[PATCH v4 19/21] dma-buf: Document dynamic locking convention

[Dmitry Osipenko]

Add documentation for the dynamic locking convention. The documentation
tells dma-buf API users when they should take the reservation lock and
when not.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
---
 Documentation/driver-api/dma-buf.rst |  6 +++
 drivers/dma-buf/dma-buf.c            | 64 ++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 36a76cbe9095..622b8156d212 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -119,6 +119,12 @@ DMA Buffer ioctls
 
 .. kernel-doc:: include/uapi/linux/dma-buf.h
 
+DMA-BUF locking convention
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: locking convention
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d9130486cb8f..97ce884fad76 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -794,6 +794,70 @@ static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
 	return sg_table;
 }
 
+/**
+ * DOC: locking convention
+ *
+ * In order to avoid deadlock situations between dma-buf exports and importers,
+ * all dma-buf API users must follow the common dma-buf locking convention.
+ *
+ * Convention for importers
+ *
+ * 1. Importers must hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_pin()
+ *     - dma_buf_unpin()
+ *     - dma_buf_map_attachment()
+ *     - dma_buf_unmap_attachment()
+ *     - dma_buf_vmap()
+ *     - dma_buf_vunmap()
+ *
+ * 2. Importers must not hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_attach()
+ *     - dma_buf_dynamic_attach()
+ *     - dma_buf_detach()
+ *     - dma_buf_export(
+ *     - dma_buf_fd()
+ *     - dma_buf_get()
+ *     - dma_buf_put()
+ *     - dma_buf_mmap()
+ *     - dma_buf_begin_cpu_access()
+ *     - dma_buf_end_cpu_access()
+ *     - dma_buf_map_attachment_unlocked()
+ *     - dma_buf_unmap_attachment_unlocked()
+ *     - dma_buf_vmap_unlocked()
+ *     - dma_buf_vunmap_unlocked()
+ *
+ * Convention for exporters
+ *
+ * 1. These &dma_buf_ops callbacks are invoked with unlocked dma-buf
+ *    reservation and exporter can take the lock:
+ *
+ *     - &dma_buf_ops.attach()
+ *     - &dma_buf_ops.detach()
+ *     - &dma_buf_ops.release()
+ *     - &dma_buf_ops.begin_cpu_access()
+ *     - &dma_buf_ops.end_cpu_access()
+ *
+ * 2. These &dma_buf_ops callbacks are invoked with locked dma-buf
+ *    reservation and exporter can't take the lock:
+ *
+ *     - &dma_buf_ops.pin()
+ *     - &dma_buf_ops.unpin()
+ *     - &dma_buf_ops.map_dma_buf()
+ *     - &dma_buf_ops.unmap_dma_buf()
+ *     - &dma_buf_ops.mmap()
+ *     - &dma_buf_ops.vmap()
+ *     - &dma_buf_ops.vunmap()
+ *
+ * 3. Exporters must hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_move_notify()
+ */
+
 /**
  * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list
  * @dmabuf:		[in]	buffer to attach device to.
-- 
2.37.2