Re: [RFC PATCH] [media]: of: move graph helpers from drivers/media/v4l2-core to drivers/of

[Russell King - ARM Linux]

On Thu, Mar 20, 2014 at 07:16:29PM +0100, Laurent Pinchart wrote:
> On Thursday 20 March 2014 17:54:31 Grant Likely wrote:
> > On Wed, 12 Mar 2014 10:25:56 +0000, Russell King - ARM Linux wrote:
> > > On Mon, Mar 10, 2014 at 02:52:53PM +0100, Laurent Pinchart wrote:
> > > > 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.
> 
> Absolutely not agreed. The whole concept of CPU towards peripherals only makes 
> sense for very simple devices and breaks as soon as the hardware gets more 
> complex. There's no such thing as CPU towards peripherals when peripherals 
> communicate directly.
> 
> Please consider use cases more complex than just a display controller and an 
> encoder, and you'll realize how messy not being able to parse the whole graph 
> at once will become. Let's try to improve things, not to make sure to prevent 
> support for future devices.

That's odd, I did.

Please draw some (ascii) diagrams of the situations you're saying this
won't work for, because at the moment all I'm seeing is some vague
hand-waving rather than anything factual that I can relate to.  Help
us to actually _see_ the problem you have with this approach so we can
understand it.

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