Em Tue, 14 Mar 2017 23:32:54 +0100
Pavel Machek [off-list ref] escreveu:

Hi!

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Even if they were merged, if we keep the same mean time to develop a
libv4l plugin, that would mean that a plugin for i.MX6 could take 2-3
years to be developed.

There's a clear message on it:
	- we shouldn't keep pushing for a solution via libv4l.    

Or:

	- userspace plugin development had a very a low priority and
	  never got the attention it needed.  

The end result is the same: we can't count on it.
  

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I know that's *my* reason. I rarely if ever looked at it. I always assumed
Sakari and/or Laurent would look at it. If this reason is also valid for
Sakari and Laurent, then it is no wonder nothing has happened in all that
time.

We're all very driver-development-driven, and userspace gets very little
attention in general. So before just throwing in the towel we should take
a good look at the reasons why there has been little or no development: is
it because of fundamental design defects, or because nobody paid attention
to it?  

No. We should look it the other way: basically, there are patches
for i.MX6 driver that sends control from videonode to subdevs. 

If we nack apply it, who will write the userspace plugin? When
such change will be merged upstream?  

Well, I believe first question is: what applications would we want to
run on complex devices? Will sending control from video to subdevs
actually help?

I would say: camorama, xawtv3, zbar, google talk, skype. If it runs
with those, it will likely run with any other application.

mplayer is useful for testing... but that one already works (after you
setup the pipeline, and configure exposure/gain).

But thats useful for testing, not really for production. Image will be
out of focus and with wrong white balance.

What I would really like is an application to get still photos. For
taking pictures with manual settings we need

a) units for controls: user wants to focus on 1m, and take picture
with ISO200, 1/125 sec. We should also tell him that lens is f/5.6 and
focal length is 20mm with 5mm chip.

But... autofocus/autogain would really be good to have. Thus we need:

b) for each frame, we need exposure settings and focus position at
time frame was taken. Otherwise autofocus/autogain will be too
slow. At least focus position is going to be tricky -- either kernel
would have to compute focus position for us (not trivial) or we'd need
enough information to compute it in userspace.

There are more problems: hardware-accelerated preview is not trivial
to set up (and I'm unsure if it can be done in generic way). Still
photos application needs to switch resolutions between preview and
photo capture. Probably hardware-accelerated histograms are needed for
white balance, auto gain and auto focus, ....

It seems like there's a _lot_ of stuff to be done before we have
useful support for complex cameras...

Taking still pictures using a hardware-accelerated preview is
a sophisticated use case. I don't know any userspace application
that does that. Ok, several allow taking snapshots, by simply
storing the image of the current frame.

(And I'm not sure... when application such as skype is running, is
there some way to run autogain/autofocus/autowhitebalance? Is that
something we want to support?)

Autofocus no. Autogain/Autowhite can be done via libv4l, provided that
it can access the device's controls via /dev/video devnode. Other
applications may be using some other similar algorithms.

Ok, they don't use histograms provided by the SoC. So, they do it in
software, with is slower. Still, it works fine when the light
conditions don't change too fast.

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If we don't have answers to any of the above questions, we should not
nack it.

That's said, that doesn't prevent merging a libv4l plugin if/when
someone can find time/interest to develop it.  

I believe other question is: will not having same control on main
video device and subdevs be confusing? Does it actually help userspace
in any way? Yes, we can make controls accessible to old application,
but does it make them more useful? 

Yes. As I said, libv4l (and some apps) have logic inside to adjust
the image via bright, contrast and white balance controls, using the
video devnode. They don't talk subdev API. So, if those controls
aren't exported, they won't be able to provide a good quality image.

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In addition, I suspect end-users of these complex devices don't really care
about a plugin: they want full control and won't typically use generic
applications. If they would need support for that, we'd have seen much more
interest. The main reason for having a plugin is to simplify testing and
if this is going to be used on cheap hobbyist devkits.  

What are the needs for a cheap hobbyist devkit owner? Do we currently
satisfy those needs? I'd say that having a functional driver when
compiled without the subdev API, that implements the ioctl's/controls  

Having different interface based on config options... is just
weird. What about poor people (like me) trying to develop complex
applications?

Well, that could be done using other mechanisms, like a modprobe
parameter or by switching the behaviour if a subdev interface is
opened. I don't see much trouble on allowing accessing a control via
both interfaces.

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[1] Yet, I might eventually do that for fun, an OMAP3 board with tvp5150
just arrived here last week. It would be nice to have xawtv3 running on it :-)
So, if I have a lot of spare time (with is very unlikely), I might eventually 
do something for it to work.
  

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I know it took me a very long time before I had a working omap3.  

My first OMAP3 board with working V4L2 source just arrived last week
:-)  

You can get Nokia N900 on aliexpress. If not, they are still available
between people :-)

I have one. Unfortunately, I never had a chance to use it, as the display
stopped working one week after I get it.

Thanks,
Mauro