From 9ce56146fbe483e09d7ccb98e3b4189c5dc41929 Mon Sep 17 00:00:00 2001
From: Daniel Golle <daniel@makrotopia.org>
Date: Mon, 19 Oct 2026 11:57:12 +0100
Subject: [PATCH] gstreamer: Negotiate downstream buffer strides

V4L2 encoders advertise row padding through allocation metadata.
Configure the camera with that stride before allocating capture buffers
so hardware consumers can import them directly. Reject padding layouts
that the camera cannot represent.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
 src/gstreamer/gstlibcamerasrc.cpp | 57 +++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

--- a/src/gstreamer/gstlibcamerasrc.cpp
+++ b/src/gstreamer/gstlibcamerasrc.cpp
@@ -32,6 +32,8 @@
 #include <libcamera/control_ids.h>
 
 #include <gst/base/base.h>
+#include <gst/video/gstvideometa.h>
+#include <gst/video/gstvideopool.h>
 
 #include "gstlibcamera-controls.h"
 #include "gstlibcamera-utils.h"
@@ -585,6 +587,31 @@ gst_libcamera_create_video_pool(GstLibca
 	return { std::exchange(pool, nullptr), 0 };
 }
 
+static int
+gst_libcamera_requested_stride(GstPad *srcpad)
+{
+	g_autoptr(GstCaps) caps = gst_pad_get_current_caps(srcpad);
+	g_autoptr(GstQuery) query = gst_query_new_allocation(caps, true);
+	const GstStructure *params = nullptr;
+	GstVideoAlignment alignment;
+	GstVideoInfo info;
+	guint index;
+
+	if (!gst_pad_peer_query(srcpad, query) ||
+	    !gst_query_find_allocation_meta(query, GST_VIDEO_META_API_TYPE, &index))
+		return 0;
+
+	gst_query_parse_nth_allocation_meta(query, index, &params);
+	if (!params || !gst_buffer_pool_config_get_video_alignment(params, &alignment))
+		return 0;
+	if (alignment.padding_top || alignment.padding_bottom || alignment.padding_left)
+		return -EINVAL;
+	if (!gst_video_info_from_caps(&info, caps) || !gst_video_info_align(&info, &alignment))
+		return -EINVAL;
+
+	return info.stride[0];
+}
+
 /* Must be called with stream_lock held. */
 static bool
 gst_libcamera_src_negotiate(GstLibcameraSrc *self)
@@ -651,6 +678,36 @@ gst_libcamera_src_negotiate(GstLibcamera
 			return false;
 	}
 
+	std::vector<unsigned int> strides(state->srcpads_.size());
+	bool strideChanged = false;
+	for (gsize i = 0; i < state->srcpads_.size(); i++) {
+		StreamConfiguration &cfg = state->config_->at(i);
+		int stride = gst_libcamera_requested_stride(state->srcpads_[i]);
+
+		if (stride < 0) {
+			GST_ELEMENT_ERROR(self, RESOURCE, SETTINGS,
+					  ("Unsupported downstream buffer padding"), (nullptr));
+			return false;
+		}
+		strides[i] = stride;
+		if (strides[i] > cfg.stride) {
+			cfg.stride = strides[i];
+			strideChanged = true;
+		}
+	}
+	if (strideChanged && state->cam_->configure(state->config_.get())) {
+		GST_ELEMENT_ERROR(self, RESOURCE, SETTINGS,
+				  ("Failed to configure downstream buffer strides"), (nullptr));
+		return false;
+	}
+	for (gsize i = 0; i < strides.size(); i++) {
+		if (state->config_->at(i).stride < strides[i]) {
+			GST_ELEMENT_ERROR(self, RESOURCE, SETTINGS,
+					  ("Camera cannot provide the requested buffer stride"), (nullptr));
+			return false;
+		}
+	}
+
 	if (self->allocator)
 		g_clear_object(&self->allocator);