[PATCH 0/5] sphinxification for dma-buf docs

[PATCH 0/5] sphinxification for dma-buf docs

[Daniel Vetter]

Hi all,

Not yet everything in this area, I still want to sprinkle nice docs around all
the fence code. Especially some text to explain implicit vs. explicit fencing
and how it's all supposed to work.

But just cleanup in the dma-buf part was quite a bit of work, and I'd like to
get feedback on that before moving on.

Cheers, Daniel

Daniel Vetter (5):
  dma-buf: Extract dma-buf.rst
  dma-buf: Update kerneldoc for sync_file_create
  dma-buf: Reorganize device dma access docs
  dma-buf: Update cpu access documentation
  dma-buf: Final bits of doc polish

 Documentation/dma-buf-sharing.txt           | 482 ----------------------------
 Documentation/driver-api/dma-buf.rst        | 165 ++++++++++
 Documentation/driver-api/index.rst          |   1 +
 Documentation/driver-api/infrastructure.rst |  70 ----
 drivers/dma-buf/dma-buf.c                   | 207 +++++++++++-
 drivers/dma-buf/sync_file.c                 |  15 +-
 include/linux/dma-buf.h                     | 224 +++++++++++--
 include/linux/dma-fence.h                   |   1 -
 8 files changed, 565 insertions(+), 600 deletions(-)
 delete mode 100644 Documentation/dma-buf-sharing.txt
 create mode 100644 Documentation/driver-api/dma-buf.rst

-- 
2.10.2

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[PATCH 1/5] dma-buf: Extract dma-buf.rst

[Daniel Vetter]

Just prep work to polish and consolidate all the dma-buf related
documenation.

Unfortunately I didn't discover a way to both integrate this new file
into the overall toc while keeping it at the current place. Work
around that by moving it into the overall driver-api/index.rst.

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/driver-api/dma-buf.rst        | 73 +++++++++++++++++++++++++++++
 Documentation/driver-api/index.rst          |  1 +
 Documentation/driver-api/infrastructure.rst | 70 ---------------------------
 include/linux/dma-fence.h                   |  1 -
 4 files changed, 74 insertions(+), 71 deletions(-)
 create mode 100644 Documentation/driver-api/dma-buf.rst

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
new file mode 100644
index 000000000000..a9b457a4b949
--- /dev/null
+++ b/Documentation/driver-api/dma-buf.rst
@@ -0,0 +1,73 @@
+Buffer Sharing and Synchronization
+==================================
+
+The dma-buf subsystem provides the framework for sharing buffers for
+hardware (DMA) access across multiple device drivers and subsystems, and
+for synchronizing asynchronous hardware access.
+
+This is used, for example, by drm "prime" multi-GPU support, but is of
+course not limited to GPU use cases.
+
+The three main components of this are: (1) dma-buf, representing a
+sg_table and exposed to userspace as a file descriptor to allow passing
+between devices, (2) fence, which provides a mechanism to signal when
+one device as finished access, and (3) reservation, which manages the
+shared or exclusive fence(s) associated with the buffer.
+
+Shared DMA Buffers
+------------------
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :export:
+
+.. kernel-doc:: include/linux/dma-buf.h
+   :internal:
+
+Reservation Objects
+-------------------
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+   :doc: Reservation Object Overview
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+   :export:
+
+.. kernel-doc:: include/linux/reservation.h
+   :internal:
+
+DMA Fences
+----------
+
+.. kernel-doc:: drivers/dma-buf/dma-fence.c
+   :export:
+
+.. kernel-doc:: include/linux/dma-fence.h
+   :internal:
+
+Seqno Hardware Fences
+~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/seqno-fence.c
+   :export:
+
+.. kernel-doc:: include/linux/seqno-fence.h
+   :internal:
+
+DMA Fence Array
+~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
+   :export:
+
+.. kernel-doc:: include/linux/dma-fence-array.h
+   :internal:
+
+DMA Fence uABI/Sync File
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/sync_file.c
+   :export:
+
+.. kernel-doc:: include/linux/sync_file.h
+   :internal:
+
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 8e259c5d0322..1d9d2fe57179 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -16,6 +16,7 @@ available subsections can be seen below.
 
    basics
    infrastructure
+   dma-buf
    message-based
    sound
    frame-buffer
diff --git a/Documentation/driver-api/infrastructure.rst b/Documentation/driver-api/infrastructure.rst
index a0d65eb49055..0bb0b5fc9512 100644
--- a/Documentation/driver-api/infrastructure.rst
+++ b/Documentation/driver-api/infrastructure.rst
@@ -46,76 +46,6 @@ Device Drivers Base
 .. kernel-doc:: drivers/base/bus.c
    :export:
 
-Buffer Sharing and Synchronization
-----------------------------------
-
-The dma-buf subsystem provides the framework for sharing buffers for
-hardware (DMA) access across multiple device drivers and subsystems, and
-for synchronizing asynchronous hardware access.
-
-This is used, for example, by drm "prime" multi-GPU support, but is of
-course not limited to GPU use cases.
-
-The three main components of this are: (1) dma-buf, representing a
-sg_table and exposed to userspace as a file descriptor to allow passing
-between devices, (2) fence, which provides a mechanism to signal when
-one device as finished access, and (3) reservation, which manages the
-shared or exclusive fence(s) associated with the buffer.
-
-dma-buf
-~~~~~~~
-
-.. kernel-doc:: drivers/dma-buf/dma-buf.c
-   :export:
-
-.. kernel-doc:: include/linux/dma-buf.h
-   :internal:
-
-reservation
-~~~~~~~~~~~
-
-.. kernel-doc:: drivers/dma-buf/reservation.c
-   :doc: Reservation Object Overview
-
-.. kernel-doc:: drivers/dma-buf/reservation.c
-   :export:
-
-.. kernel-doc:: include/linux/reservation.h
-   :internal:
-
-fence
-~~~~~
-
-.. kernel-doc:: drivers/dma-buf/dma-fence.c
-   :export:
-
-.. kernel-doc:: include/linux/dma-fence.h
-   :internal:
-
-.. kernel-doc:: drivers/dma-buf/seqno-fence.c
-   :export:
-
-.. kernel-doc:: include/linux/seqno-fence.h
-   :internal:
-
-.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
-   :export:
-
-.. kernel-doc:: include/linux/dma-fence-array.h
-   :internal:
-
-.. kernel-doc:: drivers/dma-buf/reservation.c
-   :export:
-
-.. kernel-doc:: include/linux/reservation.h
-   :internal:
-
-.. kernel-doc:: drivers/dma-buf/sync_file.c
-   :export:
-
-.. kernel-doc:: include/linux/sync_file.h
-   :internal:
-
 Device Drivers DMA Management
 -----------------------------
 
