On 24 January 2013 18:12, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote: > On Thu, Jan 24, 2013 at 1:59 PM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxxx> wrote: > >> The clock can be masked independently too via the clock enable bit in >> the clock register. >> >> However, it's interesting to note: experiments that Linus did with having >> the primecell automatically masking the clock to the card did not prove >> worthwhile (see the comment in the code). We seem to be heading towards >> doing this in software and people suggesting it is worthwhile... Why is >> this? >> >> If the hardware can gate the clock and it doesn't produce the power >> savings we expect, how does then gating the clock via software then >> become worthwhile? It doesn't make sense. > > We're talking about this I guess: > http://marc.info/?l=linux-arm-kernel&m=123790711306850&w=4 > > In these experiments I actually gated not only the > enable bit (bit 8) to the MCI clock, but also the pclk to > the PrimeCell itself as well. This was due to the U300 > clock framework doing that behind the scenes on > clk_disable(host->clk); > > And the reason the clock framework was doing that > was that the U300 had the block clock and the card > clock wired to the same clock tap ... > The equivalent patch today would also call > amba_pclk_disable() when the card was inactive. > > Thus we also got rid of all the switching leaks in the PL180 > logic. (Containing two big state machines for example.) > > It was an interesting comparison to just enable PwrSave > (bit 9) instead, and disable the other clock logic. > > It turned out that this saving was significantly bigger when > gating the entire PrimeCell than the savings for just gating > the clock out to the card itself with bit 9. > > But at this time I didn't have a card in the slot, so as to > eliminate the power consumed in the card (assuming > this would drown the overall current consumption). > > So I have second thoughts about this. The experiment > was not testing the PwrSave bit with a card inserted, this > is the wrong thing to do. To determine if that bit is a good > thing to turn on, it needs to be tested with a card in the > slot. > > The only really useful information is that the switching > logic inside the PL180 synthesized IP-core consumes > circa 200uA, so it could be useful to gate it at times. So in the runtime_pm callbacks, it you don't just want to do normal clk gating/ungating with the clock API, which is done right now. You also would like to actually gate the clock internally by using clock/power register. Does that make sense? > > Anyone wants to re-do the measurements? > > Yours, > Linus Walleij Kind regards Ulf Hansson -- 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
