Re: [PATCH v2 0/3] DCMI bridge support

Hi Sakari,

On Fri, Jul 05, 2019 at 10:55:22AM +0300, Sakari Ailus wrote:

On Thu, Jun 27, 2019 at 04:38:24PM +0300, Laurent Pinchart wrote:

quoted

On Thu, Jun 27, 2019 at 12:38:40PM +0000, Hugues FRUCHET wrote:

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On 6/26/19 7:25 PM, Laurent Pinchart wrote:

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On Mon, Jun 24, 2019 at 10:10:05AM +0000, Hugues FRUCHET wrote:

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Hi Sakari,

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- Where's the sub-device representing the bridge itself?

This is pointed by [1]: drivers/media/i2c/st-mipid02.c

quoted

- As the driver becomes MC-centric, crop configuration takes place through
  V4L2 sub-device interface, not through the video device node.
- Same goes for accessing sensor configuration: it does not take place
  through video node but through the sub-device nodes.

Our objective is to be able to support either a simple parallel sensor
or a CSI-2 sensor connected through a bridge without any changes on
userspace side because no additional processing or conversion involved,
only deserialisation is m.
With the proposed set of patches, we succeeded to do so, the same
non-regression tests campaign is passed with OV5640 parallel sensor
(STM32MP1 evaluation board) or OV5640 CSI-2 sensor (Avenger96 board with
D3 mezzanine board).

We don't want driver to be MC-centric, media controller support was
required only to get access to the set of functions needed to link and
walk trough subdevices: media_create_pad_link(),
media_entity_remote_pad(), etc...

We did a try with the v1 version of this patchset, delegating subdevices
handling to userspace, by using media-controller, but this require to
configure first the pipeline for each single change of resolution and
format before making any capture using v4l2-ctl or GStreamer, quite
heavy in fact.
Benjamin did another try using new libcamera codebase, but even for a
basic capture use-case, negotiation code is quite tricky in order to
match the right subdevices bus format to the required V4L2 format.

Why would it be trickier in userspace than in the kernel ? The V4L2
subdev operations are more or less expose verbatim through the subdev
userspace API.

quoted

Moreover, it was not clear how to call libcamera library prior to any
v4l2-ctl or GStreamer calls.

libcamera isn't meant to be called before v4l2-ctl or GStreamer.
Applications are supposed to be based directly on libcamera, or, for
existing userspace APIs such as V4L2 or GStreamer, compatibility layers
are supposed to be developed. For V4L2 it will take the form of a
LD_PRELOAD-able .so that will intercept the V4L2 API calls, making most
V4L2 applications work with libcamera unmodified (I said most as 100%
compatibility will likely not be achievable). For GStreamer it will take
the form of a GStreamer libcamera element that will replace the V4L2
source element.

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Adding 100 lines of code into DCMI to well configure resolution and
formats fixes the point and allows us to keep backward compatibility
as per our objective, so it seems far more reasonable to us to do so
even if DCMI controls more than the subdevice it is connected to.
Moreover we found similar code in other video interfaces code like
qcom/camss/camss.c and xilinx/xilinx-dma.c, controlling the whole
pipeline, so it seems to us quite natural to go this way.

I can't comment on the qcom-camss driver as I'm not aware of its
internals, but where have you found such code in the Xilinx V4L2 drivers
?

For ex. in xilinx/xilinx-dma.c, stream on/off is propagated to all 
subdevices within pipeline:
  * Walk the entities chain starting at the pipeline output video node 
static int xvip_pipeline_start_stop(struct xvip_pipeline *pipe, bool start)

Same for qcom/camss/camss-video.c:
static int video_start_streaming(struct vb2_queue *q, unsigned int count)

For stream start/stop, that's expected. Userspace only controls the
stream start/stop on the video node, and the kernel propagates that
along the pipeline. There is no VIDIOC_STREAMON or VIDIOC_STREAMOFF
ioctl exposed to userspace for V4L2 subdevs. What is not propagated in
the kernel for MC-centric devices is the pipeline configuration (formats
and selection rectangles).

quoted

For resolution/format, in exynos4-is/fimc-capture.c:
static int fimc_pipeline_try_format(struct fimc_ctx *ctx,
...
	while (1) {
...
		/* set format on all pipeline subdevs */
		while (me != &fimc->vid_cap.subdev.entity) {
...
			ret = v4l2_subdev_call(sd, pad, set_fmt, NULL, &sfmt);

As explained below, propagating formats is fine for video node-centric
drivers, but comes with limitations.

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To summarize, if we cannot do the negotiation within kernel, delegating
this to userspace implies far more complexity and breaks compatibility
with existing applications without adding new functionalities.

Having all that in mind, what should be reconsidered in your opinion
Sakari ? Do you have some alternatives ?

First of all, let's note that your patch series performs to related but
still independent changes: it enables MC support, *and* enables the V4L2
subdev userspace API. The former is clearly needed and will allow you to
use the MC API internally in the kernel, simplifying pipeline traversal.
The latter then enables the V4L2 subdev userspace API, moving the
pipeline configuration responsibility to userspace.

You could in theory move to the MC API inside the kernel, without
enabling support for the V4L2 subdev userspace API. Configuring the
pipeline and propagating the formats would then be the responsibility of
the kernel driver.

Yes this is exactly what we want to do.
If I understand well, to disable the V4L2 subdev userspace API, I just 
have to remove the media device registry:

-	/* Register the media device */
-	ret = media_device_register(&dcmi->mdev);
-	if (ret) {
-		dev_err(dcmi->dev, "Failed to register media device (%d)\n",
-			ret);
-		goto err_media_device_cleanup;
-	}

Do you see any additional things to do ?

That should be it. Note that in that case pipeline configuration has to
be handled by the master driver (DCMI in this case), the external
subdevs involved (such as the CSI-2 to parallel bridge) must not handle
any propagation of formats or selection rectangles.

I wonder what we'd do in the case when someone needs to connect something
else to the pipeline, such as a sensor with more than one sub-device, or a
flash or a lens controller.

For future-proofness, I'd just use MC for hardware that may be part of a
complex pipeline. In this case, if you think backwards compatibility is
important (and for most hardware it probably is), I don't think there are
perfect solutions if your existing driver is not MC-enabled.

Oh, I fully agree with you, which is why I mentioned in another e-mail
that using a video node-centric approach would come with limitations,
such as not being able to support more complex pipelines, ever.

A reasonable compromise would be to add a Kconfig option that allows
enabling MC. This way you can provide backwards compatibility and allow
making use of the full potential of the hardware. That's also why hardware
that may be part of a non-trivial MC pipeline should start with MC-enabled
so we wouldn't run into this.

I really don't like this, as it introduces additional complexity. My
recommendation is to go for an MC-centric approach. Going for a video
node-centric approach is really shooting oneself in the foot regarding
future extensions. But that being said, if there's a strong desire to go
for foot self-shooting, the way to go is explained above.

-- 
Regards,

Laurent Pinchart

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