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authorDaniel Vetter <daniel.vetter@ffwll.ch>2016-12-09 19:53:07 +0100
committerSumit Semwal <sumit.semwal@linaro.org>2016-12-13 17:09:51 +0530
commit2904a8c1311f02896635fd35744262413a0b2726 (patch)
treece61b0e4799617f22b315ffa83cd04beb6d97afb /drivers/dma-buf
parent24a367348a017555f982a9ee137070a7a821fa97 (diff)
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
Diffstat (limited to 'drivers/dma-buf')
-rw-r--r--drivers/dma-buf/dma-buf.c64
-rw-r--r--drivers/dma-buf/sync_file.c1
2 files changed, 59 insertions, 6 deletions
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,
};
-