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) { @@ -458,7 +491,12 @@ EXPORT_SYMBOL_GPL(dma_buf_get); * 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) { @@ -475,8 +513,17 @@ EXPORT_SYMBOL_GPL(dma_buf_put); * @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) @@ -519,6 +566,7 @@ EXPORT_SYMBOL_GPL(dma_buf_attach); * @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) { @@ -543,7 +591,12 @@ EXPORT_SYMBOL_GPL(dma_buf_detach); * @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) @@ -571,6 +624,7 @@ EXPORT_SYMBOL_GPL(dma_buf_map_attachment); * @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, diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c index d5179d7e8575..07cb9b908f30 100644 --- a/drivers/dma-buf/sync_file.c +++ b/drivers/dma-buf/sync_file.c @@ -462,4 +462,3 @@ static const struct file_operations sync_file_fops = { .unlocked_ioctl = sync_file_ioctl, .compat_ioctl = sync_file_ioctl, }; - diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h index 8daeb3ce0016..6df170fb243f 100644 --- a/include/linux/dma-buf.h +++ b/include/linux/dma-buf.h @@ -39,19 +39,6 @@ struct dma_buf_attachment;
/** * 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. @@ -72,25 +59,109 @@ struct dma_buf_attachment; * @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); @@ -124,6 +195,15 @@ struct dma_buf_ops { * @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; @@ -160,6 +240,11 @@ struct dma_buf { * 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; @@ -192,9 +277,11 @@ struct dma_buf_export_info { };
/** - * 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, \