Re: [10/13] drm/fbdev-generic: Fix locking with drm_client_buffer_vmap_local()

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Hi,


On 2024/2/27 18:14, Thomas Zimmermann wrote:
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@xxxxxxx>
---
  drivers/gpu/drm/drm_client.c        | 68 +++++++++++++++++++++++++----
  drivers/gpu/drm/drm_fbdev_generic.c |  4 +-
  drivers/gpu/drm/drm_gem.c           | 12 +++++
  include/drm/drm_client.h            | 10 +++++
  include/drm/drm_gem.h               |  3 ++
  5 files changed, 87 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_client.c b/drivers/gpu/drm/drm_client.c
index 9403b3f576f7b..2cc81831236b5 100644
--- a/drivers/gpu/drm/drm_client.c
+++ b/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;
diff --git a/drivers/gpu/drm/drm_fbdev_generic.c b/drivers/gpu/drm/drm_fbdev_generic.c
index d647d89764cb9..be357f926faec 100644
--- a/drivers/gpu/drm/drm_fbdev_generic.c
+++ b/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;

Then, please remove the local variable 'dst' (struct iosys_map) at here.
As you said, the returned iosys_map is another copy of the original backup,
we can play with this local variable at will, there no need to duplicate
another time again.

I have modified and tested with fbdev generic, no problem. With this trivial
issue resolved. For fbdev-generic:


Acked-by: Sui Jingfeng <sui.jingfeng@xxxxxxxxx>





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