[libcamera-devel] [PATCH 3/3] cam: kms_sink: Scale the frame buffer to full screen if supported

[Laurent Pinchart]

The KMS sink currently displays the frame buffer on the top-left corner
of the screen, resulting in either a black area on the bottom and right
sides (if the frame buffer is smaller than the display resolution) of in
a restricted field of view (if the frame buffer is larger than the
display resolution). Improve this by scaling the frame buffer to full
screen if supported, and aligning the crop rectangle to the frame buffer
center if the field of view needs to be restricted.

The implementation test three possible composition options, from best to
worst. The tests are performed when the camera is started, as testing
atomic commits requires access to frame buffer objects, which are not
available at configure time. Changing this would require either a large
refactoring of the cam application to provide frame buffers earlier, or
extending the KMS API to support testing commits with dummy buffer
objects. Both are candidates for later development.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 src/cam/kms_sink.cpp | 92 ++++++++++++++++++++++++++++++++++++++++----
 src/cam/kms_sink.h   |  8 ++++
 2 files changed, 92 insertions(+), 8 deletions(-)

diff --git a/src/cam/kms_sink.cpp b/src/cam/kms_sink.cpp
index 16435ede6b6a..2f306955cf51 100644
--- a/src/cam/kms_sink.cpp
+++ b/src/cam/kms_sink.cpp
@@ -284,6 +284,77 @@ int KMSSink::stop()
 	return FrameSink::stop();
 }
 
+bool KMSSink::testModeSet(DRM::FrameBuffer *drmBuffer,
+			  const libcamera::Rectangle &src,
+			  const libcamera::Rectangle &dst)
+{
+	DRM::AtomicRequest drmRequest{ &dev_ };
+
+	drmRequest.addProperty(connector_, "CRTC_ID", crtc_->id());
+
+	drmRequest.addProperty(crtc_, "ACTIVE", 1);
+	drmRequest.addProperty(crtc_, "MODE_ID", mode_->toBlob(&dev_));
+
+	drmRequest.addProperty(plane_, "CRTC_ID", crtc_->id());
+	drmRequest.addProperty(plane_, "FB_ID", drmBuffer->id());
+	drmRequest.addProperty(plane_, "SRC_X", src.x << 16);
+	drmRequest.addProperty(plane_, "SRC_Y", src.y << 16);
+	drmRequest.addProperty(plane_, "SRC_W", src.width << 16);
+	drmRequest.addProperty(plane_, "SRC_H", src.height << 16);
+	drmRequest.addProperty(plane_, "CRTC_X", dst.x);
+	drmRequest.addProperty(plane_, "CRTC_Y", dst.y);
+	drmRequest.addProperty(plane_, "CRTC_W", dst.width);
+	drmRequest.addProperty(plane_, "CRTC_H", dst.height);
+
+	return !drmRequest.commit(DRM::AtomicRequest::FlagAllowModeset |
+				  DRM::AtomicRequest::FlagTestOnly);
+}
+
+bool KMSSink::setupComposition(DRM::FrameBuffer *drmBuffer)
+{
+	/*
+	 * Test composition options, from most to least desirable, to select the
+	 * best one.
+	 */
+	const libcamera::Rectangle framebuffer{ size_ };
+	const libcamera::Rectangle display{ 0, 0, mode_->hdisplay, mode_->vdisplay };
+
+	/* 1. Scale the frame buffer to full screen. */
+	libcamera::Rectangle src = libcamera::Rectangle{ size_ };
+	libcamera::Rectangle dst = display;
+
+	if (testModeSet(drmBuffer, src, dst)) {
+		std::cout << "KMS: full-screen scaled output" << std::endl;
+		src_ = src;
+		dst_ = dst;
+		return true;
+	}
+
+	/* 2. Center the frame buffer on the display. */
+	src = display.size().centeredTo(framebuffer.center()).boundedTo(framebuffer);
+	dst = framebuffer.size().centeredTo(display.center()).boundedTo(display);
+
+	if (testModeSet(drmBuffer, src, dst)) {
+		std::cout << "KMS: centered output" << std::endl;
+		src_ = src;
+		dst_ = dst;
+		return true;
+	}
+
+	/* 3. Align the frame buffer on the top-left of the display. */
+	src = framebuffer.boundedTo(display);
+	dst = display.boundedTo(framebuffer);
+
+	if (testModeSet(drmBuffer, src, dst)) {
+		std::cout << "KMS: top-left aligned output" << std::endl;
+		src_ = src;
+		dst_ = dst;
+		return true;
+	}
+
+	return false;
+}
+
 bool KMSSink::processRequest(libcamera::Request *camRequest)
 {
 	/*
@@ -307,20 +378,25 @@ bool KMSSink::processRequest(libcamera::Request *camRequest)
 
