"Rafael J. Wysocki" <rjw@xxxxxxx> writes: > On Monday, September 20, 2010, Kevin Hilman wrote: >> "Rafael J. Wysocki" <rjw@xxxxxxx> writes: >> >> [...] >> >> >> >> In terms of the lifetime rules on the nodes in the list: >> >> >> The list is expected to be maintained once created, entries are expected >> >> >> to be added optimally and not expected to be destroyed, the choice of >> >> >> list implementation was for reducing the complexity of the code itself >> >> >> and not yet meant as a mechanism to dynamically add and delete nodes on >> >> >> the fly.. Essentially, it was intended for the SOC framework to ensure >> >> >> it plugs in the OPP entries optimally and not create a humongous list of >> >> >> all possible OPPs for all families of the vendor SOCs - even though it >> >> >> is possible to use the OPP layer so - it just wont be smart to do so >> >> >> considering list scan latencies on hot paths such as cpufreq transitions >> >> >> or idle transitions. >> >> > >> >> > If the list nodes are not supposed to be added and removed dynamically, >> >> > it probably would make sense to create data structures containing >> >> > the "available" OPPs only, once they are known, and simply free the object >> >> > representing the other ones. >> >> I covered the usage in my reply here: >> >> http://marc.info/?l=linux-arm-kernel&m=128476570300466&w=2 >> >> but to repeat, the list is dynamic during initialization but remains >> >> static after initialization based on SOC framework implementation - this >> >> is best implemented with a list (we had started with an original array >> >> implementation which evolved to the current list implementation >> >> http://marc.info/?l=linux-omap&m=125912217718770&w=2) >> > >> > Well, my point is, since the _final_ set of OPPs doesn't really >> > change, there's no need to use a list for storing it in principle. >> > >> > Your current algorithm seems to be: >> > (1) Create a list of all _possible_ OPPs. >> > (2) Mark the ones that can actually be used on the given hardware as >> > "available". >> > (3) Whenever we need to find an OPP to use, browse the entire list. >> > This isn't optimal, because the OPPs that are not marked as "available" in (2) >> > will never be used, although they _will_ be inspected while browsing the list. >> >> A little clarificaion about "will never be used" below... >> >> > So, I think a better algorithm would be: >> > (1) Create a list of all possible OPPs. >> > (2) Drop the nonavailable OPPs from the list. >> > (3) Whenever we need to find an OPP to use, browse the entire list. >> > >> > But then, it may be better to simply move the list we get in (2) into an >> > array, because the browsing is going to require fewer memory accesses in >> > that case (also, an array would use less memory than the list). So, perhaps, >> > it's better to change the algorithm even further: >> > (1) Create a list of all possible OPPs. >> > (2) Drop the nonavailable OPPs from the list. >> > (3) Move the list we got in (2) into a sorted array. >> > (4) Whenever we need to find an OPP to use, browse the array >> > (perhaps using binary search). >> >> Just a little clarification on "available." The intended use of this flag >> was not just a one-time "available on hardware X." It was also intended >> to be able to add/remove availbale OPPs dynamically at run-time. >> >> More specifically, it's intended for use to *temporarily* remove an OPP >> from being selected. The production usage of this would primarily for >> thermal considerations (e.g. don't use OPPx until the temperature drops) >> >> However, for PM development & debug, we also use this to temporarily >> take a class of OPPs out of the running for test/debug purposes >> (e.g. driver X runs great at OPPx and OPPy, but not OPPz.) So the >> ability to temporarily be selective about OPPs at runtime for >> debug/development is extremely useful. >> >> So, to summarize, "most of the time", all the OPPs that were added (via >> opp_add()) will be "available". Ones that are !availble will likely >> only be so temporarily, so I'm not sure that the overhead of keeping a >> separate structure for the available and !available OPPs is worth it. >> Especially, since OPP changes are relatively infrequent. > > Well, the Nishanth's description doesn't match this, so thanks for the > clarification. Agreed, we need to update the doc file to reflect this. > In that case you might consider using a red-black tree for storing the > "available" OPPs, so that you can add-remove them dynamically, but > you can avoid a linear search through the entire list every time you need to > find and available OPP. Since we have standard helpers for handling rbtrees, > that shouldn't be a big deal. That's a possibility, but do you think rbtrees are worth it for a relatively small number of OPPs for any given device? We're using this to track a list of OPPs for any struct device, so there may be multiple independent OPP lists, but each would have a small number of OPPs. For example, on OMAP, while the CPU might have a larger number of OPPs (5-6), most devices will have a small number of OPPs (1-3.) I gather this is similar for many of the embedded SoCs available today. Considering such a small number of OPPs, is the extra complexity of an rbtree worth it? Kevin -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
