On Thu, 20 Mar 2014 15:38:04 -0300, Mauro Carvalho Chehab <m.chehab@xxxxxxxxxxx> wrote: > Em Thu, 20 Mar 2014 17:54:31 +0000 > Grant Likely <grant.likely@xxxxxxxxxx> escreveu: > > > On Wed, 12 Mar 2014 10:25:56 +0000, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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. We're talking about wiring up device drivers here, not data flow. Yes, I completely understand that data flow is often not even remotely cpu-centric. However, device drivers are, and the kernel needs to know the dependency graph for choosing what devices depend on other devices. g. -- 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