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index d51a7d23c358..c683c4e908d2 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -163,7 +163,6 @@ struct dma_fence_cb {
  * destruction of the fence. Can be called from irq context.
  * If pointer is set to NULL, kfree will get called instead.
  */
-
 struct dma_fence_ops {
 	const char * (*get_driver_name)(struct dma_fence *fence);
 	const char * (*get_timeline_name)(struct dma_fence *fence);
-- 
2.10.2

[PATCH 2/5] dma-buf: Update kerneldoc for sync_file_create

[Daniel Vetter]

This was missed when adding a dma_fence_get call. While at it also
remove the kerneldoc for the static inline helper - no point
documenting internals down to every detail.

Fixes: 30cd85dd6edc ("dma-buf/sync_file: hold reference to fence when creating sync_file")
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
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>
---
 drivers/dma-buf/sync_file.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 6d802f2d2881..d5179d7e8575 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -67,9 +67,10 @@ static void fence_check_cb_func(struct dma_fence *f, struct dma_fence_cb *cb)
  * sync_file_create() - creates a sync file
  * @fence:	fence to add to the sync_fence
  *
- * Creates a sync_file containg @fence. Once this is called, the sync_file
- * takes ownership of @fence. The sync_file can be released with
- * fput(sync_file->file). Returns the sync_file or NULL in case of error.
+ * Creates a sync_file containg @fence. This function acquires and additional
+ * reference of @fence for the newly-created &sync_file, if it succeeds. The
+ * sync_file can be released with fput(sync_file->file). Returns the
+ * sync_file or NULL in case of error.
  */
 struct sync_file *sync_file_create(struct dma_fence *fence)
 {
@@ -90,13 +91,6 @@ struct sync_file *sync_file_create(struct dma_fence *fence)
 }
 EXPORT_SYMBOL(sync_file_create);
 
-/**
- * sync_file_fdget() - get a sync_file from an fd
- * @fd:		fd referencing a fence
- *
- * Ensures @fd references a valid sync_file, increments the refcount of the
- * backing file. Returns the sync_file or NULL in case of error.
- */
 static struct sync_file *sync_file_fdget(int fd)
 {
 	struct file *file = fget(fd);
-- 
2.10.2

[PATCH 3/5] dma-buf: Reorganize device dma access docs

[Daniel Vetter]

- 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, \
-- 
2.10.2

[PATCH 4/5] dma-buf: Update cpu access documentation

[Daniel Vetter]

- Again move the information relevant for driver writers next to the
  callbacks.
- Put the overview and userspace interface documentation into a DOC:
  section within the code.
- Remove the text that mmap needs to be coherent - since the
  DMA_BUF_IOCTL_SYNC landed that's no longer the case. But keep the text
  that for pte zapping exporters need to adjust the address space.
- Add a FIXME that kmap and the new begin/end stuff used by the SYNC
  ioctl don't really mix correctly. That's something I just realized
  while doing this doc rework.
- Augment function and structure docs like usual.

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    | 213 -----------------------------------
 Documentation/driver-api/dma-buf.rst |   6 +
 drivers/dma-buf/dma-buf.c            | 121 ++++++++++++++++++++
 include/linux/dma-buf.h              |  91 +++++++++++++--
 4 files changed, 210 insertions(+), 221 deletions(-)

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index dca2fb7ac3b4..74c99edb7976 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -6,205 +6,6 @@
                  <sumit dot semwal at ti dot com>
 
