libcamera: control_serializer: Separate the handles space

Two independent instances of the ControlSerializer class are in use at
the IPC boundaries, one in the Proxy class that serializes data from the
pipeline handler to the IPA, and one in the ProxyWorker which serializes
data in the opposite direction.

Each instance operates autonomously, without any centralized point of
control, and each one assigns a numerical handle to each ControlInfoMap
it serializes. This creates a risk of potential collision on the handle
values, as both instances will use the same numerical space and
are not aware of what handles has been already used by the instance "on
the other side".

To fix that, partition the handles numerical space by initializing the
control serializer with a seed according to the role of the component
that creates the serializer and increment the handle number by 2, to
avoid any collision risk.

While this is temporary and rather hacky solution, it solves an issue
with isolated IPA modules without too much complexity added.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 58 insertions, 5 deletions

diff --git a/src/libcamera/control_serializer.cpp b/src/libcamera/control_serializer.cpp
index e1ba6d14..77b77448 100644
--- a/src/libcamera/control_serializer.cpp
+++ b/src/libcamera/control_serializer.cpp
@@ -62,6 +62,14 @@ LOG_DEFINE_CATEGORY(Serializer)
  * corresponding ControlInfoMap handle in the binary data, and when
  * deserializing to retrieve the corresponding ControlInfoMap.
  *
+ * As independent ControlSerializer instances are used on both sides of the IPC
+ * boundary, and the two instances operate without a shared point of control,
+ * there is a potential risk of collision of the numerical handles assigned to
+ * each serialized ControlInfoMap. For this reason the control serializer is
+ * initialized with a seed and the handle is incremented by 2, so that instances
+ * initialized with a different seed operate on a separate numerical space,
+ * avoiding any collision risk.
+ *
  * In order to perform those tasks, the serializer keeps an internal state that
  * needs to be properly populated. This mechanism requires the ControlInfoMap
  * corresponding to a ControlList to have been serialized or deserialized
@@ -77,9 +85,45 @@ LOG_DEFINE_CATEGORY(Serializer)
  * proceed with care to avoid stale references.
  */
 
-ControlSerializer::ControlSerializer()
-	: serial_(0)
+/**
+ * \enum ControlSerializer::Role
+ * \brief Define the role of the IPC component using the control serializer
+ *
+ * The role of the component that creates the serializer is used to initialize
+ * the handles numerical space.
+ *
+ * \var ControlSerializer::Role::Proxy
+ * \brief The control serializer is used by the IPC Proxy classes
+ *
+ * \var ControlSerializer::Role::Worker
+ * \brief The control serializer is used by the IPC ProxyWorker classes
+ */
+
+/**
+ * \brief Construct a new ControlSerializer
+ * \param[in] role The role of the IPC component using the serializer
+ */
+ControlSerializer::ControlSerializer(Role role)
 {
+	/*
+	 * Initialize the handle numerical space using the role of the
+	 * component that created the instance.
+	 *
+	 * Instances initialized for a different role will use a different
+	 * numerical handle space, avoiding any collision risk when, in example,
+	 * two instances of the ControlSerializer class are used at the IPC
+	 * boundaries.
+	 *
+	 * Start counting handles from '1' as '0' is a special value used as
+	 * place holder when serializing lists that do not have a ControlInfoMap
+	 * associated (in example list of libcamera controls::controls).
+	 *
+	 * \todo This is a temporary hack and should probably be better
+	 * engineered, but for the time being it avoids collisions on the handle
+	 * value when using IPC.
+	 */
+	serialSeed_ = role == Role::Proxy ? 1 : 2;
+	serial_ = serialSeed_;
 }
 
 /**
@@ -90,7 +134,7 @@ ControlSerializer::ControlSerializer()
  */
 void ControlSerializer::reset()
 {
-	serial_ = 0;
+	serial_ = serialSeed_;
 
 	infoMapHandles_.clear();
 	infoMaps_.clear();
@@ -200,10 +244,10 @@ int ControlSerializer::serialize(const ControlInfoMap &infoMap,
 	else
 		idMapType = IPA_CONTROL_ID_MAP_V4L2;
 
-	/* Prepare the packet header, assign a handle to the ControlInfoMap. */
+	/* Prepare the packet header. */
 	struct ipa_controls_header hdr;
 	hdr.version = IPA_CONTROLS_FORMAT_VERSION;
-	hdr.handle = ++serial_;
+	hdr.handle = serial_;
 	hdr.entries = infoMap.size();
 	hdr.size = sizeof(hdr) + entriesSize + valuesSize;
 	hdr.data_offset = sizeof(hdr) + entriesSize;
@@ -212,6 +256,15 @@ int ControlSerializer::serialize(const ControlInfoMap &infoMap,
 	buffer.write(&hdr);
 
 	/*
+	 * Increment the handle for the ControlInfoMap by 2 to keep the handles
+	 * numerical space partitioned between instances initialized for a
+	 * different role.
+	 *
+	 * \sa ControlSerializer::Role
+	 */
+	serial_ += 2;
+
+	/*
 	 * Serialize all entries.
 	 * \todo Serialize the control name too
 	 */