Hi Mauro,

On Tuesday 09 August 2011 22:05:47 Mauro Carvalho Chehab wrote:

Em 29-07-2011 05:36, Laurent Pinchart escreveu:

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On Friday 29 July 2011 06:02:54 Mauro Carvalho Chehab wrote:

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Em 28-07-2011 19:57, Sylwester Nawrocki escreveu:

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On 07/28/2011 03:20 PM, Mauro Carvalho Chehab wrote:

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Accumulating sub-dev controls at the video node is the right thing to
do.

An MC-aware application will need to handle with that, but that
doesn't sound to be hard. All such application would need to do is to
first probe the subdev controls, and, when parsing the videodev
controls, not register controls with duplicated ID's, or to mark them
with some special attribute.

IMHO it's not a big issue in general. Still, both subdev and the host
device may support same control id. And then even though the control
ids are same on the subdev and the host they could mean physically
different controls (since when registering a subdev at the host driver
the host's controls take precedence and doubling subdev controls are
skipped).

True, but, except for specific usecases, the host control is enough.

Not for embedded devices. In many case the control won't even be
implemented by the host. If your system has two sensors connected to the
same host, they will both expose an exposure time control. Which one do
you configure through the video node ? The two sensors will likely have
different bounds for the same control, how do you report that ?

If the device has two sensors that are mutually exclusive, they should be
mapped as two different inputs. The showed control should be the one used
by the currently active input.

If the sensors aren't mutually exclusive, then two different video nodes
will be shown in userspace.

It's more complex than that. The OMAP3 ISP driver exposes 7 video nodes 
regardless of the number of sensors. Sensors can be mutually exclusive or not, 
depending on the board. S_INPUT has its use cases, but is less useful on 
embedded hardware.

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Those controls are also quite useless for generic V4L2 applications,
which will want auto-exposure anyway. This needs to be implemented in
userspace in libv4l.

Several webcams export exposure controls. Why shouldn't those controls be
exposed to userspace anymore?

Ok, if the hardware won't support 3A algorithm, libv4l will implement it,
eventually using an extra hardware-aware code to get the best performance
for that specific device, but this doesn't mean that the user should always
use it.

Btw, the 3A algorithm is one of the things I don't like on my cell phone:
while it works most of the time, sometimes I want to disable it and
manually adjust, as it produces dark images, when there's a very bright
light somewhere on the image background. Manually adjusting the exposure
time and aperture is something relevant for some users.

It is, but on embedded devices that usually requires the application to be 
hardware-aware. Exposure time limits depend on blanking, which in turn 
influences the frame rate along with the pixel clock (often configurable as 
well). Programming those settings wrong can exceed the ISP available 
bandwidth.

The world unfortunately stopped being simple some time ago :-)

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Also there might be some preference at user space, at which stage of
the pipeline to apply some controls. This is where the subdev API
helps, and plain video node API does not.

Again, this is for specific usecases. On such cases, what is expected is
that the more generic control will be exported via V4L2 API.

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Thus it's a bit hard to imagine that we could do something like
"optionally not to inherit controls" as the subdev/MC API is
optional.

:)

This was actually implemented. There are some cases at ivtv/cx18
driver where both the bridge and a subdev provides the same control
(audio volume, for example). The idea is to allow the bridge driver
to touch at the subdev control without exposing it to userspace,
since the desire was that the bridge driver itself would expose such
control, using a logic that combines changing the subdev and the
bridge registers for volume.

This seem like hard coding a policy in the driver;) Then there is no
way (it might not be worth the effort though) to play with volume
level at both devices, e.g. to obtain optimal S/N ratio.

In general, playing with just one control is enough. Andy had a
different opinion when this issue were discussed, and he thinks that
playing with both is better. At the end, this is a developers decision,
depending on how much information (and bug reports) he had.

ivtv/cx18 is a completely different use case, where hardware
configurations are known, and experiments possible to find out which
control(s) to use and how. In this case you can't know in advance what
the sensor and host drivers will be used for.

Why not?

My point is that the ISP driver developer can't know in advance which sensor 
will be used systems that don't exist yet.

I never saw an embedded hardware that allows physically changing the sensor.

Beagleboard + pluggable sensor board.

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Even if you did, fine image quality tuning requires accessing pretty much
all controls individually anyway.

The same is also true for non-embedded hardware. The only situation where
V4L2 API is not enough is when there are two controls of the same type
active. For example, 2 active volume controls, one at the audio demod, and
another at the bridge. There may have some cases where you can do the same
thing at the sensor or at a DSP block. This is where MC API gives an
improvement, by allowing changing both, instead of just one of the
controls.

To be precise it's the V4L2 subdev userspace API that allows that, not the MC 
API.

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This is a hack...sorry, just joking ;-) Seriously, I think the
situation with the userspace subdevs is a bit different. Because with
one API we directly expose some functionality for applications, with
other we code it in the kernel, to make the devices appear uniform at
user space.

