dma-buf: Remove kmap kerneldoc vestiges

Also try to clarify a bit when dma_buf_begin/end_cpu_access should
be called.
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/20201211155843.3348718-1-daniel.vetter@ffwll.ch

drivers/dma-buf/dma-buf.c

@@ -1001,15 +1001,15 @@ EXPORT_SYMBOL_GPL(dma_buf_move_notify);
  *   vmalloc space might be limited and result in vmap calls failing.
  *
  *   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.
+ *   it runs out of vmalloc space. 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
@@ -1098,6 +1098,11 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
  * dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
  * it guaranteed to be coherent with other DMA access.
  *
+ * This function will also wait for any DMA transactions tracked through
+ * implicit synchronization in &dma_buf.resv. For DMA transactions with explicit
+ * synchronization this function will only ensure cache coherency, callers must
+ * ensure synchronization with such DMA transactions on their own.
+ *
  * Can return negative error values, returns 0 on success.
  */
 int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
@@ -1199,7 +1204,10 @@ EXPORT_SYMBOL_GPL(dma_buf_mmap);
  * This call may fail due to lack of virtual mapping address space.
  * These calls are optional in drivers. The intended use for them
  * is for mapping objects linear in kernel space for high use objects.
- * Please attempt to use kmap/kunmap before thinking about these interfaces.
+ *
+ * To ensure coherency users must call dma_buf_begin_cpu_access() and
+ * dma_buf_end_cpu_access() around any cpu access performed through this
+ * mapping.
  *
  * Returns 0 on success, or a negative errno code otherwise.
  */

include/linux/dma-buf.h

@@ -183,24 +183,19 @@ struct dma_buf_ops {
 	 * @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
+	 * exporter to ensure that the memory is actually coherent for cpu
+	 * access. 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.
+	 * Note that this is both called through the DMA_BUF_IOCTL_SYNC IOCTL
+	 * command for userspace mappings established through @mmap, and also
+	 * for kernel mappings established with @vmap.
 	 *
-	 * 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.
+	 * This callback is optional.
 	 *
 	 * Returns:
 	 *
@@ -216,9 +211,7 @@ struct dma_buf_ops {
 	 *
 	 * 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.
+	 * undo anything else done in @begin_cpu_access.
 	 *
 	 * This callback is optional.
 	 *