 
-Kernel cpu access to a dma-buf buffer object
---------------------------------------------
-
-The motivation to allow cpu access from the kernel to a dma-buf object from the
-importers side are:
-- fallback operations, e.g. if the devices is connected to a usb bus and the
-  kernel needs to shuffle the data around first before sending it away.
-- full transparency for existing users on the importer side, i.e. userspace
-  should not notice the difference between a normal object from that subsystem
-  and an imported one backed by a dma-buf. This is really important for drm
-  opengl drivers that expect to still use all the existing upload/download
-  paths.
-
-Access to a dma_buf from the kernel context involves three steps:
-
-1. Prepare access, which invalidate any necessary caches and make the object
-   available for cpu access.
-2. Access the object page-by-page with the dma_buf map apis
-3. Finish access, which will flush any necessary cpu caches and free reserved
-   resources.
-
-1. Prepare access
-
-   Before an importer can access a dma_buf object with the cpu from the kernel
-   context, it needs to notify the exporter of the access that is about to
-   happen.
-
-   Interface:
-      int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
-				   enum dma_data_direction direction)
-
-   This allows the exporter to ensure that the memory is actually available for
-   cpu access - the exporter might need to allocate or swap-in and pin the
-   backing storage. 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).
-
-   This step might fail, e.g. in oom conditions.
-
-2. Accessing the buffer
-
-   To support dma_buf objects residing in highmem cpu access is page-based using
-   an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
-   PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
-   a pointer in kernel virtual address space. Afterwards the chunk needs to be
-   unmapped again. There is no limit on how often a given chunk can be mapped
-   and unmapped, i.e. the importer does not need to call begin_cpu_access again
-   before mapping the same chunk again.
-
-   Interfaces:
-      void *dma_buf_kmap(struct dma_buf *, unsigned long);
-      void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
-
-   There are also atomic variants of these interfaces. Like for kmap they
-   facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
-   the callback) is allowed to block when using these.
-
-   Interfaces:
-      void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
-      void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
-
-   For importers all the restrictions of using kmap apply, like the limited
-   supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
-   atomic dma_buf kmaps at the same time (in any given process context).
-
-   dma_buf kmap calls outside of the range specified in begin_cpu_access are
-   undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
-   the partial chunks at the beginning and end but may return stale or bogus
-   data outside of the range (in these partial chunks).
-
-   Note that these calls need to always succeed. The exporter needs to complete
-   any preparations that might fail in begin_cpu_access.
-
-   For some cases the overhead of kmap can be too high, a vmap interface
-   is introduced. This interface should be used very carefully, as vmalloc
-   space is a limited resources on many architectures.
-
-   Interfaces:
-      void *dma_buf_vmap(struct dma_buf *dmabuf)
-      void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
-
-   The vmap call can fail if there is no vmap support in the exporter, or if it
-   runs out of vmalloc space. Fallback to kmap should be implemented. Note that
-   the dma-buf layer keeps a reference count for all vmap access and calls down
-   into the exporter's vmap function only when no vmapping exists, and only
-   unmaps it once. Protection against concurrent vmap/vunmap calls is provided
-   by taking the dma_buf->lock mutex.
-
-3. Finish access
-
-   When the importer is done accessing the CPU, it needs to announce this to
-   the exporter (to facilitate cache flushing and unpinning of any pinned
-   resources). The result of any dma_buf kmap calls after end_cpu_access is
-   undefined.
-
-   Interface:
-      void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
-				  enum dma_data_direction dir);
-
-
-Direct Userspace Access/mmap Support
-------------------------------------
-
-Being able to mmap an export dma-buf buffer object has 2 main use-cases:
-- CPU fallback processing in a pipeline and
-- supporting existing mmap interfaces in importers.
-
-1. CPU fallback processing in a pipeline
-
-   In many processing pipelines it is sometimes required that the cpu can access
-   the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
-   the need to handle this specially in userspace frameworks for buffer sharing
-   it's ideal if the dma_buf fd itself can be used to access the backing storage
-   from userspace using mmap.
-
-   Furthermore Android's ION framework already supports this (and is otherwise
-   rather similar to dma-buf from a userspace consumer side with using fds as
-   handles, too). So it's beneficial to support this in a similar fashion on
-   dma-buf to have a good transition path for existing Android userspace.
-
-   No special interfaces, userspace simply calls mmap on the dma-buf fd, making
-   sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
-   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
-   -EAGAIN or -EINTR, in which case it must be restarted.
-
-   Some systems might need some sort of cache coherency management e.g. when
-   CPU and GPU domains are being accessed through dma-buf at the same time. To
-   circumvent this problem there are begin/end coherency markers, that forward
-   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
-   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
-   would be used like following:
-     - mmap dma-buf fd
-     - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
-       to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
-       want (with the new data being consumed by the GPU or say scanout device)
-     - munmap once you don't need the buffer any more
-
-    For correctness and optimal performance, it is always required to use
-    SYNC_START and SYNC_END before and after, respectively, when accessing the
-    mapped address. Userspace cannot rely on coherent access, even when there
-    are systems where it just works without calling these ioctls.
-
-2. Supporting existing mmap interfaces in importers
-
-   Similar to the motivation for kernel cpu access it is again important that
-   the userspace code of a given importing subsystem can use the same interfaces
-   with a imported dma-buf buffer object as with a native buffer object. This is
-   especially important for drm where the userspace part of contemporary OpenGL,
-   X, and other drivers is huge, and reworking them to use a different way to
-   mmap a buffer rather invasive.
-
-   The assumption in the current dma-buf interfaces is that redirecting the
-   initial mmap is all that's needed. A survey of some of the existing
-   subsystems shows that no driver seems to do any nefarious thing like syncing
-   up with outstanding asynchronous processing on the device or allocating
-   special resources at fault time. So hopefully this is good enough, since
-   adding interfaces to intercept pagefaults and allow pte shootdowns would
-   increase the complexity quite a bit.
-
-   Interface:
-      int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
-		       unsigned long);
-
-   If the importing subsystem simply provides a special-purpose mmap call to set
-   up a mapping in userspace, calling do_mmap with dma_buf->file will equally
-   achieve that for a dma-buf object.
-
-3. Implementation notes for exporters
-
-   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.
-
-   Because existing importing subsystems might presume coherent mappings for
-   userspace, the exporter needs to set up a coherent mapping. If that's not
-   possible, it needs to fake coherency by manually shooting down ptes when
-   leaving the cpu domain and flushing caches at fault time. Note that all the
-   dma_buf files share the same anon inode, hence the exporter needs to replace
-   the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
-   required. This is because the kernel uses the underlying inode's address_space
-   for vma tracking (and hence pte tracking at shootdown time with
-   unmap_mapping_range).
-
-   If the above shootdown dance turns out to be too expensive in certain
-   scenarios, we can extend dma-buf with a more explicit cache tracking scheme
-   for userspace mappings. But the current assumption is that using mmap is
-   always a slower path, so some inefficiencies should be acceptable.
-
-   Exporters that shoot down mappings (for any reasons) shall not do any
-   synchronization at fault time with outstanding device operations.
-   Synchronization is an orthogonal issue to sharing the backing storage of a
-   buffer and hence should not be handled by dma-buf itself. This is explicitly
-   mentioned here because many people seem to want something like this, but if
-   different exporters handle this differently, buffer sharing can fail in
-   interesting ways depending upong the exporter (if userspace starts depending
-   upon this implicit synchronization).
-
 Other Interfaces Exposed to Userspace on the dma-buf FD
 ------------------------------------------------------
 
@@ -240,20 +41,6 @@ Miscellaneous notes
   the exporting driver to create a dmabuf fd must provide a way to let
   userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
 
-- 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.
-
 References:
 [1] struct dma_buf_ops in include/linux/dma-buf.h
 [2] All interfaces mentioned above defined in include/linux/dma-buf.h
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 906d1532efad..92e417035e16 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -52,6 +52,12 @@ Basic Operation and Device DMA Access
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :doc: dma buf device access
 
+CPU Access to DMA Buffer Objects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: cpu access
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 09f948fd62ad..0933c01e6d8c 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -640,6 +640,121 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
 }
 EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
 
