drm/fbdev-generic: Fix locking with drm_client_buffer_vmap_local()

Temporarily lock the fbdev buffer object during updates to prevent
memory managers from evicting/moving the buffer. Moving a buffer
object while update its content results in undefined behaviour.

Fbdev-generic updates its buffer object from a shadow buffer. Gem-shmem
and gem-dma helpers do not move buffer objects, so they are safe to be
used with fbdev-generic. Gem-vram and qxl are based on TTM, but pin
buffer objects are part of the vmap operation. So both are also safe
to be used with fbdev-generic.

Amdgpu and nouveau do not pin or lock the buffer object during an
update. Their TTM-based memory management could move the buffer object
while the update is ongoing.

The new vmap_local and vunmap_local helpers hold the buffer object's
reservation lock during the buffer update. This prevents moving the
buffer object on all memory managers.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-11-tzimmermann@suse.de

drivers/gpu/drm/drm_client.c

@@ -304,6 +304,66 @@ drm_client_buffer_create(struct drm_client_dev *client, u32 width, u32 height,
 	return ERR_PTR(ret);
 }
 
+/**
+ * drm_client_buffer_vmap_local - Map DRM client buffer into address space
+ * @buffer: DRM client buffer
+ * @map_copy: Returns the mapped memory's address
+ *
+ * This function maps a client buffer into kernel address space. If the
+ * buffer is already mapped, it returns the existing mapping's address.
+ *
+ * Client buffer mappings are not ref'counted. Each call to
+ * drm_client_buffer_vmap_local() should be closely followed by a call to
+ * drm_client_buffer_vunmap_local(). See drm_client_buffer_vmap() for
+ * long-term mappings.
+ *
+ * The returned address is a copy of the internal value. In contrast to
+ * other vmap interfaces, you don't need it for the client's vunmap
+ * function. So you can modify it at will during blit and draw operations.
+ *
+ * Returns:
+ *	0 on success, or a negative errno code otherwise.
+ */
+int drm_client_buffer_vmap_local(struct drm_client_buffer *buffer,
+				 struct iosys_map *map_copy)
+{
+	struct drm_gem_object *gem = buffer->gem;
+	struct iosys_map *map = &buffer->map;
+	int ret;
+
+	drm_gem_lock(gem);
+
+	ret = drm_gem_vmap(gem, map);
+	if (ret)
+		goto err_drm_gem_vmap_unlocked;
+	*map_copy = *map;
+
+	return 0;
+
+err_drm_gem_vmap_unlocked:
+	drm_gem_unlock(gem);
+	return 0;
+}
+EXPORT_SYMBOL(drm_client_buffer_vmap_local);
+
+/**
+ * drm_client_buffer_vunmap_local - Unmap DRM client buffer
+ * @buffer: DRM client buffer
+ *
+ * This function removes a client buffer's memory mapping established
+ * with drm_client_buffer_vunmap_local(). Calling this function is only
+ * required by clients that manage their buffer mappings by themselves.
+ */
+void drm_client_buffer_vunmap_local(struct drm_client_buffer *buffer)
+{
+	struct drm_gem_object *gem = buffer->gem;
+	struct iosys_map *map = &buffer->map;
+
+	drm_gem_vunmap(gem, map);
+	drm_gem_unlock(gem);
+}
+EXPORT_SYMBOL(drm_client_buffer_vunmap_local);
+
 /**
  * drm_client_buffer_vmap - Map DRM client buffer into address space
  * @buffer: DRM client buffer
@@ -331,14 +391,6 @@ drm_client_buffer_vmap(struct drm_client_buffer *buffer,
 	struct iosys_map *map = &buffer->map;
 	int ret;
 
-	/*
-	 * FIXME: The dependency on GEM here isn't required, we could
-	 * convert the driver handle to a dma-buf instead and use the
-	 * backend-agnostic dma-buf vmap support instead. This would
-	 * require that the handle2fd prime ioctl is reworked to pull the
-	 * fd_install step out of the driver backend hooks, to make that
-	 * final step optional for internal users.
-	 */
 	ret = drm_gem_vmap_unlocked(buffer->gem, map);
 	if (ret)
 		return ret;

drivers/gpu/drm/drm_fbdev_generic.c

@@ -197,14 +197,14 @@ static int drm_fbdev_generic_damage_blit(struct drm_fb_helper *fb_helper,
 	 */
 	mutex_lock(&fb_helper->lock);
 
-	ret = drm_client_buffer_vmap(buffer, &map);
+	ret = drm_client_buffer_vmap_local(buffer, &map);
 	if (ret)
 		goto out;
 
 	dst = map;
 	drm_fbdev_generic_damage_blit_real(fb_helper, clip, &dst);
 
-	drm_client_buffer_vunmap(buffer);
+	drm_client_buffer_vunmap_local(buffer);
 
 out:
 	mutex_unlock(&fb_helper->lock);

drivers/gpu/drm/drm_gem.c

@@ -1227,6 +1227,18 @@ void drm_gem_vunmap(struct drm_gem_object *obj, struct iosys_map *map)
 }
 EXPORT_SYMBOL(drm_gem_vunmap);
 
+void drm_gem_lock(struct drm_gem_object *obj)
+{
+	dma_resv_lock(obj->resv, NULL);
+}
+EXPORT_SYMBOL(drm_gem_lock);
+
+void drm_gem_unlock(struct drm_gem_object *obj)
+{
+	dma_resv_unlock(obj->resv);
+}
+EXPORT_SYMBOL(drm_gem_unlock);
+
 int drm_gem_vmap_unlocked(struct drm_gem_object *obj, struct iosys_map *map)
 {
 	int ret;

include/drm/drm_client.h

@@ -141,6 +141,13 @@ struct drm_client_buffer {
 
 	/**
 	 * @gem: GEM object backing this buffer
+	 *
+	 * FIXME: The dependency on GEM here isn't required, we could
+	 * convert the driver handle to a dma-buf instead and use the
+	 * backend-agnostic dma-buf vmap support instead. This would
+	 * require that the handle2fd prime ioctl is reworked to pull the
+	 * fd_install step out of the driver backend hooks, to make that
+	 * final step optional for internal users.
 	 */
 	struct drm_gem_object *gem;
 
@@ -159,6 +166,9 @@ struct drm_client_buffer *
 drm_client_framebuffer_create(struct drm_client_dev *client, u32 width, u32 height, u32 format);
 void drm_client_framebuffer_delete(struct drm_client_buffer *buffer);
 int drm_client_framebuffer_flush(struct drm_client_buffer *buffer, struct drm_rect *rect);
+int drm_client_buffer_vmap_local(struct drm_client_buffer *buffer,
+				 struct iosys_map *map_copy);
+void drm_client_buffer_vunmap_local(struct drm_client_buffer *buffer);
 int drm_client_buffer_vmap(struct drm_client_buffer *buffer,
 			   struct iosys_map *map);
 void drm_client_buffer_vunmap(struct drm_client_buffer *buffer);

include/drm/drm_gem.h

@@ -527,6 +527,9 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj);
 void drm_gem_put_pages(struct drm_gem_object *obj, struct page **pages,
 		bool dirty, bool accessed);
 
+void drm_gem_lock(struct drm_gem_object *obj);
+void drm_gem_unlock(struct drm_gem_object *obj);
+
 int drm_gem_vmap_unlocked(struct drm_gem_object *obj, struct iosys_map *map);
 void drm_gem_vunmap_unlocked(struct drm_gem_object *obj, struct iosys_map *map);