On 09/23/2014 01:10 PM, Laurent Pinchart wrote: > 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. > But you do not need to traverse backwards. You just wait when the source start to communicate with the destination, at this moment destination can build back-link dynamically. Regards Andrzej -- 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