+/**
+ * DOC: cpu access
+ *
+ * There are mutliple reasons for supporting CPU access to a dma buffer object:
+ *
+ * - Fallback operations in the kernel, for example when a device is connected
+ *   over USB and the kernel needs to shuffle the data around first before
+ *   sending it away. Cache coherency is handled by braketing any transactions
+ *   with calls to dma_buf_begin_cpu_access() and dma_buf_end_cpu_access()
+ *   access.
+ *
+ *   To support dma_buf objects residing in highmem cpu access is page-based using
+ *   an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
+ *   PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
+ *   a pointer in kernel virtual address space. Afterwards the chunk needs to be
+ *   unmapped again. There is no limit on how often a given chunk can be mapped
+ *   and unmapped, i.e. the importer does not need to call begin_cpu_access again
+ *   before mapping the same chunk again.
+ *
+ *   Interfaces::
+ *      void \*dma_buf_kmap(struct dma_buf \*, unsigned long);
+ *      void dma_buf_kunmap(struct dma_buf \*, unsigned long, void \*);
+ *
+ *   There are also atomic variants of these interfaces. Like for kmap they
+ *   facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
+ *   the callback) is allowed to block when using these.
+ *
+ *   Interfaces::
+ *      void \*dma_buf_kmap_atomic(struct dma_buf \*, unsigned long);
+ *      void dma_buf_kunmap_atomic(struct dma_buf \*, unsigned long, void \*);
+ *
+ *   For importers all the restrictions of using kmap apply, like the limited
+ *   supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
+ *   atomic dma_buf kmaps at the same time (in any given process context).
+ *
+ *   dma_buf kmap calls outside of the range specified in begin_cpu_access are
+ *   undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
+ *   the partial chunks at the beginning and end but may return stale or bogus
+ *   data outside of the range (in these partial chunks).
+ *
+ *   Note that these calls need to always succeed. The exporter needs to complete
+ *   any preparations that might fail in begin_cpu_access.
+ *
+ *   For some cases the overhead of kmap can be too high, a vmap interface
+ *   is introduced. This interface should be used very carefully, as vmalloc
+ *   space is a limited resources on many architectures.
+ *
+ *   Interfaces::
+ *      void \*dma_buf_vmap(struct dma_buf \*dmabuf)
+ *      void dma_buf_vunmap(struct dma_buf \*dmabuf, void \*vaddr)
+ *
+ *   The vmap call can fail if there is no vmap support in the exporter, or if it
+ *   runs out of vmalloc space. Fallback to kmap should be implemented. Note that
+ *   the dma-buf layer keeps a reference count for all vmap access and calls down
+ *   into the exporter's vmap function only when no vmapping exists, and only
+ *   unmaps it once. Protection against concurrent vmap/vunmap calls is provided
+ *   by taking the dma_buf->lock mutex.
+ *
+ * - For full compatibility on the importer side with existing userspace
+ *   interfaces, which might already support mmap'ing buffers. This is needed in
+ *   many processing pipelines (e.g. feeding a software rendered image into a
+ *   hardware pipeline, thumbnail creation, snapshots, ...). Also, Android's ION
+ *   framework already supported this and for DMA buffer file descriptors to
+ *   replace ION buffers mmap support was needed.
+ *
+ *   There is no special interfaces, userspace simply calls mmap on the dma-buf
+ *   fd. But like for CPU access there's a need to braket the actual access,
+ *   which is handled by the ioctl (DMA_BUF_IOCTL_SYNC). Note that
+ *   DMA_BUF_IOCTL_SYNC can fail with -EAGAIN or -EINTR, in which case it must
+ *   be restarted.
+ *
+ *   Some systems might need some sort of cache coherency management e.g. when
+ *   CPU and GPU domains are being accessed through dma-buf at the same time. To
+ *   circumvent this problem there are begin/end coherency markers, that forward
+ *   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
+ *   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
+ *   would be used like following:
+ *
+ *     - mmap dma-buf fd
+ *     - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
+ *       to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
+ *       want (with the new data being consumed by the GPU or say scanout device)
+ *     - munmap once you don't need the buffer any more
+ *
+ *    For correctness and optimal performance, it is always required to use
+ *    SYNC_START and SYNC_END before and after, respectively, when accessing the
+ *    mapped address. Userspace cannot rely on coherent access, even when there
+ *    are systems where it just works without calling these ioctls.
+ *
+ * - And as a CPU fallback in userspace processing pipelines.
+ *
+ *   Similar to the motivation for kernel cpu access it is again important that
+ *   the userspace code of a given importing subsystem can use the same interfaces
+ *   with a imported dma-buf buffer object as with a native buffer object. This is
+ *   especially important for drm where the userspace part of contemporary OpenGL,
+ *   X, and other drivers is huge, and reworking them to use a different way to
+ *   mmap a buffer rather invasive.
+ *
+ *   The assumption in the current dma-buf interfaces is that redirecting the
+ *   initial mmap is all that's needed. A survey of some of the existing
+ *   subsystems shows that no driver seems to do any nefarious thing like syncing
+ *   up with outstanding asynchronous processing on the device or allocating
+ *   special resources at fault time. So hopefully this is good enough, since
+ *   adding interfaces to intercept pagefaults and allow pte shootdowns would
+ *   increase the complexity quite a bit.
+ *
+ *   Interface::
+ *      int dma_buf_mmap(struct dma_buf \*, struct vm_area_struct \*,
+ *		       unsigned long);
+ *
+ *   If the importing subsystem simply provides a special-purpose mmap call to set
+ *   up a mapping in userspace, calling do_mmap with dma_buf->file will equally
+ *   achieve that for a dma-buf object.
+ */
+
 static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 				      enum dma_data_direction direction)
 {
@@ -665,6 +780,10 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
  * @dmabuf:	[in]	buffer to prepare cpu access for.
  * @direction:	[in]	length of range for cpu access.
  *
+ * After the cpu access is complete the caller should call
+ * dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
+ * it guaranteed to be coherent with other DMA access.
+ *
  * Can return negative error values, returns 0 on success.
  */
 int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
@@ -697,6 +816,8 @@ EXPORT_SYMBOL_GPL(dma_buf_begin_cpu_access);
  * @dmabuf:	[in]	buffer to complete cpu access for.
  * @direction:	[in]	length of range for cpu access.
  *
+ * This terminates CPU access started with dma_buf_begin_cpu_access().
+ *
  * Can return negative error values, returns 0 on success.
  */
 int dma_buf_end_cpu_access(struct dma_buf *dmabuf,
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 6df170fb243f..57828154e440 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -39,10 +39,6 @@ struct dma_buf_attachment;
 
