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From afb22c4844a4b841ee8ecadf65ab8244069ef2af Mon Sep 17 00:00:00 2001
From: Stefan Klug <stefan.klug@ideasonboard.com>
Date: Wed, 25 Mar 2026 16:13:55 +0100
Subject: [PATCH] libcamera: internal: Add SequenceSyncHelper class
On a V4L2 buffer the assigned sequence is not known until the buffer is
dequeued. But for per frame controls we have to prepare other data like
sensor controls and ISP params in advance. So we try to anticipate the
sequence number a given buffer will be. In a perfect world this works
well as long as the initial sequence is assigned correctly. But it
breaks as soon as things like running out of buffers or incomplete
images happen. To make things even more complicated, in most cases
more than one buffer is queued to the kernel at a time.
So as soon as a sequence number doesn't match the expected one after
dequeuing, most likely all the already queued buffers will be dequeued
with the same error. It is not sufficient to simply add the correction
after dequeuing because the error on all queued frames would accumulate
and the whole system starts to oscillate. To work around that add a
SequenceSyncHelper class that tracks the expected error and allows to
easily query the necessary correction when queuing new buffers.
Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
---
include/libcamera/internal/meson.build | 1 +
.../libcamera/internal/sequence_sync_helper.h | 76 +++++++++++++++++++
src/libcamera/meson.build | 1 +
src/libcamera/sequence_sync_helper.cpp | 21 +++++
4 files changed, 99 insertions(+)
create mode 100644 include/libcamera/internal/sequence_sync_helper.h
create mode 100644 src/libcamera/sequence_sync_helper.cpp
--- a/include/libcamera/internal/meson.build
+++ b/include/libcamera/internal/meson.build
@@ -40,6 +40,7 @@ libcamera_internal_headers = files([
'process.h',
'pub_key.h',
'request.h',
+ 'sequence_sync_helper.h',
'shared_mem_object.h',
'source_paths.h',
'sysfs.h',
--- /dev/null
+++ b/include/libcamera/internal/sequence_sync_helper.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2025, Ideas on Board
+ *
+ * Sequence sync helper
+ */
+
+#pragma once
+
+#include <queue>
+
+#include <libcamera/base/log.h>
+
+namespace libcamera {
+
+LOG_DECLARE_CATEGORY(SequenceSyncHelper)
+
+class SequenceSyncHelper
+{
+public:
+ int gotFrame(size_t expectedSequence, size_t actualSequence)
+ {
+ ASSERT(!corrections_.empty());
+ int diff = actualSequence - expectedSequence;
+ int corr = corrections_.front();
+ corrections_.pop();
+ expectedOffset_ -= corr;
+ int necessaryCorrection = diff - expectedOffset_;
+ correctionToApply_ += necessaryCorrection;
+
+ LOG(SequenceSyncHelper, Debug) << "Sync frame "
+ << "expected: " << expectedSequence
+ << " actual: " << actualSequence
+ << " correction: " << corr
+ << " expectedOffset: " << expectedOffset_
+ << " correctionToApply " << correctionToApply_;
+
+ expectedOffset_ += necessaryCorrection;
+ return necessaryCorrection;
+ }
+
+ void cancelFrame()
+ {
+ int corr = corrections_.front();
+ corrections_.pop();
+ expectedOffset_ -= corr;
+ }
+
+ /* Value to be added to the source sequence */
+ int correction()
+ {
+ return correctionToApply_;
+ }
+
+ void pushCorrection(int correction)
+ {
+ corrections_.push(correction);
+ correctionToApply_ -= correction;
+ LOG(SequenceSyncHelper, Debug)
+ << "Push correction " << correction
+ << " correctionToApply " << correctionToApply_;
+ }
+
+ void reset()
+ {
+ corrections_ = {};
+ correctionToApply_ = 0;
+ expectedOffset_ = 0;
+ }
+
+ std::queue<int> corrections_;
+ int correctionToApply_ = 0;
+ int expectedOffset_ = 0;
+};
+
+} /* namespace libcamera */
--- a/src/libcamera/meson.build
+++ b/src/libcamera/meson.build
@@ -49,6 +49,7 @@ libcamera_internal_sources = files([
'pipeline_handler.cpp',
'process.cpp',
'pub_key.cpp',
+ 'sequence_sync_helper.cpp',
'shared_mem_object.cpp',
'source_paths.cpp',
'sysfs.cpp',
--- /dev/null
+++ b/src/libcamera/sequence_sync_helper.cpp
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2026, Ideas on Board.
+ *
+ * Helper to easily record debug metadata inside libcamera.
+ */
+
+#include "libcamera/internal/sequence_sync_helper.h"
+
+#include <libcamera/base/log.h>
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(SequenceSyncHelper)
+
+/**
+ * \file debug_controls.h
+ * \brief Helper to synchronize buffer sequences
+ */
+
+} /* namespace libcamera */
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