Re: [linux-sunxi] [PATCH v4 0/2] Initial Allwinner V3s CSI Support

["Ondřej Jirman" <megous@megous.com>]

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Hello,

On Mon, Dec 25, 2017 at 11:15:26AM +0800, Yong wrote:
> Hi,
> 
> On Fri, 22 Dec 2017 14:46:48 +0100
> Ondřej Jirman <megous@megous.com> wrote:
> 
> > Hello,
> > 
> > Yong Deng píše v Pá 22. 12. 2017 v 17:32 +0800:
> > > 
> > > Test input 0:
> > > 
> > >         Control ioctls:
> > >                 test VIDIOC_QUERY_EXT_CTRL/QUERYMENU: OK (Not Supported)
> > >                 test VIDIOC_QUERYCTRL: OK (Not Supported)
> > >                 test VIDIOC_G/S_CTRL: OK (Not Supported)
> > >                 test VIDIOC_G/S/TRY_EXT_CTRLS: OK (Not Supported)
> > >                 test VIDIOC_(UN)SUBSCRIBE_EVENT/DQEVENT: OK (Not Supported)
> > >                 test VIDIOC_G/S_JPEGCOMP: OK (Not Supported)
> > >                 Standard Controls: 0 Private Controls: 0
> > 
> > I'm not sure if your driver passes control queries to the subdev. It
> > did not originally, and I'm not sure you picked up the change from my
> > version of the driver. "Not supported" here seems to indicate that it
> > does not.
> > 
> > I'd be interested what's the recommended practice here. It sure helps
> > with some apps that expect to be able to modify various input controls
> > directly on the /dev/video# device. These are then supported out of the
> > box.
> > 
> > It's a one-line change. See:
> > 
> > https://www.kernel.org/doc/html/latest/media/kapi/v4l2-controls.html#in
> > heriting-controls
> 
> I think this is a feature and not affect the driver's main function.
> I just focused on making the CSI main function to work properly in 
> the initial version. Is this feature mandatory or most commonly used?

I grepped the platform/ code and it seems, that inheriting controls
from subdevs is pretty common for input drivers. (there are varying
approaches though, some inherit by hand in the link function, some
just register and empty ctrl_handler on the v4l2_dev and leave the
rest to the core).

Practically, I haven't found a common app that would allow me to enter
both /dev/video0 and /dev/v4l-subdevX. I'm sure anyone can write one
themselves, but it would be better if current controls were available
at the /dev/video0 device automatically.

It's much simpler for the userspace apps than the alternative, which
is trying to identify the correct subdev that is currently
associated with the CSI driver at runtime, which is not exactly
straightforward and requires much more code, than a few lines in
the kernel, that are required to inherit controls:


	ret = v4l2_ctrl_handler_init(&csi->ctrl_handler, 0);
	if (ret) {
		dev_err(csi->dev,
			"V4L2 controls handler init failed (%d)\n",
			ret);
		goto handle_error;
	}

	csi->v4l2_dev.ctrl_handler = &csi->ctrl_handler;

See: https://github.com/megous/linux/blob/linux-tbs/drivers/media/platform/sun6i-csi/sun6i_csi.c#L1005

regards,
  o.j.

> Thanks,
> Yong

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