 /**
  * struct dma_buf_ops - operations possible on struct dma_buf
- * @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.
- * @end_cpu_access: [optional] called after cpu access to flush caches.
  * @kmap_atomic: maps a page from the buffer into kernel address
  * 		 space, users may not block until the subsequent unmap call.
  * 		 This callback must not sleep.
@@ -50,10 +46,6 @@ struct dma_buf_attachment;
  * 		   This Callback must not sleep.
  * @kmap: maps a page from the buffer into kernel address space.
  * @kunmap: [optional] unmaps a page from the buffer.
- * @mmap: used to expose the backing storage to userspace. Note that the
- * 	  mapping needs to be coherent - if the exporter doesn't directly
- * 	  support this, it needs to fake coherency by shooting down any ptes
- * 	  when transitioning away from the cpu domain.
  * @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
@@ -164,13 +156,96 @@ struct dma_buf_ops {
 	 */
 	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 available for cpu
+	 * access - the exporter might need to allocate or swap-in and pin the
+	 * backing storage. 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).
+	 *
+	 * This callback is optional.
+	 *
+	 * FIXME: This is both called through the DMA_BUF_IOCTL_SYNC command
+	 * from userspace (where storage shouldn't be pinned to avoid handing
+	 * de-factor mlock rights to userspace) and for the kernel-internal
+	 * users of the various kmap interfaces, where the backing storage must
+	 * be pinned to guarantee that the atomic kmap calls can succeed. Since
+	 * there's no in-kernel users of the kmap interfaces yet this isn't a
+	 * real problem.
+	 *
+	 * 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
+	 * unpin any resources pinned in @begin_cpu_access.
+	 * The result of any dma_buf kmap calls after end_cpu_access is
+	 * undefined.
+	 *
+	 * 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);
 	void *(*kmap_atomic)(struct dma_buf *, unsigned long);
 	void (*kunmap_atomic)(struct dma_buf *, unsigned long, void *);
 	void *(*kmap)(struct dma_buf *, unsigned long);
 	void (*kunmap)(struct dma_buf *, unsigned long, void *);
 
+	/**
+	 * @mmap:
+	 *
+	 * This callback is used by the dma_buf_mmap() function
+	 *
+	 * Note that the mapping needs to be incoherent, userspace is expected
+	 * to braket 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);
 
 	void *(*vmap)(struct dma_buf *);
-- 
2.10.2

[PATCH 5/5] dma-buf: Final bits of doc polish

[Daniel Vetter]

- Put all the remaing bits of the old doc into suitable places in the
  new sphinx world.
- Also document the poll support, we forgot to do that.
- Delete dma-buf-sharing.txt.

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    | 47 -----------------------------------
 Documentation/driver-api/dma-buf.rst | 48 ++++++++++++++++++++++++++++++++++++
 drivers/dma-buf/dma-buf.c            | 22 +++++++++++++++++
 3 files changed, 70 insertions(+), 47 deletions(-)
 delete mode 100644 Documentation/dma-buf-sharing.txt

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
deleted file mode 100644
index 74c99edb7976..000000000000
--- a/Documentation/dma-buf-sharing.txt
+++ /dev/null
@@ -1,47 +0,0 @@
-                    DMA Buffer Sharing API Guide
-                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-                            Sumit Semwal
-                <sumit dot semwal at linaro dot org>
-                 <sumit dot semwal at ti dot com>
-
-
-Other Interfaces Exposed to Userspace on the dma-buf FD
-------------------------------------------------------
-
-- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
-  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
-  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
-  llseek operation will report -EINVAL.
-
-  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
-  cases. Userspace can use this to detect support for discovering the dma-buf
-  size using llseek.
-
-Miscellaneous notes
--------------------
-
-- Any exporters or users of the dma-buf buffer sharing framework must have
-  a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
-
-- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
-  on the file descriptor.  This is not just a resource leak, but a
-  potential security hole.  It could give the newly exec'd application
-  access to buffers, via the leaked fd, to which it should otherwise
-  not be permitted access.
-
-  The problem with doing this via a separate fcntl() call, versus doing it
-  atomically when the fd is created, is that this is inherently racy in a
-  multi-threaded app[3].  The issue is made worse when it is library code
-  opening/creating the file descriptor, as the application may not even be
-  aware of the fd's.
-
-  To avoid this problem, userspace must have a way to request O_CLOEXEC
-  flag be set when the dma-buf fd is created.  So any API provided by
-  the exporting driver to create a dmabuf fd must provide a way to let
-  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
-
-References:
-[1] struct dma_buf_ops in include/linux/dma-buf.h
-[2] All interfaces mentioned above defined in include/linux/dma-buf.h
-[3] https://lwn.net/Articles/236486/
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 92e417035e16..31671b469627 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -46,6 +46,48 @@ The buffer-user
    same area of memory. This interface is provided by :c:type:`struct
    dma_buf_attachment <dma_buf_attachment>`.
 
+Any exporters or users of the dma-buf buffer sharing framework must have a
+'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+Userspace Interface Notes
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
+and hence the generic interface exposed is very minimal. There's a few things to
+consider though:
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
+- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+  on the file descriptor.  This is not just a resource leak, but a
+  potential security hole.  It could give the newly exec'd application
+  access to buffers, via the leaked fd, to which it should otherwise
+  not be permitted access.
+
+  The problem with doing this via a separate fcntl() call, versus doing it
+  atomically when the fd is created, is that this is inherently racy in a
+  multi-threaded app[3].  The issue is made worse when it is library code
+  opening/creating the file descriptor, as the application may not even be
+  aware of the fd's.
+
+  To avoid this problem, userspace must have a way to request O_CLOEXEC
+  flag be set when the dma-buf fd is created.  So any API provided by
+  the exporting driver to create a dmabuf fd must provide a way to let
+  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
+
+- Memory mapping the contents of the DMA buffer is also supported. See the
+  discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
+
+- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
+  details.
+
 Basic Operation and Device DMA Access
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -58,6 +100,12 @@ CPU Access to DMA Buffer Objects
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :doc: cpu access
 
+Fence Poll Support
+~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: fence polling
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 0933c01e6d8c..0775fc947d73 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -124,6 +124,28 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
 	return base + offset;
 }
 
