On Thursday 20 March 2014 18:43:16 Russell King - ARM Linux wrote: > 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 escreveu: > > > 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. > > > > 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. DT has been designed to represent a control-based view of the system. It does so pretty well using its tree-based model. However, it doesn't have a native way to represent a flow-based graph, hence the OF graph solution we're discussing. The whole point of this proposal is to represent the topology of the media device, not how each entity is controlled. -- Regards, Laurent Pinchart -- 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