 	if (!active_ && !queued_) {
 		/* Enable the display pipeline on the first frame. */
+		if (!setupComposition(drmBuffer)) {
+			std::cerr << "Failed to setup composition" << std::endl;
+			return true;
+		}
+
 		drmRequest->addProperty(connector_, "CRTC_ID", crtc_->id());
 
 		drmRequest->addProperty(crtc_, "ACTIVE", 1);
 		drmRequest->addProperty(crtc_, "MODE_ID", mode_->toBlob(&dev_));
 
 		drmRequest->addProperty(plane_, "CRTC_ID", crtc_->id());
-		drmRequest->addProperty(plane_, "SRC_X", 0 << 16);
-		drmRequest->addProperty(plane_, "SRC_Y", 0 << 16);
-		drmRequest->addProperty(plane_, "SRC_W", size_.width << 16);
-		drmRequest->addProperty(plane_, "SRC_H", size_.height << 16);
-		drmRequest->addProperty(plane_, "CRTC_X", 0);
-		drmRequest->addProperty(plane_, "CRTC_Y", 0);
-		drmRequest->addProperty(plane_, "CRTC_W", size_.width);
-		drmRequest->addProperty(plane_, "CRTC_H", size_.height);
+		drmRequest->addProperty(plane_, "SRC_X", src_.x << 16);
+		drmRequest->addProperty(plane_, "SRC_Y", src_.y << 16);
+		drmRequest->addProperty(plane_, "SRC_W", src_.width << 16);
+		drmRequest->addProperty(plane_, "SRC_H", src_.height << 16);
+		drmRequest->addProperty(plane_, "CRTC_X", dst_.x);
+		drmRequest->addProperty(plane_, "CRTC_Y", dst_.y);
+		drmRequest->addProperty(plane_, "CRTC_W", dst_.width);
+		drmRequest->addProperty(plane_, "CRTC_H", dst_.height);
 
 		flags |= DRM::AtomicRequest::FlagAllowModeset;
 	}
diff --git a/src/cam/kms_sink.h b/src/cam/kms_sink.h
index 8f5f08667cea..76c4e611bf85 100644
--- a/src/cam/kms_sink.h
+++ b/src/cam/kms_sink.h
@@ -50,6 +50,11 @@ private:
 
 	int selectPipeline(const libcamera::PixelFormat &format);
 	int configurePipeline(const libcamera::PixelFormat &format);
+	bool testModeSet(DRM::FrameBuffer *drmBuffer,
+			 const libcamera::Rectangle &src,
+			 const libcamera::Rectangle &dst);
+	bool setupComposition(DRM::FrameBuffer *drmBuffer);
+
 	void requestComplete(DRM::AtomicRequest *request);
 
 	DRM::Device dev_;
@@ -63,6 +68,9 @@ private:
 	libcamera::Size size_;
 	unsigned int stride_;
 
+	libcamera::Rectangle src_;
+	libcamera::Rectangle dst_;
+
 	std::map<libcamera::FrameBuffer *, std::unique_ptr<DRM::FrameBuffer>> buffers_;
 
 	std::mutex lock_;
-- 
Regards,

Laurent Pinchart