[PATCH 1/8] drm/gem: Write down some rules for vmap usage

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Mapping a GEM object's buffer into kernel address space prevents the
buffer from being evicted from VRAM, which in turn may result in
out-of-memory errors. It's therefore required to only vmap GEM BOs for
short periods of time; unless the GEM implementation provides additional
guarantees.

Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
---
 drivers/gpu/drm/drm_prime.c |  6 ++++++
 include/drm/drm_gem.h       | 16 ++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 7db55fce35d8..9c9ece9833e0 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -669,6 +669,12 @@ EXPORT_SYMBOL(drm_gem_unmap_dma_buf);
  * callback. Calls into &drm_gem_object_funcs.vmap for device specific handling.
  * The kernel virtual address is returned in map.
  *
+ * To prevent the GEM object from being relocated, callers must hold the GEM
+ * object's reservation lock from when calling this function until releasing the
+ * mapping. Holding onto a mapping and the associated reservation lock for an
+ * unbound time may result in out-of-memory errors. Calls to drm_gem_dmabuf_vmap()
+ * should therefore be accompanied by a call to drm_gem_dmabuf_vunmap().
+ *
  * Returns 0 on success or a negative errno code otherwise.
  */
 int drm_gem_dmabuf_vmap(struct dma_buf *dma_buf, struct dma_buf_map *map)
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 5e6daa1c982f..7c34cd5ec261 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -137,7 +137,21 @@ struct drm_gem_object_funcs {
 	 * Returns a virtual address for the buffer. Used by the
 	 * drm_gem_dmabuf_vmap() helper.
 	 *
+	 * Notes to implementors:
+	 *
+	 * - Implementations must expect pairs of @vmap and @vunmap to be
+	 *   called frequently and should optimize for this case.
+	 *
+	 * - Implemenations may expect the caller to hold the GEM object's
+	 *   reservation lock to protect against concurrent calls and relocation
+	 *   of the GEM object.
+	 *
+	 * - Implementations may provide additional guarantees (e.g., working
+	 *   without holding the reservation lock).
+	 *
 	 * This callback is optional.
+	 *
+	 * See also drm_gem_dmabuf_vmap()
 	 */
 	int (*vmap)(struct drm_gem_object *obj, struct dma_buf_map *map);
 
@@ -148,6 +162,8 @@ struct drm_gem_object_funcs {
 	 * drm_gem_dmabuf_vunmap() helper.
 	 *
 	 * This callback is optional.
+	 *
+	 * See also @vmap.
 	 */
 	void (*vunmap)(struct drm_gem_object *obj, struct dma_buf_map *map);
 
-- 
2.29.2

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