Message-ID: <20140815044811.GP16460@valkosipuli.retiisi.org.uk>
Date:	Fri, 15 Aug 2014 07:48:12 +0300
From:	Sakari Ailus <sakari.ailus@....fi>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	kyungmin.park@...sung.com, b.zolnierkie@...sung.com,
	laurent.pinchart@...asonboard.com
Subject: Re: [PATCH/RFC v4 00/21] LED / flash API integration

On Thu, Aug 14, 2014 at 12:35:05PM +0200, Jacek Anaszewski wrote:
> On 08/14/2014 07:03 AM, Sakari Ailus wrote:
> >Hi Jacek,
> >
> >On Thu, Aug 07, 2014 at 10:21:14AM +0200, Jacek Anaszewski wrote:
> >>On 08/06/2014 08:53 AM, Sakari Ailus wrote:
> >>>Hi Jacek,
> >>>
> >>>On Fri, Jul 11, 2014 at 04:04:03PM +0200, Jacek Anaszewski wrote:
> >>>...
> >>>>1) Who should register V4L2 Flash sub-device?
> >>>>
> >>>>LED Flash Class devices, after introduction of the Flash Manager,
> >>>>are not tightly coupled with any media controller. They are maintained
> >>>>by the Flash Manager and made available for dynamic assignment to
> >>>>any media system they are connected to through multiplexing devices.
> >>>>
> >>>>In the proposed rough solution, when support for V4L2 Flash sub-devices
> >>>>is enabled, there is a v4l2_device created for them to register in.
> >>>>This however implies that V4L2 Flash device will not be available
> >>>>in any media controller, which calls its existence into question.
> >>>>
> >>>>Therefore I'd like to consult possible ways of solving this issue.
> >>>>The option I see is implementing a mechanism for moving V4L2 Flash
> >>>>sub-devices between media controllers. A V4L2 Flash sub-device
> >>>>would initially be assigned to one media system in the relevant
> >>>>device tree binding, but it could be dynamically reassigned to
> >>>>the other one. However I'm not sure if media controller design
> >>>>is prepared for dynamic modifications of its graph and how many
> >>>>modifications in the existing drivers this solution would require.
> >>>
> >>>Do you have a use case where you would need to strobe a flash from multiple
> >>>media devices at different times, or is this entirely theoretical? Typically
> >>>flash controllers are connected to a single source of hardware strobe (if
> >>>there's one) since the flash LEDs are in fact mounted next to a specific
> >>>camera sensor.
> >>
> >>I took into account such arrangements in response to your message
> >>[1], where you were considering configurations like "one flash but
> >>two
> >>cameras", "one camera and two flashes". And you also called for
> >>proposing generic solution.
> >>
> >>One flash and two (or more) cameras case is easily conceivable -
> >>You even mentioned stereo cameras. One camera and many flashes
> >>arrangement might be useful in case of some professional devices which
> >>might be designed so that they would be able to apply different scene
> >>lighting. I haven't heard about such devices, but as you said
> >>such a configuration isn't unthinkable.
> >>
> >>>If this is a real issue the way to solve it would be to have a single media
> >>>device instead of many.
> >>
> >>I was considering adding media device, that would be a representation
> >>of a flash manager, gathering all the registered flashes. Nonetheless,
> >>finally I came to conclusion that a v4l2-device alone should suffice,
> >>just to provide a Flash Manager representation allowing for
> >>v4l2-flash sub-devices to register in.
> >>All the features provided by the media device are useless in case
> >>of a set of V4L2 Flash sub-devices. They couldn't have any linkage
> >>in such a device. The only benefit from having media device gathering
> >>V4L2 Flash devices would be possibility of listing them.
> >
> >Not quite so. The flash is associated to the sensor (and lens) using the
> >group ID in the Media controller. The user space doesn't need to "know" this
> >association.
> >
> >More complex use cases such as the above may need extensions to the Media
> >controller API.
> 
> I think that I have unnecessarily complicated the issue. Generally
> there will be always one media controller created for all camera
> sensors available in the system. If there is a single media controller
> then we can easily use async subdev registration API. A media-dev
> driver would have to parse list of flash device phandles from
> the ISP device's DT node and register them as async sub-devices.

Currently the media device is created by a driver which is most of the time
the ISP driver on embedded systems. This driver is also responsible for
registering the flash device (nodes) to the media device. So "will" is the
word, I think.

-- 
Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk
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