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. -- FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly improving, and getting towards what was expected from it. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
