[libcamera-devel] [RFC PATCH v1] v4l2: Add support for PREPARE_BUF as one of the supported ioctl

[Vedant Paranjape]

Add support for PREPARE_BUF as one of the ioctl. This needs adding the
prepare_buf function to the v4l2_camera class as this operation involves
passing on the ownership of buffers to the driver.

Signed-off-by: Vedant Paranjape <vedantparanjape160201 at gmail.com>
---
I have not cargo copied the error checking or the function, I actually
went to kernel source and checked what parameters are being checked to
give out a error and checked those in my definition as well.

https://elixir.bootlin.com/linux/latest/source/drivers/media/common/videobuf2/videobuf2-v4l2.c#L729

Please review the v4l2_camera_proxy part, if the function looks good
enough hypothetically assuming vcam_->prepare_buf() has a correct
implementation.

As for the part in v4l2_camera, prepare_buf. Seeing the pattern of code,
any operation that needs exchanging buffers with the drivers needs to
have a function to be implemented inside v4l2_camera just like qbuf is.
Let me know if my assumption is wrong, and this change is absolutely not
needed. 

I am unable to figure out what is the equivalent of transfering ownership 
of the buffer in the libcamera compat world. Just like queueing a buffer
has an equivalent, camera_->queueRequest(request) (not comparing apples
to apples, but to some extent).
---
src/v4l2/v4l2_camera.cpp       | 10 ++++++++++
 src/v4l2/v4l2_camera.h         |  1 +
 src/v4l2/v4l2_camera_proxy.cpp | 32 ++++++++++++++++++++++++++++++++
 src/v4l2/v4l2_camera_proxy.h   |  1 +
 4 files changed, 44 insertions(+)

diff --git a/src/v4l2/v4l2_camera.cpp b/src/v4l2/v4l2_camera.cpp
index e922b9e6aab2..2c155a63f836 100644
--- a/src/v4l2/v4l2_camera.cpp
+++ b/src/v4l2/v4l2_camera.cpp
@@ -275,6 +275,16 @@ int V4L2Camera::qbuf(unsigned int index)
 	return 0;
 }
 
+int V4L2Camera::prepare_buf(unsigned int index, unsigned int memory)
+{
+	// just a stub for now
+	// referring to
+	// https://elixir.bootlin.com/linux/latest/source/drivers/media/common/videobuf2/videobuf2-v4l2.c#L729
+	// https://elixir.bootlin.com/linux/latest/source/drivers/media/common/videobuf2/videobuf2-core.c#L1341
+
+	return 0;
+}
+
 void V4L2Camera::waitForBufferAvailable()
 {
 	MutexLocker locker(bufferMutex_);
diff --git a/src/v4l2/v4l2_camera.h b/src/v4l2/v4l2_camera.h
index 03e741180e8f..b4e4b65ed701 100644
--- a/src/v4l2/v4l2_camera.h
+++ b/src/v4l2/v4l2_camera.h
@@ -57,6 +57,7 @@ public:
 	int streamOff();
 
 	int qbuf(unsigned int index);
+	int prepare_buf(unsigned int index, unsigned int memory);
 
 	void waitForBufferAvailable();
 	bool isBufferAvailable();
diff --git a/src/v4l2/v4l2_camera_proxy.cpp b/src/v4l2/v4l2_camera_proxy.cpp
index 4d529bc29a4d..ae0b0691ca4e 100644
--- a/src/v4l2/v4l2_camera_proxy.cpp
+++ b/src/v4l2/v4l2_camera_proxy.cpp
@@ -544,6 +544,37 @@ int V4L2CameraProxy::vidioc_querybuf(V4L2CameraFile *file, struct v4l2_buffer *a
 	return 0;
 }
 
+int V4L2CameraProxy::vidioc_prepare_buf(V4L2CameraFile *file, struct v4l2_buffer *arg)
+{
+	LOG(V4L2Compat, Debug) << "Servicing vidioc_prepare_buf, index = "
+			       << arg->index << " fd = " << file->efd();
+
+	if (arg->index >= bufferCount_)
+		return -EINVAL;
+
+	if (arg->flags & V4L2_BUF_FLAG_REQUEST_FD ||
+	    arg->flags & V4L2_BUF_FLAG_PREPARED)
+		return -EINVAL;
+
+	if (!hasOwnership(file))
+		return -EBUSY;
+
+	if (!validateBufferType(arg->type) ||
+	    !validateMemoryType(arg->memory))
+		return -EINVAL;
+
+	/* \todo when DMABUF memory type is supported, add support here as well */
+	int ret = vcam_->prepare_buf(arg->index, arg->memory);
+	if (ret < 0)
+		return ret;
+
+	buffers_[arg->index].flags |= V4L2_BUF_FLAG_PREPARED;
+
+	arg->flags = buffers_[arg->index].flags;
+
+	return ret;
+}
+
 int V4L2CameraProxy::vidioc_qbuf(V4L2CameraFile *file, struct v4l2_buffer *arg)
 {
 	LOG(V4L2Compat, Debug) << "Servicing vidioc_qbuf, index = "
@@ -709,6 +740,7 @@ const std::set<unsigned long> V4L2CameraProxy::supportedIoctls_ = {
 	VIDIOC_S_INPUT,
 	VIDIOC_REQBUFS,
 	VIDIOC_QUERYBUF,
+	VIDIOC_PREPARE_BUF,
 	VIDIOC_QBUF,
 	VIDIOC_DQBUF,
 	VIDIOC_EXPBUF,
diff --git a/src/v4l2/v4l2_camera_proxy.h b/src/v4l2/v4l2_camera_proxy.h
index 14e027c3e7d1..6baba94262a9 100644
--- a/src/v4l2/v4l2_camera_proxy.h
+++ b/src/v4l2/v4l2_camera_proxy.h
@@ -57,6 +57,7 @@ private:
 	int vidioc_s_input(V4L2CameraFile *file, int *arg);
 	int vidioc_reqbufs(V4L2CameraFile *file, struct v4l2_requestbuffers *arg);
 	int vidioc_querybuf(V4L2CameraFile *file, struct v4l2_buffer *arg);
+	int vidioc_prepare_buf(V4L2CameraFile *file, struct v4l2_buffer *arg);
 	int vidioc_qbuf(V4L2CameraFile *file, struct v4l2_buffer *arg);
 	int vidioc_dqbuf(V4L2CameraFile *file, struct v4l2_buffer *arg,
 			 libcamera::Mutex *lock) LIBCAMERA_TSA_REQUIRES(*lock);
-- 
2.25.1