Not sure if I understood you. V4L2 export drivers functionality to
userspace in an uniform way. MC api is for special applications that
might need to access some internal functions on embedded devices.

Of course, there are some cases where it doesn't make sense to export a
subdev control via V4L2 API.

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Also, the sensor subdev can be configured in the video node driver as
well as through the subdev device node. Both APIs can do the same
thing but in order to let the subdev API work as expected the video
node driver must be forbidden to configure the subdev.

Why? For the sensor, a V4L2 API call will look just like a bridge
driver call. The subdev will need a mutex anyway, as two MC
applications may be opening it simultaneously. I can't see why it
should forbid changing the control from the bridge driver call.

Please do not forget there might be more than one subdev to configure
and that the bridge itself is also a subdev (which exposes a scaler
interface, for instance). A situation pretty much like in Figure 4.4
[1] (after the scaler there is also a video node to configure, but we
may assume that pixel resolution at the scaler pad 1 is same as at the
video node). Assuming the format and crop configuration flow is from
sensor to host scaler direction, if we have tried to configure _all_
subdevs when the last stage of the pipeline is configured (i.e. video
node) the whole scaler and crop/composition configuration we have been
destroyed at that time. And there is more to configure than
VIDIOC_S_FMT can do.

Think from users perspective: all user wants is to see a video of a
given resolution. S_FMT (and a few other VIDIOC_* calls) have
everything that the user wants: the desired resolution, framerate and
format.

Specialized applications indeed need more, in order to get the best
images for certain types of usages. So, MC is there.

Such applications will probably need to know exactly what's the sensor,
what are their bugs, how it is connected, what are the DSP blocks in the
patch, how the DSP algorithms are implemented, etc, in order to obtain
the the perfect image.

Even on embedded devices like smartphones and tablets, I predict that
both types of applications will be developed and used: people may use a
generic application like flash player, and an specialized application
provided by the manufacturer. Users can even develop their own
applications generic apps using V4L2 directly, at the devices that
allow that.

As I said before: both application types are welcome. We just need to
warrant that a pure V4L application will work reasonably well.

That's why we have libv4l. The driver simply doesn't receive enough
information to configure the hardware correctly from the VIDIOC_* calls.
And as mentioned above, 3A algorithms, required by "simple" V4L2
applications, need to be implemented in userspace anyway.

It is OK to improve users experience via libv4l. What I'm saying is that it
is NOT OK to remove V4L2 API support from the driver, forcing users to use
some hardware plugin at libv4l.

Let me be clear on this. I'm *NOT* advocating removing V4L2 API support from 
any driver (well, on the drivers I can currently think of, if you show me a 
wifi driver that implements a V4L2 interface I might change my mind :-)).

The V4L2 API has been designed mostly for desktop hardware. Thanks to its 
clean design we can use it for embedded hardware, even though it requires 
extensions. What we need to do is to define which parts of the whole API apply 
as-is to embedded hardware, and which should better be left unused.

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Allowing the bridge driver to configure subdevs at all times would
prevent the subdev/MC API to work.

Well, then we need to think on an alternative for that. It seems an
interesting theme for the media workshop at the Kernel Summit/2011.

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There is a conflict there that in order to use
'optional' API the 'main' API behaviour must be affected....

It is optional from userspace perspective. A V4L2-only application
should be able to work with all drivers. However, a MC-aware
application will likely be specific for some hardware, as it will need
to know some device-specific stuff.

Both kinds of applications are welcome, but dropping support for
V4L2-only applications is the wrong thing to do.

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And I really cant use V4L2 API only as is because it's too limited.

Why?

For instance there is really yet no support for scaler and composition
onto a target buffer in the Video Capture Interface (we also use
sensors with built in scalers). It's difficult to efficiently manage
capture/preview pipelines. It is impossible to discover the system
topology.

Scaler were always supported by V4L2: if the resolution specified by
S_FMT is not what the sensor provides, then scale. All non-embedded
drivers with sensor or bridge supports scale does that.

Composition is not properly supported yet. It could make sense to add it
to V4L. How do you think MC API would help with composite?

Managing capture/preview pipelines will require some support at V4L2
level. This is a problem that needs to be addressed there anyway, as
buffers for preview/capture need to be allocated. There's an RFC about
that, but I don't think it covers the pipelines for it.

Managing pipelines is a policy decision, and needs to be implemented in
userspace. Once again, the solution here is libv4l.

If V4L2 API is not enough, implementing it on libv4l won't solve, as
userspace apps will use V4L2 API for requresting it.

We need to let userspace configure the pipeline. That's what the MC + V4L2 
APIs are for. The driver must no make policy decisions though, that must be 
left to userspace.

-- 
Regards,

Laurent Pinchart