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

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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On Mon, Dec 25, 2017 at 09:58:02AM +0100, Ondřej Jirman wrote:
> 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:

And it becomes much more complicated once you have the same controls
on the v4l2 device and subdevice, which is not that uncommon.

Maxime



-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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