Quoting Ulf Hansson (2013-03-18 03:36:29) > On 15 March 2013 20:38, Stephen Warren <swar...@wwwdotorg.org> wrote: > > On 03/15/2013 06:33 AM, Ulf Hansson wrote: > >> On 15 March 2013 13:06, Bill Huang <bilhu...@nvidia.com> wrote: > >>> On Fri, 2013-03-15 at 18:08 +0800, Ulf Hansson wrote: > > ... > >>>> Some prerequisites; I think am in favor of using the clk API to > >>>> trigger DVFS changes and then I agree on that clk_prepare|unprepare > >>>> needs to be possible to track from a DVFS perspective. clk_set_rate is > >>>> not enough. > >>>> > >>>> So if we decide to do the above (using the clk API to trigger DVFS > >>>> changes), I believe we should discuss two possible solutions; > >>>> - clk notifiers or.. > >>>> - dvfs clock type. > >>>> > >>>> I am trying to make up my mind of what I think is the best solution. > >>>> Have you considered "dvfs clock type"? > >>>> I put some comments about this for "[PATCH 2/5] clk: notifier handler > >>>> for dynamic voltage scaling" recently as well. > >>>> > >>>> What could the advantages/disadvantages be between the two options? > >>> > >>> I personally prefer clk notifiers since that's easy and all the existing > >>> device drivers don't need to be modified, a new clock or API might be > >>> more thoroughly considered (and hence maybe more graceful) but that > >>> means we need more time to cook and many drivers need to plug into that > >>> API when it comes out, a lot of test/verification or maybe chaos > >>> follows, I'm not sure will that be a little overkill. > >> > >> I guess you did not fully got what I meant with "dvfs clock type". It > >> will not affect the clock API. But instead the dvfs is handled by > >> implementing a specific clk hw type. So the same thing is accomplished > >> as with clk notifiers, no changes should be needed to device drivers. > >> > >> The difference is only that no notifiers will be needed, and all the > >> dvfs stuff will be handled in the clk hw instead. It will mean that we > >> will bundle dvfs stuff into the clock drivers, instead of separating > >> the code outside the clock drivers. But, on the other hand no > >> notifiers will be needed. > > > > The advantage here is that I assume that a notifier would continually > > have to check whether the clock being modified was one that the DVFS > > notifier cared about. By integrating the CVFS logic into the clk_hw > > itself, it'll only ever get executed for clocks that really care about > > DVFS. Presumably, the code that implements the clk_hw could also use > > some common DVFS library as part of the implementation, and still share > > code. Or perhaps, what about putting DVFS "ops" into a clk_hw alongside > > any other existing ops, and having the clock core call them whenever > > appropriate? > > Thanks for your comment Stephen. > > I agree to your reflections as well. It will probably be a more > optimized solution going this direction and we don't have to add more > "clk notifier code" to the clk API, which I guess is good. > It would be interesting to get some input from some of the maintainers > to this discussion as well, let's see. >
I do not like the dvfs clock type at all. For the set of DVFS problems that we are trying to solve, voltage scaling is more of a function of a device's requirements than of a clock. Put another way, we don't scale voltage because a clock runs at a rate, we scale voltage because a device runs at a rate. That is why the clock rate change notifiers were interesting for dvfs: the device scales the voltage in response to a clock frequency change. However it seems the rate-change notifiers are a bit messy for dvfs. As such I'm currently hacking on a new rfc to introduce a separate dvfs api. Based on recent discussions and some face-to-face feedback I think that kicking off DVFS transitions from the clock framework (which looks very enticing at first glance) is coming at the problem from the wrong direction. A new api that builds on top of clocks, regulators and opps is straight-forward, requires no weird cross-layer calls and is generally cleaner. The downside is that driver authors will have to choose between using clk_set_rate or magic_dvfs_set_rate apis. That dilemma is entirely analogous to the runtime pm versus clk_enable/clk_disable dilemma and hopefully won't be a big hurdle to acceptance. I'll post in the v2 thread as well, specifically addressing the devfreq/clk_set_rate idea. Regards, Mike > Kind regards > Ulf Hansson > > _______________________________________________ > linaro-dev mailing list > linaro-dev@lists.linaro.org > http://lists.linaro.org/mailman/listinfo/linaro-dev _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
