Hi Mauro,

On Friday 26 August 2011 17:09:02 Mauro Carvalho Chehab wrote:

Em 26-08-2011 11:16, Hans Verkuil escreveu:

quoted

On Friday, August 26, 2011 15:45:30 Laurent Pinchart wrote:

quoted

On Thursday 25 August 2011 14:43:56 Mauro Carvalho Chehab wrote:

quoted

Em 24-08-2011 19:29, Sakari Ailus escreveu:

[snip]

quoted

quoted

The question I still have on this is that how should the user know
which video node to access on an embedded system with a camera: the
OMAP 3 ISP, for example, contains some eight video nodes which have
different ISP blocks connected to them. Likely two of these nodes are
useful for a general purpose application based on which image format
it requests. It would make sense to provide generic applications
information only on those devices they may meaningfully use.

IMO, we should create a namespace device mapping for video devices.
What I

mean is that we should keep the "raw" V4L2 devices as:
	/dev/video??

But also recommend the creation of a new userspace map, like:
	/dev/webcam??
	/dev/tv??
	...

with is an alias for the actual device.

Something similar to dvd/cdrom aliases that already happen on most
distros:

lrwxrwxrwx   1 root root           3 Ago 24 12:14 cdrom -> sr0
lrwxrwxrwx   1 root root           3 Ago 24 12:14 cdrw -> sr0
lrwxrwxrwx   1 root root           3 Ago 24 12:14 dvd -> sr0
lrwxrwxrwx   1 root root           3 Ago 24 12:14 dvdrw -> sr0

I've been toying with a similar idea. libv4l currently wraps /dev/video*
device nodes and assumes a 1:1 relationship between a video device node
and a video device. Should this assumption be somehow removed, replaced
by a video device concept that wouldn't be tied to a single video
device node ?

Just as background information: the original idea was always that all v4l
drivers would have a MC and that libv4l would use the information
contained there as a helper (such as deciding which nodes would be the
'default' nodes for generic applications).

This is something that libv4l won't do: it is up to the userspace
application to choose the device node to open.

I think this is one of our fundamental issues. Most applications are actually 
not interested in video nodes at all. What they want is a video device. 
Shouldn't libv4l should allow applications to enumerate video devices (as 
opposed to video nodes) and open them without caring about video nodes ?

Ok, libv4l can have helper APIs for that, like the one I wrote, but even
adding MC support on it may not solve the issues.

quoted

Since there is only one MC device node for each piece of video hardware
that would make it much easier to discover what hardware there is and
what video nodes to use.

I always liked that idea, although I know Mauro is opposed to having a MC
for all v4l drivers.

It doesn't make sense to add MC for all V4L drivers. Not all devices are
like ivtv with lots of device drivers. In a matter of fact, most supported
devices create just one video node. Adding MC support for those devices
will just increase the drivers complexity without _any_ reason, as those
devices are fully configurable using the existing ioctl's.

Hans' proposal is to handle this in the V4L2 core for most drivers, so those 
drivers won't become more complex as they won't be modified at all. The MC API 
for those devices will only offer read-only enumeration, not link 
configuration.

Also, as I said before, and implemented at xawtv and at a v4l-utils library,
the code may use sysfs for simpler devices. It shouldn't be hard to
implement a mc aware code there, although I don't think that MC API is
useful to discover what nodes are meant to be used for TV, encoder, decoder,
webcams, etc. The only type information it currently provides is:

#define MEDIA_ENT_T_DEVNODE_V4L		(MEDIA_ENT_T_DEVNODE + 1)
#define MEDIA_ENT_T_DEVNODE_FB		(MEDIA_ENT_T_DEVNODE + 2)
#define MEDIA_ENT_T_DEVNODE_ALSA	(MEDIA_ENT_T_DEVNODE + 3)
#define MEDIA_ENT_T_DEVNODE_DVB		(MEDIA_ENT_T_DEVNODE + 4)

So, a MC aware application also needs to be a hardware-dependent
application, as it will need to use something else, like the media entity
name, to discover for what purpose a media node is meant to be used.

As Hans pointed out, this is because we haven't implemented more detailed 
information *yet*. It has always been a goal to provide more details through 
the MC API.

quoted

While I am not opposed to creating such userspace maps I also think it is
a bit of a poor-man's solution.

The creation of per-type devices is part of the current API: radio
and vbi nodes are examples of that (except that they aren't aliases, but
real devices, but the idea is the same: different names for different
types of usage).

This would only work in a black-and-white world. Devices are often not just 
webcams or tv tuners.

quoted

In particular I am worried that we get a lot of those mappings (just think
of ivtv with its 8 or 9 devices).

I can think of: webcam, tv, compressed (mpeg), tv-out, compressed-out,
mem2mem.

But a 'tv' node might also be able to handle compressed video (depending
on how the hardware is organized), so how do you handle that?

Well, What you've called as "compressed" is, in IMO, "encoder". It probably
makes sense to have, also "decoder". I'm in doubt about "webcam", as there
are some grabber devices with analog camera inputs for video surveillance.
Maybe "camera" is a better name for it.

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It can all be solved, I'm sure, but I'm not sure if such userspace
mappings will scale that well with the increasing hardware complexity.

Not all video nodes would need an alias. Just the ones where it makes sense
for an application to open it.

If it doesn't make sense for an application to open a video node, you can 
remove it completely :-)

-- 
Regards,

Laurent Pinchart