dma-buf: Document non-dynamic exporter expectations better

Christian and me realized we have a pretty massive disconnect about
different interpretations of what dma_resv is used for by different
drivers. The discussion is much, much bigger than this change here,
but this is an important one:

Non-dynamic exporters must guarantee that the memory they return is
ready for use. They cannot expect importers to wait for the exclusive
fence. Only dynamic importers are required to obey the dma_resv fences
strictly (and more patches are needed to define exactly what this
means).

Christian has patches to update nouvea, radeon and amdgpu. The only
other driver using both ttm and supporting dma-buf export is qxl,
which only uses synchronous ttm_bo_move.

v2: To hammer this in document that dynamic importers _must_ wait for
the exclusive fence after having called dma_buf_map_attachment.
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Christian König <ckoenig.leichtzumerken@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210621151758.2347474-1-daniel.vetter@ffwll.ch

drivers/dma-buf/dma-buf.c

@@ -951,6 +951,9 @@ EXPORT_SYMBOL_GPL(dma_buf_unpin);
  * 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.
+ *
+ * Important: Dynamic importers must wait for the exclusive fence of the struct
+ * dma_resv attached to the DMA-BUF first.
  */
 struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 					enum dma_data_direction direction)

include/linux/dma-buf.h

@@ -96,6 +96,12 @@ struct dma_buf_ops {
 	 * This is called automatically for non-dynamic importers from
 	 * dma_buf_attach().
 	 *
+	 * Note that similar to non-dynamic exporters in their @map_dma_buf
+	 * callback the driver must guarantee that the memory is available for
+	 * use and cleared of any old data by the time this function returns.
+	 * Drivers which pipeline their buffer moves internally must wait for
+	 * all moves and clears to complete.
+	 *
 	 * Returns:
 	 *
 	 * 0 on success, negative error code on failure.
@@ -144,6 +150,15 @@ struct dma_buf_ops {
 	 * This is always called with the dmabuf->resv object locked when
 	 * the dynamic_mapping flag is true.
 	 *
+	 * Note that for non-dynamic exporters the driver must guarantee that
+	 * that the memory is available for use and cleared of any old data by
+	 * the time this function returns.  Drivers which pipeline their buffer
+	 * moves internally must wait for all moves and clears to complete.
+	 * Dynamic exporters do not need to follow this rule: For non-dynamic
+	 * importers the buffer is already pinned through @pin, which has the
+	 * same requirements. Dynamic importers otoh are required to obey the
+	 * dma_resv fences.
+	 *
 	 * Returns:
 	 *
 	 * A &sg_table scatter list of or the backing storage of the DMA buffer,