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diff --git a/src/ipa/libipa/fc_queue.cpp b/src/ipa/libipa/fc_queue.cpp
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+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2022, Google Inc.
+ *
+ * fc_queue.cpp - IPA Frame context queue
+ */
+
+#include "fc_queue.h"
+
+#include <libcamera/base/log.h>
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(FCQueue)
+
+namespace ipa {
+
+/**
+ * \file fc_queue.h
+ * \brief Queue of per-frame contexts
+ */
+
+/**
+ * \class FCQueue
+ * \brief A support class for managing FrameContext instances in IPA modules
+ * \tparam FrameContext The IPA module-specific FrameContext derived class type
+ *
+ * Along with the Module and Algorithm classes, the frame context queue is a
+ * core component of the libipa infrastructure. It stores per-frame contexts
+ * used by the Algorithm operations. By centralizing the lifetime management of
+ * the contexts and implementing safeguards against underflows and overflows, it
+ * simplifies IPA modules and improves their reliability.
+ *
+ * The queue references frame contexts by a monotonically increasing sequence
+ * number. The FCQueue design assumes that this number matches both the sequence
+ * number of the corresponding frame, as generated by the camera sensor, and the
+ * sequence number of the request. This allows IPA modules to obtain the frame
+ * context from any location where a request or a frame is available.
+ *
+ * A frame context normally begins its lifetime when the corresponding request
+ * is queued, way before the frame is captured by the camera sensor. IPA modules
+ * allocate the context from the queue at that point, calling alloc() using the
+ * request number. The queue initializes the context, and the IPA module then
+ * populates it with data from the request. The context can be later retrieved
+ * with a call to get(), typically when the IPA module is requested to provide
+ * sensor or ISP parameters or receives statistics for a frame. The frame number
+ * is used at that point to identify the context.
+ *
+ * If an application fails to queue requests to the camera fast enough, frames
+ * may be produced by the camera sensor and processed by the IPA module without
+ * a corresponding request having been queued to the IPA module. This creates an
+ * underrun condition, where the IPA module will try to get a frame context that
+ * hasn't been allocated. In this case, the get() function will allocate and
+ * initialize a context for the frame, and log a message. Algorithms will not
+ * apply the controls associated with the late request, but should otherwise
+ * behave correctly.
+ *
+ * \todo Mark the frame context with a per-frame control error flag in case of
+ * underrun, and research how algorithms should handle this.
+ *
+ * At its core, the queue uses a circular buffer to avoid dynamic memory
+ * allocation at runtime. The buffer is pre-allocated with a maximum number of
+ * entries when the FCQueue instance is constructed. Entries are initialized on
+ * first use by alloc() or, in underrun conditions, get(). The queue is not
+ * allowed to overflow, which must be ensured by pipeline handlers never
+ * queuing more in-flight requests to the IPA module than the queue size. If an
+ * overflow condition is detected, the queue will log a fatal error.
+ */
+
+/**
+ * \fn FCQueue::FCQueue(unsigned int size)
+ * \brief Construct a frame contexts queue of a specified size
+ * \param[in] size The number of contexts in the queue
+ */
+
+/**
+ * \fn FCQueue::clear()
+ * \brief Clear the contexts queue
+ *
+ * IPA modules must clear the frame context queue at the beginning of a new
+ * streaming session, in IPAModule::start().
+ *
+ * \todo Fix any issue this may cause with requests queued before the camera is
+ * started.
+ */
+
+/**
+ * \fn FCQueue::alloc(uint32_t frame)
+ * \brief Allocate and return a FrameContext for the \a frame
+ * \param[in] frame The frame context sequence number
+ *
+ * The first call to obtain a FrameContext from the FCQueue should be handled
+ * through this function. The FrameContext will be initialised, if not
+ * initialised already, and returned to the caller.
+ *
+ * If the FrameContext was already initialized for this \a frame, a warning will
+ * be reported and the previously initialized FrameContext is returned.
+ *
+ * Frame contexts are expected to be initialised when a Request is first passed
+ * to the IPA module in IPAModule::queueRequest().
+ *
+ * \return A reference to the FrameContext for sequence \a frame
+ */
+
+/**
+ * \fn FCQueue::get(uint32_t frame)
+ * \brief Obtain the FrameContext for the \a frame
+ * \param[in] frame The frame context sequence number
+ *
+ * If the FrameContext is not correctly initialised for the \a frame, it will be
+ * initialised.
+ *
+ * \return A reference to the FrameContext for sequence \a frame
+ */
+
+} /* namespace ipa */
+
+} /* namespace libcamera */