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? Regards Andrzej > > Tomi > > -- 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
