Message-Id: <20250206-hotplug-drm-bridge-v6-18-9d6f2c9c3058@bootlin.com>
Date: Thu, 06 Feb 2025 19:14:33 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Simona Vetter <simona@...ll.ch>, Inki Dae <inki.dae@...sung.com>, 
 Jagan Teki <jagan@...rulasolutions.com>, 
 Marek Szyprowski <m.szyprowski@...sung.com>, 
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Cc: Paul Kocialkowski <contact@...lk.fr>, 
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 linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
 Paul Kocialkowski <paul.kocialkowski@...tlin.com>, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH v6 18/26] drm/bridge: add documentation of refcounted
 bridges

Document in detail the new refcounted bridges as well as the "legacy"
bridges.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

---

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          | 118 ++++++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 79c8d3e63f7e06136440ed38972697b5f057d5d1..027c6ab65aa5c3848c4afab6fbc8ab93f9a285ba 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 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 92ce40adfaa59a278a972ac862bebee06970ff83..fc44a5d227a89a12b5c3299a29776cfddb36ce27 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -62,6 +62,124 @@
  * encoder chain.
  */
 
+/**
+ * DOC: bridge lifecycle
+ *
+ * Allocation, initialization and teardown of a bridge can be implemented
+ * in one of two ways: *refcounted* mode and *legacy* mode.
+ *
+ * In **refcounted** mode:
+ *
+ * - each &struct drm_bridge is reference counted since its allocation
+ * - any code taking a pointer to a bridge has get and put APIs to refcount
+ *   it and so ensure 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 it
+ * - deallocation is done when the last put happens, dropping the refcount
+ *   to zero
+ *
+ * A bridge using refcounted mode is called a *refcounted bridge*.
+ *
+ * In **legacy** mode the &struct drm_bridge lifetime is tied to the device
+ * instantiating it: it is allocated on probe and freed on removal. Any
+ * other kernel code holding a pointer to the bridge could incur in
+ * use-after-free in case the bridge is deallocated at runtime.
+ *
+ * Legacy mode used to be the only one until refcounted bridges were
+ * introduced, hence the name. It is still fine in case the bridges are a
+ * fixed part of the pipeline, i.e. if the bridges are removed only when
+ * tearing down the entire card. Refcounted bridges support both that case
+ * and the case of more dynamic hardware with bridges that can be removed
+ * at runtime without tearing down the entire card.
+ *
+ * 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**, in both refcounted and legacy modes the common
+ * and expected pattern is that the bridge driver declares a
+ * driver-specific struct embedding a &struct drm_bridge. E.g.::
+ *
+ *   struct my_bridge {
+ *       ...
+ *       struct drm_bridge bridge;
+ *       ...
+ *   };
+ *
+ * When using refcounted mode, the driver should 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();
+ *         ...
+ *     }
+ *
+ *     static void my_bridge_remove()
+ *     {
+ *         struct my_bridge *mybr = ...;
+ *
+ *         drm_bridge_remove(&mybr->bridge);
+ *         // Free resources
+ *         // ... NO kfree here!
+ *     }
+ *
+ * In legacy mode, the driver can either use ``devm_`` allocation or
+ * equivalently free ``struct my_bridge`` in their remove function::
+ *
+ *     static int my_bridge_probe(...)
+ *     {
+ *         struct device *dev = ...;
+ *         struct my_bridge *mybr;
+ *
+ *         mybr = devm_kzalloc(dev, sizeof(*mybr), GFP_KERNEL);
+ *         if (!mybr)
+ *             return -ENOMEM;
+ *
+ *         // Get resources, initialize my_bridge members...
+ *         mybr->funcs = &my_bridge_funcs;
+ *         drm_bridge_add();
+ *         ...
+ *     }
+ *
+ *     static void my_bridge_remove()
+ *     {
+ *         struct my_bridge *mybr = ...;
+ *
+ *         drm_bridge_remove(&mybr->bridge);
+ *         // Free resources
+ *         // kfree(mybr) if not using devm_*() for allocation
+ *     }
+ *
+ * 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 in this case the consumers do not have to
+ * do it.
+ *
+ * Calling drm_bridge_get() and drm_bridge_put() on a bridge that is not
+ * refcounted does nothing, so code using these two APIs will work both on
+ * refcounted bridges and legacy bridges.
+ */
+
 /**
  * DOC:	display driver integration
  *

-- 
2.34.1

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