Quoting Ulf Hansson (2015-01-15 02:04:04) > On 15 January 2015 at 10:20, Krzysztof Kozlowski > <k.kozlowski@xxxxxxxxxxx> wrote: > > On czw, 2015-01-15 at 09:20 +0100, Ulf Hansson wrote: > >> + Mike, Stephen (Clock maintainers) > >> > >> On 12 January 2015 at 10:23, Krzysztof Kozlowski > >> <k.kozlowski@xxxxxxxxxxx> wrote: > >> > Hi, > >> > > >> > > >> > I would like to hear some comments about idea of scaling MMC clock > >> > frequency. The basic idea is to lower the clock when device is > >> > completely idle or not busy enough. > >> > >> We already have host drivers that implements runtime PM support. > >> Typically that would mean the clock will be gated once the device > >> becomes runtime PM suspended. > >> > >> Why should we decrease the frequency of an already gated clock? > > > > In case of idle state you're right that clkgate would be better. But > > what about finding a compromise between high performance (high > > frequency) and energy saving for different loads on MMC? > > I guess a compromise could be beneficial for some SOC and use cases. > At least I remember, ST-Ericsson's UX500 SOC had such an out of tree > hack to track MMC load. It is very important to model when resources are not needed, since this has some system-wide effects. There are two main use-cases I have in mind: 1) MMC clk is a leaf clock of some complex hierarchy (e.g. a PLL at the top of a clock sub-tree). If MMC is always "locked" at some fast rate (e.g. 48MHz instead of 24MHz or 12MHz) then that constraint prevents the rest of the hierarchy from transitioning to a lower frequency. Even if the MMC clock is aggressively gating, maximum system-level power savings may not be achieved since the rest of the clock sub-tree (starting at the PLL) will be "stuck" at a higher frequency than necessary. Thus, aggressive clock gating might give good power savings for the MMC case, but may be a blocker for other system components. 2) Wake-up latency constraints might make it impossible to clock gate, and thus the only power-saving option is to run at a lower frequency. This is not quite what is described above, but the point is that clock frequency scaling and clock gating are complementary power saving options, but we rely on the driver to model resource requirements accurately to get the best results. Regards, Mike > > > > > The frequency scaling could help in that case. Anyway I should prepare > > some more benchmarks for such conditions. > > Seems reasonable and please do! > > Kind regards > Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
