Date:   Mon, 8 Jan 2018 11:21:17 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Ondřej Jirman <megous@...ous.com>
Cc:     Yong <yong.deng@...ewell.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Chen-Yu Tsai <wens@...e.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Hugues Fruchet <hugues.fruchet@...com>,
        Yannick Fertre <yannick.fertre@...com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Ramesh Shanmugasundaram <ramesh.shanmugasundaram@...renesas.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Rick Chang <rick.chang@...iatek.com>,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com
Subject: Re: [linux-sunxi] [PATCH v4 0/2] Initial Allwinner V3s CSI Support

Hi,

On Thu, Jan 04, 2018 at 04:27:41PM +0100, Ondřej Jirman wrote:
> On Thu, Jan 04, 2018 at 03:06:25PM +0100, Maxime Ripard wrote:
> > 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@...ous.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.
> 
> I don't think you understand the issue. In your hypothetical situation, if the
> CSI device will have any controls in the future, the merging of controls from
> subdev will be done automatically anyway, it's not some optional feature.
> 
> Also userspace will not get any more complicated than without my proposed change
> to the driver. It will be at most the same as without the change if any subdev
> controls are masked by the CSI device controls.
> 
> This CSI driver has no controls anyway. All my change does is create an empty
> handler for future controls of the CSI driver, so that apps can depend on this
> merging behavior right now, and not wait until someone adds the first control
> to the CSI driver.

My point is slightly different though. In more complex pipelines like
we have (which is even more complicated due to the fact that the ISP
is largely unknown), you cannot just have an application that rely on
the controls exposed on the v4l2 device, but they should take the
subdevices into account as well.

Maxime

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

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