libcamera: v4l2_videodevice: Document plane handling in createBuffer()

The V4L2VideoDevice::createBuffer() calculates offsets manually when
using a multi-planar pixel format and a single-planar V4L2 format. The
process isn't trivial, document it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
index 1483181a..88535f5a 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -1334,10 +1334,19 @@ std::unique_ptr<FrameBuffer> V4L2VideoDevice::createBuffer(unsigned int index)
 	}
 
 	/*
+	 * If we have a multi-planar format with a V4L2 single-planar buffer,
+	 * split the single V4L2 plane into multiple FrameBuffer planes by
+	 * computing the offsets manually.
+	 *
 	 * The format info is not guaranteed to be valid, as there are no
 	 * PixelFormatInfo for metadata formats, so check it first.
 	 */
 	if (formatInfo_->isValid() && formatInfo_->numPlanes() != numPlanes) {
+		/*
+		 * There's no valid situation where the number of colour planes
+		 * differs from the number of V4L2 planes and the V4L2 buffer
+		 * has more than one plane.
+		 */
 		ASSERT(numPlanes == 1u);
 
 		planes.resize(formatInfo_->numPlanes());