dma-buf: doc polish for pin/unpin

Motivated by a discussion with Christian and Thomas: Try to untangle a
bit what's relevant for importers and what's relevant for exporters.

Also add an assert that really only dynamic importers use the api
function, anything else doesn't make sense.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201211155843.3348718-4-daniel.vetter@ffwll.ch

drivers/dma-buf/dma-buf.c

@@ -809,9 +809,15 @@ EXPORT_SYMBOL_GPL(dma_buf_detach);
 
 /**
  * dma_buf_pin - Lock down the DMA-buf
- *
  * @attach:	[in]	attachment which should be pinned
  *
+ * Only dynamic importers (who set up @attach with dma_buf_dynamic_attach()) may
+ * call this, and only for limited use cases like scanout and not for temporary
+ * pin operations. It is not permitted to allow userspace to pin arbitrary
+ * amounts of buffers through this interface.
+ *
+ * Buffers must be unpinned by calling dma_buf_unpin().
+ *
  * Returns:
  * 0 on success, negative error code on failure.
  */
@@ -820,6 +826,8 @@ int dma_buf_pin(struct dma_buf_attachment *attach)
 	struct dma_buf *dmabuf = attach->dmabuf;
 	int ret = 0;
 
+	WARN_ON(!dma_buf_attachment_is_dynamic(attach));
+
 	dma_resv_assert_held(dmabuf->resv);
 
 	if (dmabuf->ops->pin)
@@ -830,14 +838,19 @@ int dma_buf_pin(struct dma_buf_attachment *attach)
 EXPORT_SYMBOL_GPL(dma_buf_pin);
 
 /**
- * dma_buf_unpin - Remove lock from DMA-buf
- *
+ * dma_buf_unpin - Unpin a DMA-buf
  * @attach:	[in]	attachment which should be unpinned
+ *
+ * This unpins a buffer pinned by dma_buf_pin() and allows the exporter to move
+ * any mapping of @attach again and inform the importer through
+ * &dma_buf_attach_ops.move_notify.
  */
 void dma_buf_unpin(struct dma_buf_attachment *attach)
 {
 	struct dma_buf *dmabuf = attach->dmabuf;
 
+	WARN_ON(!dma_buf_attachment_is_dynamic(attach));
+
 	dma_resv_assert_held(dmabuf->resv);
 
 	if (dmabuf->ops->unpin)

include/linux/dma-buf.h

@@ -86,13 +86,15 @@ struct dma_buf_ops {
 	 * @pin:
 	 *
 	 * This is called by dma_buf_pin() and lets the exporter know that the
-	 * DMA-buf can't be moved any more.
+	 * DMA-buf can't be moved any more. The exporter should pin the buffer
+	 * into system memory to make sure it is generally accessible by other
+	 * devices.
 	 *
 	 * This is called with the &dmabuf.resv object locked and is mutual
 	 * exclusive with @cache_sgt_mapping.
 	 *
-	 * This callback is optional and should only be used in limited use
-	 * cases like scanout and not for temporary pin operations.
+	 * This is called automatically for non-dynamic importers from
+	 * dma_buf_attach().
 	 *
 	 * Returns:
 	 *