dma-buf: Reorganize device dma access docs

- 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>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161209185309.1682-4-daniel.vetter@ffwll.ch

1 files changed, 110 insertions, 23 deletions

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, \