On Wed, 21 Oct 2009, Valentin Longchamp wrote:

Sascha Hauer wrote:

quoted

On Mon, Oct 19, 2009 at 06:41:13PM +0200, Valentin Longchamp wrote:

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Hi Guennadi,

Guennadi Liakhovetski wrote:

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Hi

On Thu, 15 Oct 2009, Valentin Longchamp wrote:

quoted

We have two mt9t031 cameras that have a muxed bus on the robot.
We can control which one we are using with gpio outputs. This
currently is not optimal

So, what prevents you from registering two platform devices for your two
cameras? Is there a problem with that?

The lack of time until now to do it properly. I sent those patches as
initial RFC (and by the way thanks for your comment).

I would like to have one video interface only and that I can switch
between the two physical camera using a quite simple system call. Would
that be compatible with registering the two platform devices ?

Wouldn't it be better to have /dev/video[01] for two cameras? How do
keep the registers synchron between both cameras otherwise?

Well, from my experimentations, most initializations are done when you open
the device. So if you close the device, switch camera and open it again, the
registers are initialized with the need values. Of course there is a problem
is you switch camera while the device is open.

It could be ok with /dev/video[01], but I would need something that would
prevent one device to be opened when the other already is open (a mutex, but
where ?).

Besides, I have read a slide from Dongsoo Kim
(http://www.celinuxforum.org/CelfPubWiki/ELC2009Presentations?action=AttachFile&do=get&target=Framework_for_digital_camera_in_linux-in_detail.ppt
slides 41-47) and the cleanest solution would be to have the two chips
enumerated with VIDIOC_ENUMINPUT as proposed. What would then be the v4l2 call
to switch from one device to each other ? How to "link" it with the kernel
code that make the real hardware switching ?

Ok, I don't have a definite answer to this, so, just my thoughts:

1. soc-camera currently registers one struct v4l2_device device per _host_ 
immediately upon its registration, and one struct video_device per 
_client_ platform device.

2. we currently have 1 or 2 boards in the mainline with two video client 
devices on one interface: arch/sh/boards/mach-migor/ and (unsure about) 
arch/sh/boards/board-ap325rxa.c. At least the first of them exports two 
platform devices and thus gets /dev/video[01]. Accesses are synchronised 
with a mutex (I don't actually like that, I'd prefer to get a -EBUSY back 
instead of hanging in D in open()), and a successful acquisition of the 
mutex switches the respective camera on. See code for details. So, this 
approach is supported and it works. In this case we have one v4l2_device 
and two video_device instances, don't know whether this matches how this 
is supposed to be done, but it works so far:-)

3. to support switching inputs, significant modifications to soc_camera.c 
would be required. I read Nate's argument before, that as long as clients 
can only be accessed one at a time, this should be presented by multiple 
inputs rather than multiple device nodes. Somebody else from the V4L folk 
has also confirmed this opinion. In principle I don't feel strongly either 
way. But currently soc-camera uses a one i2c client to one device node 
model, and I'm somewhat reluctant to change this before we're done with 
the v4l2-subdev conversion.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/