On Tuesday 23 September 2014 12:02:45 Andrzej Hajda wrote: > On 09/23/2014 11:30 AM, Tomi Valkeinen wrote: > > On 23/09/14 09:21, Thierry Reding wrote: > >>> Well, I can write almost any kind of bindings, and then evidently my > >>> device would work. For me, on my board. > >> > >> Well, that's the whole problem with DT. For many devices we only have a > >> single setup to test against. And even when we have several they often > >> are derived from each other. But the alternative would be to defer > >> (possibly indefinitely) merging support for a device until a second, > >> wildly different setup shows up. That's completely unreasonable and we > >> need to start somewhere. > > > > Yes, but in this case we know of existing boards that have complex > > setups. It's not theoretical. > > > > I'm not saying we should stop everything until we have a 100% solution > > for the rare complex cases. But we should keep them in mind and, when > > possible, solve problems in a way that will work for the complex cases > > also.> > >>> I guess non-video devices haven't had need for those. I have had lots of > >>> boards with video setup that cannot be represented with simple phandles. > >>> I'm not sure if I have just been unlucky or what, but my understand is > >>> that other people have encountered such boards also. Usually the > >>> problems encountered there have been circumvented with some hacky video > >>> driver for that specific board, or maybe a static configuration handled > >>> by the boot loader. > >> > >> I have yet to encounter such a setup. Can you point me at a DTS for one > >> such setup? I do remember a couple of hypothetical cases being discussed > >> at one time or another, but I haven't seen any actual DTS content where > >> this was needed. > > > > No, I can't point to them as they are not in the mainline (at least the > > ones I've been working on), for obvious reasons. > > > > With a quick glance, I have the following devices in my cabinet that > > have more complex setups: OMAP 4430 SDP, BeagleBoneBlack + LCD, AM43xx > > EVM. Many Nokia devices used to have such setups, usually so that the > > LCD and tv-out were connected to the same video source. > > > >>> Do we have a standard way of representing the video pipeline with simple > >>> phandles? Or does everyone just do their own version? If there's no > >>> standard way, it sounds it'll be a mess to support in the future. > >> > >> It doesn't matter all that much whether the representation is standard. > > > > Again, I disagree. > > > >> phandles should simply point to the next element in the pipeline and the > >> OS abstractions should be good enough to handle the details about how to > >> chain the elements. > > > > I, on the other hand, would rather see the links the other way around. > > Panel having a link to the video source, etc. > > > > The video graphs have two-way links, which of course is the safest > > options, but also more verbose and redundant. > > > > When this was discussed earlier, it was unclear which way the links > > should be. It's true that only links to one direction are strictly > > needed, but the question raised was that if in the drivers we end up > > always going the links the other way, the performance penalty may be > > somewhat big. (If I recall right). > > I do not see why performance may drop significantly? > If the link is one-way it should probably work as below: > - the destination registers itself in some framework, > - the source looks for the destination in this framework using phandle, > - the source starts to communicate with the destination - since now full > two way link can be established dynamically. > > Where do you see here big performance penalty? The performance-related problems arise when you need to locate the remote device in the direction opposite to the phandle link direction. Traversing a link forward just involves a phandle lookup, but traversing it backwards isn't possible the same way. -- 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
