Document in detail the DRM bridge allocation and refcounting process based
on the recently introduced devm_drm_bridge_alloc().
Signed-off-by: Luca Ceresoli <luca.ceres...@bootlin.com>
---
Changes in v7:
- remove mention of "legacy mode", we now support only refcounted
bridges
- rename patch title from "drm/bridge: add documentation of refcounted
bridges", we now support only refcounted bridges
Changes in v6:
- update to the new devm_drm_bridge_alloc() API
- rewrite and improve various sentences for clarity
- fix typos (Randy Dunlap)
This patch was added in v5.
---
Documentation/gpu/drm-kms-helpers.rst | 6 +++
drivers/gpu/drm/drm_bridge.c | 73 +++++++++++++++++++++++++++++++++++
2 files changed, 79 insertions(+)
diff --git a/Documentation/gpu/drm-kms-helpers.rst
b/Documentation/gpu/drm-kms-helpers.rst
index
5139705089f200b189876a5a61bf2a935cec433a..393cd0e4cb5af3fe98674e7a96c853ffb2556c97
100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -151,6 +151,12 @@ Overview
.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
:doc: overview
+Bridge allocation and lifecycle
+-------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
+ :doc: bridge lifecycle
+
Display Driver Integration
--------------------------
diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index
b4c89ec01998b849018ce031c7cd84614e65e710..b7e1ad761dad52bdb2ec09d425e69ee23a18fd36
100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -61,6 +61,79 @@
* encoder chain.
*/
+/**
+ * DOC: bridge lifecycle
+ *
+ * In some use cases such as hot-plugging a DRM bridge device can
+ * physically disappear and reappear at runtime. To handle such cases
+ * without destroying and recreating the entire DRM pipeline, DRM bridge
+ * lifetime is managed using reference counting:
+ *
+ * - each &struct drm_bridge is reference counted since its allocation
+ * - any code taking a pointer to a bridge has APIs to get a reference and
+ * put it when done, to ensure the memory allocated for the bridge won't
+ * be deallocated while there is still a reference to it
+ * - the driver implementing the bridge also holds a reference, but the
+ * allocated struct can survive the driver in case other references still
+ * exist
+ * - deallocation is done when the last put happens, dropping the refcount
+ * to zero
+ *
+ * Usage of refcounted bridges happens in two sides: the bridge *provider*
+ * and the bridge *consumers*. The bridge provider is the driver
+ * implementing the bridge. The bridge consumers are all parts of the
+ * kernel taking a &struct drm_bridge pointer, including other bridges,
+ * encoders and the DRM core.
+ *
+ * For bridge **providers**, the bridge driver declares a driver-specific
+ * struct embedding a &struct drm_bridge. E.g.::
+ *
+ * struct my_bridge {
+ * ...
+ * struct drm_bridge bridge;
+ * ...
+ * };
+ *
+ * The driver must allocate and initialize ``struct my_bridge`` using
+ * devm_drm_bridge_alloc(), as in this example::
+ *
+ * static int my_bridge_probe(...)
+ * {
+ * struct device *dev = ...;
+ * struct my_bridge *mybr;
+ *
+ * mybr = devm_drm_bridge_alloc(dev, struct my_bridge, bridge,
&my_bridge_funcs);
+ * if (IS_ERR(mybr))
+ * return PTR_ERR(mybr);
+ *
+ * // Get resources, initialize my_bridge members...
+ * drm_bridge_add(&mybr->bridge);
+ * ...
+ * }
+ *
+ * static void my_bridge_remove(...)
+ * {
+ * struct my_bridge *mybr = ...;
+ *
+ * drm_bridge_remove(&mybr->bridge);
+ * // Free resources
+ * // ... NO kfree here!
+ * }
+ *
+ * Bridge **consumers** need to handle the case of a bridge being removed
+ * while they have a pointer to it. As this can happen at any time, such
+ * code can incur in use-after-free. To avoid that, consumers have to call
+ * drm_bridge_get() when taking a pointer and drm_bridge_put() after they
+ * are done using it. This will extend the allocation lifetime of the
+ * bridge struct until the last reference has been put, potentially a long
+ * time after the bridge device has been removed from the kernel.
+ *
+ * Functions that return a pointer to a bridge, such as
+ * of_drm_find_bridge(), internally call drm_bridge_get() on the bridge
+ * they are about to return, so users using such functions to get a bridge
+ * pointer only have to take care of calling drm_bridge_put().
+ */
+
/**
* DOC: display driver integration
*
--
2.49.0
