On Thu, Mar 20, 2014 at 03:38:04PM -0300, Mauro Carvalho Chehab wrote:

Em Thu, 20 Mar 2014 17:54:31 +0000
Grant Likely [off-list ref] escreveu:

quoted

On Wed, 12 Mar 2014 10:25:56 +0000, Russell King - ARM Linux [off-list ref] wrote:

quoted

On Mon, Mar 10, 2014 at 02:52:53PM +0100, Laurent Pinchart wrote:

quoted

In theory unidirectional links in DT are indeed enough. However, let's not 
forget the following.

- There's no such thing as single start points for graphs. Sure, in some 
simple cases the graph will have a single start point, but that's not a 
generic rule. For instance the camera graphs 
http://ideasonboard.org/media/omap3isp.ps and 
http://ideasonboard.org/media/eyecam.ps have two camera sensors, and thus two 
starting points from a data flow point of view.

I think we need to stop thinking of a graph linked in terms of data
flow - that's really not useful.

Consider a display subsystem.  The CRTC is the primary interface for
the CPU - this is the "most interesting" interface, it's the interface
which provides access to the picture to be displayed for the CPU.  Other
interfaces are secondary to that purpose - reading the I2C DDC bus for
the display information is all secondary to the primary purpose of
displaying a picture.

For a capture subsystem, the primary interface for the CPU is the frame
grabber (whether it be an already encoded frame or not.)  The sensor
devices are all secondary to that.

So, the primary software interface in each case is where the data for
the primary purpose is transferred.  This is the point at which these
graphs should commence since this is where we would normally start
enumeration of the secondary interfaces.

V4L2 even provides interfaces for this: you open the capture device,
which then allows you to enumerate the capture device's inputs, and
this in turn allows you to enumerate their properties.  You don't open
a particular sensor and work back up the tree.

I believe trying to do this according to the flow of data is just wrong.
You should always describe things from the primary device for the CPU
towards the peripheral devices and never the opposite direction.

Agreed.

I don't agree, as what's the primary device is relative. 

Actually, in the case of a media data flow, the CPU is generally not
the primary device.

Even on general purpose computers, if the full data flow is taken into
the account, the CPU is a mere device that will just be used to copy
data either to GPU and speakers or to disk, eventually doing format
conversions, when the hardware is cheap and don't provide format
converters.

On more complex devices, like the ones we want to solve with the
media controller, like an embedded hardware like a TV or a STB, the CPU
is just an ancillary component that could even hang without stopping 
TV reception, as the data flow can be fully done inside the chipset.

The CPU is the _controlling_ component - it's the component that has to
configure the peripherals so they all talk to each other in the right
way.  Therefore, the view of it needs to be CPU centric.

If we were providing a DT description for consumption by some other
device in the system, then the view should be as seen from that device
instead.

Think about this.  Would you describe a system starting at, say, the
system keyboard, and branching all the way through just becuase that's
how you interact with it, or would you describe it from the CPUs point
of view because that's what has to be in control of the system.

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