+/**
+ * DOC: fence polling
+ *
+ * To support cross-device and cross-driver synchronization of buffer access
+ * implicit fences (represented internally in the kernel with struct &fence) can
+ * be attached to a &dma_buf. The glue for that and a few related things are
+ * provided in the &reservation_object structure.
+ *
+ * Userspace can query the state of these implicitly tracked fences using poll()
+ * and related system calls:
+ *
+ * - Checking for POLLIN, i.e. read access, can be use to query the state of the
+ *   most recent write or exclusive fence.
+ *
+ * - Checking for POLLOUT, i.e. write access, can be used to query the state of
+ *   all attached fences, shared and exclusive ones.
+ *
+ * Note that this only signals the completion of the respective fences, i.e. the
+ * DMA transfers are complete. Cache flushing and any other necessary
+ * preparations before CPU access can begin still need to happen.
+ */
+
 static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
 	struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
-- 
2.10.2

✓ Fi.CI.BAT: success for sphinxification for dma-buf docs

[Patchwork]

== Series Details ==

Series: sphinxification for dma-buf docs
URL   : https://patchwork.freedesktop.org/series/16629/
State : success

== Summary ==

Series 16629v1 sphinxification for dma-buf docs
https://patchwork.freedesktop.org/api/1.0/series/16629/revisions/1/mbox/


fi-bdw-5557u     total:247  pass:233  dwarn:0   dfail:0   fail:0   skip:14 
fi-bsw-n3050     total:247  pass:208  dwarn:0   dfail:0   fail:0   skip:39 
fi-bxt-t5700     total:247  pass:220  dwarn:0   dfail:0   fail:0   skip:27 
fi-byt-j1900     total:247  pass:220  dwarn:0   dfail:0   fail:0   skip:27 
fi-byt-n2820     total:247  pass:216  dwarn:0   dfail:0   fail:0   skip:31 
fi-hsw-4770      total:247  pass:228  dwarn:0   dfail:0   fail:0   skip:19 
fi-hsw-4770r     total:247  pass:228  dwarn:0   dfail:0   fail:0   skip:19 
fi-ilk-650       total:247  pass:195  dwarn:0   dfail:0   fail:0   skip:52 
fi-ivb-3520m     total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-ivb-3770      total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-kbl-7500u     total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-skl-6260u     total:247  pass:234  dwarn:0   dfail:0   fail:0   skip:13 
fi-skl-6700hq    total:247  pass:227  dwarn:0   dfail:0   fail:0   skip:20 
fi-skl-6700k     total:247  pass:224  dwarn:3   dfail:0   fail:0   skip:20 
fi-skl-6770hq    total:247  pass:234  dwarn:0   dfail:0   fail:0   skip:13 
fi-snb-2520m     total:247  pass:216  dwarn:0   dfail:0   fail:0   skip:31 
fi-snb-2600      total:247  pass:215  dwarn:0   dfail:0   fail:0   skip:32 

de43f4e755c6bf50ad53b4ccacedf9850f42eda4 drm-tip: 2016y-12m-09d-09h-01m-58s UTC integration manifest
fc9e8d1 dma-buf: Final bits of doc polish
17e7091 dma-buf: Update cpu access documentation
c443a97 dma-buf: Reorganize device dma access docs
dc8213d dma-buf: Update kerneldoc for sync_file_create
dafe989 dma-buf: Extract dma-buf.rst

== Logs ==

For more details see: https://intel-gfx-ci.01.org/CI/Patchwork_3257/
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx

Re: [PATCH 0/5] sphinxification for dma-buf docs

[Jonathan Corbet]

On Fri,  9 Dec 2016 19:53:04 +0100
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> Not yet everything in this area, I still want to sprinkle nice docs around all
> the fence code. Especially some text to explain implicit vs. explicit fencing
> and how it's all supposed to work.
> 
> But just cleanup in the dma-buf part was quite a bit of work, and I'd like to
> get feedback on that before moving on.

No complaints here - except that I had to go looking around to find this
0/5 posting explaining what the overall goal was...:)

It seems like just the sort of thing we want to be doing to pull the docs
together in a more rational way.

jon

[PATCH] dma-buf: Final bits of doc polish

[Daniel Vetter]

- Put all the remaing bits of the old doc into suitable places in the
  new sphinx world.
- Also document the poll support, we forgot to do that.
- Delete dma-buf-sharing.txt.

v2: Don't forget to update MAINTAINERS.

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    | 47 -----------------------------------
 Documentation/driver-api/dma-buf.rst | 48 ++++++++++++++++++++++++++++++++++++
 MAINTAINERS                          |  2 +-
 drivers/dma-buf/dma-buf.c            | 22 +++++++++++++++++
 4 files changed, 71 insertions(+), 48 deletions(-)
 delete mode 100644 Documentation/dma-buf-sharing.txt

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
deleted file mode 100644
index 74c99edb7976..000000000000
--- a/Documentation/dma-buf-sharing.txt
+++ /dev/null
@@ -1,47 +0,0 @@
-                    DMA Buffer Sharing API Guide
-                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-                            Sumit Semwal
-                <sumit dot semwal at linaro dot org>
-                 <sumit dot semwal at ti dot com>
-
-
-Other Interfaces Exposed to Userspace on the dma-buf FD
-------------------------------------------------------
-
-- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
-  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
-  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
-  llseek operation will report -EINVAL.
-
-  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
-  cases. Userspace can use this to detect support for discovering the dma-buf
-  size using llseek.
-
-Miscellaneous notes
--------------------
-
-- Any exporters or users of the dma-buf buffer sharing framework must have
-  a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
-
-- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
-  on the file descriptor.  This is not just a resource leak, but a
-  potential security hole.  It could give the newly exec'd application
-  access to buffers, via the leaked fd, to which it should otherwise
-  not be permitted access.
-
-  The problem with doing this via a separate fcntl() call, versus doing it
-  atomically when the fd is created, is that this is inherently racy in a
-  multi-threaded app[3].  The issue is made worse when it is library code
-  opening/creating the file descriptor, as the application may not even be
-  aware of the fd's.
-
-  To avoid this problem, userspace must have a way to request O_CLOEXEC
-  flag be set when the dma-buf fd is created.  So any API provided by
-  the exporting driver to create a dmabuf fd must provide a way to let
-  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
-
-References:
-[1] struct dma_buf_ops in include/linux/dma-buf.h
-[2] All interfaces mentioned above defined in include/linux/dma-buf.h
-[3] https://lwn.net/Articles/236486/
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 92e417035e16..31671b469627 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -46,6 +46,48 @@ The buffer-user
    same area of memory. This interface is provided by :c:type:`struct
    dma_buf_attachment <dma_buf_attachment>`.
 
+Any exporters or users of the dma-buf buffer sharing framework must have a
+'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+Userspace Interface Notes
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
+and hence the generic interface exposed is very minimal. There's a few things to
+consider though:
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
+- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+  on the file descriptor.  This is not just a resource leak, but a
+  potential security hole.  It could give the newly exec'd application
+  access to buffers, via the leaked fd, to which it should otherwise
+  not be permitted access.
+
+  The problem with doing this via a separate fcntl() call, versus doing it
+  atomically when the fd is created, is that this is inherently racy in a
+  multi-threaded app[3].  The issue is made worse when it is library code
+  opening/creating the file descriptor, as the application may not even be
+  aware of the fd's.
+
+  To avoid this problem, userspace must have a way to request O_CLOEXEC
+  flag be set when the dma-buf fd is created.  So any API provided by
+  the exporting driver to create a dmabuf fd must provide a way to let
+  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
+
+- Memory mapping the contents of the DMA buffer is also supported. See the
+  discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
+
+- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
+  details.
+
 Basic Operation and Device DMA Access
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -58,6 +100,12 @@ CPU Access to DMA Buffer Objects
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :doc: cpu access
 
+Fence Poll Support
+~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: fence polling
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/MAINTAINERS b/MAINTAINERS
index 1a7a773114aa..d19872eed4de 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3917,7 +3917,7 @@ F:	drivers/dma-buf/
 F:	include/linux/dma-buf*
 F:	include/linux/reservation.h
 F:	include/linux/*fence.h
-F:	Documentation/dma-buf-sharing.txt
+F:	Documentation/driver-api/dma-buf.rst
 T:	git git://anongit.freedesktop.org/drm/drm-misc
 
 SYNC FILE FRAMEWORK
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 0933c01e6d8c..0775fc947d73 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -124,6 +124,28 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
 	return base + offset;
 }
 
+/**
+ * DOC: fence polling
+ *
+ * To support cross-device and cross-driver synchronization of buffer access
+ * implicit fences (represented internally in the kernel with struct &fence) can
+ * be attached to a &dma_buf. The glue for that and a few related things are
+ * provided in the &reservation_object structure.
+ *
+ * Userspace can query the state of these implicitly tracked fences using poll()
+ * and related system calls:
+ *
+ * - Checking for POLLIN, i.e. read access, can be use to query the state of the
+ *   most recent write or exclusive fence.
+ *
+ * - Checking for POLLOUT, i.e. write access, can be used to query the state of
+ *   all attached fences, shared and exclusive ones.
+ *
+ * Note that this only signals the completion of the respective fences, i.e. the
+ * DMA transfers are complete. Cache flushing and any other necessary
+ * preparations before CPU access can begin still need to happen.
+ */
+
 static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
 	struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
-- 
2.10.2

✓ Fi.CI.BAT: success for sphinxification for dma-buf docs (rev2)

[Patchwork]

== Series Details ==

Series: sphinxification for dma-buf docs (rev2)
URL   : https://patchwork.freedesktop.org/series/16629/
State : success

== Summary ==

Series 16629v2 sphinxification for dma-buf docs
https://patchwork.freedesktop.org/api/1.0/series/16629/revisions/2/mbox/


fi-bdw-5557u     total:247  pass:233  dwarn:0   dfail:0   fail:0   skip:14 
fi-bsw-n3050     total:247  pass:208  dwarn:0   dfail:0   fail:0   skip:39 
fi-bxt-t5700     total:247  pass:220  dwarn:0   dfail:0   fail:0   skip:27 
fi-byt-j1900     total:247  pass:220  dwarn:0   dfail:0   fail:0   skip:27 
fi-byt-n2820     total:247  pass:216  dwarn:0   dfail:0   fail:0   skip:31 
fi-hsw-4770      total:247  pass:228  dwarn:0   dfail:0   fail:0   skip:19 
fi-hsw-4770r     total:247  pass:228  dwarn:0   dfail:0   fail:0   skip:19 
fi-ilk-650       total:247  pass:195  dwarn:0   dfail:0   fail:0   skip:52 
fi-ivb-3520m     total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-ivb-3770      total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-kbl-7500u     total:247  pass:226  dwarn:0   dfail:0   fail:0   skip:21 
fi-skl-6260u     total:247  pass:234  dwarn:0   dfail:0   fail:0   skip:13 
fi-skl-6700hq    total:247  pass:227  dwarn:0   dfail:0   fail:0   skip:20 
fi-skl-6700k     total:247  pass:224  dwarn:3   dfail:0   fail:0   skip:20 
fi-skl-6770hq    total:247  pass:234  dwarn:0   dfail:0   fail:0   skip:13 
fi-snb-2600      total:247  pass:215  dwarn:0   dfail:0   fail:0   skip:32 

de43f4e755c6bf50ad53b4ccacedf9850f42eda4 drm-tip: 2016y-12m-09d-09h-01m-58s UTC integration manifest
be45f8c dma-buf: Final bits of doc polish
f225b6f dma-buf: Update cpu access documentation
c22e662 dma-buf: Reorganize device dma access docs
b379c89 dma-buf: Update kerneldoc for sync_file_create
e7a4d8b dma-buf: Extract dma-buf.rst

== Logs ==

For more details see: https://intel-gfx-ci.01.org/CI/Patchwork_3258/
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Re: [PATCH 0/5] sphinxification for dma-buf docs

[Sumit Semwal]

Hi Daniel,

On 10 December 2016 at 02:45, Jonathan Corbet <corbet@lwn.net> wrote:
> On Fri,  9 Dec 2016 19:53:04 +0100
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
>> Not yet everything in this area, I still want to sprinkle nice docs around all
>> the fence code. Especially some text to explain implicit vs. explicit fencing
>> and how it's all supposed to work.
>>
Thanks for the patch series; I had something in the works too, but you
beat me to it! :)

Looks good to me, so please feel free to add my
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>

to the series.

>> But just cleanup in the dma-buf part was quite a bit of work, and I'd like to
>> get feedback on that before moving on.
>
> No complaints here - except that I had to go looking around to find this
> 0/5 posting explaining what the overall goal was...:)
>
> It seems like just the sort of thing we want to be doing to pull the docs
> together in a more rational way.
>
> jon

Best regards,
Sumit.

Re: [PATCH 0/5] sphinxification for dma-buf docs

[Daniel Vetter]

On Fri, Dec 9, 2016 at 10:15 PM, Jonathan Corbet <corbet@lwn.net> wrote:
> On Fri,  9 Dec 2016 19:53:04 +0100
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
>> Not yet everything in this area, I still want to sprinkle nice docs around all
>> the fence code. Especially some text to explain implicit vs. explicit fencing
>> and how it's all supposed to work.
>>
>> But just cleanup in the dma-buf part was quite a bit of work, and I'd like to
>> get feedback on that before moving on.
>
> No complaints here - except that I had to go looking around to find this
> 0/5 posting explaining what the overall goal was...:)
>
> It seems like just the sort of thing we want to be doing to pull the docs
> together in a more rational way.

Ok if we pull this in through gfx trees? Will miss 4.10 though, that's
already finished and in bugfix-only mode.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH 0/5] sphinxification for dma-buf docs

[Jonathan Corbet]

On Sun, 11 Dec 2016 13:35:49 +0100
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> > It seems like just the sort of thing we want to be doing to pull the docs
> > together in a more rational way.  
> 
> Ok if we pull this in through gfx trees? Will miss 4.10 though, that's
> already finished and in bugfix-only mode.

I've pretty much closed up 4.10 as well.

Here's a thought, though: how about if we slip in a little version of
dma-buf.rst now with a "coming soon, don't miss it!!" message?  Then the
rest of the set could go through your tree without touching
driver-api/index.rst and, thus, avoiding the otherwise inevitable
conflicts?

jon
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Re: [PATCH 0/5] sphinxification for dma-buf docs

[Daniel Vetter]

On Sun, Dec 11, 2016 at 4:11 PM, Jonathan Corbet <corbet@lwn.net> wrote:
> On Sun, 11 Dec 2016 13:35:49 +0100
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
>> > It seems like just the sort of thing we want to be doing to pull the docs
>> > together in a more rational way.
>>
>> Ok if we pull this in through gfx trees? Will miss 4.10 though, that's
>> already finished and in bugfix-only mode.
>
> I've pretty much closed up 4.10 as well.
>
> Here's a thought, though: how about if we slip in a little version of
> dma-buf.rst now with a "coming soon, don't miss it!!" message?  Then the
> rest of the set could go through your tree without touching
> driver-api/index.rst and, thus, avoiding the otherwise inevitable
> conflicts?

So just patch 1 still for 4.10 through your tree? Sounds good to me.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH 0/5] sphinxification for dma-buf docs

[Jonathan Corbet]

On Sun, 11 Dec 2016 18:35:42 +0100
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> > Here's a thought, though: how about if we slip in a little version of
> > dma-buf.rst now with a "coming soon, don't miss it!!" message?  Then the
> > rest of the set could go through your tree without touching
> > driver-api/index.rst and, thus, avoiding the otherwise inevitable
> > conflicts?  
> 
> So just patch 1 still for 4.10 through your tree? Sounds good to me.

Yeah, that will work just fine, I'll slip it in.

Thanks,

jon

Re: [PATCH 1/5] dma-buf: Extract dma-buf.rst

[Jonathan Corbet]

On Fri,  9 Dec 2016 19:53:05 +0100
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> Just prep work to polish and consolidate all the dma-buf related
> documenation.
> 
> Unfortunately I didn't discover a way to both integrate this new file
> into the overall toc while keeping it at the current place. Work
> around that by moving it into the overall driver-api/index.rst.

OK, I've applied this, with one change:

> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index d51a7d23c358..c683c4e908d2 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -163,7 +163,6 @@ struct dma_fence_cb {
>   * destruction of the fence. Can be called from irq context.
>   * If pointer is set to NULL, kfree will get called instead.
>   */
> -
>  struct dma_fence_ops {
>  	const char * (*get_driver_name)(struct dma_fence *fence);
>  	const char * (*get_timeline_name)(struct dma_fence *fence);

This hunk didn't apply to my tree, but I figured that, under duress, we
could probably manage to do without it for now...:)

jon

Re: [PATCH 1/5] dma-buf: Extract dma-buf.rst

[Gustavo Padovan]

Hi Daniel,

2016-12-09 Daniel Vetter <daniel.vetter@ffwll.ch>:

> Just prep work to polish and consolidate all the dma-buf related
> documenation.
> 
> Unfortunately I didn't discover a way to both integrate this new file
> into the overall toc while keeping it at the current place. Work
> around that by moving it into the overall driver-api/index.rst.
> 
> 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/driver-api/dma-buf.rst        | 73 +++++++++++++++++++++++++++++
>  Documentation/driver-api/index.rst          |  1 +
>  Documentation/driver-api/infrastructure.rst | 70 ---------------------------
>  include/linux/dma-fence.h                   |  1 -
>  4 files changed, 74 insertions(+), 71 deletions(-)
>  create mode 100644 Documentation/driver-api/dma-buf.rst

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

Gustavo
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Re: [Intel-gfx] [PATCH 2/5] dma-buf: Update kerneldoc for sync_file_create

[Gustavo Padovan]

2016-12-09 Daniel Vetter <daniel.vetter@ffwll.ch>:

> This was missed when adding a dma_fence_get call. While at it also
> remove the kerneldoc for the static inline helper - no point
> documenting internals down to every detail.
> 
> Fixes: 30cd85dd6edc ("dma-buf/sync_file: hold reference to fence when creating sync_file")
> Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
> Cc: Sean Paul <seanpaul@chromium.org>
> 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>
> ---
>  drivers/dma-buf/sync_file.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

Gustavo
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Re: [PATCH 0/5] sphinxification for dma-buf docs

[Sumit Semwal]

Thanks Jonathan!

On 12 December 2016 at 01:14, Jonathan Corbet <corbet@lwn.net> wrote:
> On Sun, 11 Dec 2016 18:35:42 +0100
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
>> > Here's a thought, though: how about if we slip in a little version of
>> > dma-buf.rst now with a "coming soon, don't miss it!!" message?  Then the
>> > rest of the set could go through your tree without touching
>> > driver-api/index.rst and, thus, avoiding the otherwise inevitable
>> > conflicts?
>>
>> So just patch 1 still for 4.10 through your tree? Sounds good to me.
>
> Yeah, that will work just fine, I'll slip it in.
Pushed the rest of the patches in drm-misc-next, queued for 4.11.
>
> Thanks,
>
> jon
Best,
Sumit.
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