Arjan van de Ven wrote: >>> The data we have from this patch is that it saves typically a Watt of >>> power (depends on the machine of course, but the range is 0.5W to >>> 1.5W). If you want to also have an even more agressive thing where >>> you want to start disabling the entire controller... I don't see how >>> this is in conflict with saving power on the link level by "just" >>> enabling a hardware feature .... >> >> Well, both implement about the same thing. I prefer software >> implementation because it's more generic and ALPE/ASP seems too >> aggressive to me. > > Too aggressive in what way? There are devices which lock up hard if PHY enters PS mode (only physical power removal can reset it) and I wouldn't be surprised if some devices aren't happy with PS being too aggressive. Well, I actually expect to see such devices. It's ATA after all. This is unknown territory and that's why I was using 'seems ... to me'. > There are tradeoffs on either side. Doing things in software is more > work for the cpu, and depending on the implementation, will consume more > power on the CPU side. (for example if you need regular timers that just > consumes the power you are saving back up). The hardware can obviously > switch very fast (because it's independent of any software), yet of > course the software has higher level knowledge about how idle the link > really is (like it knows if any files are open etc etc). > > To be honest, I would be surprised if software could do significantly > better than hardware though; it seems a simple problem: Idle -> go to > low power, and estimating idle isn't all that hard on a link level... > there's not all THAT much the kernel can estimate better I suspect. I don't think the end result will vary in any significant way. My biggest argument for sw implementation is it can be used for other controllers. > This debate is very similar to the cpufreq debate from 4 years ago, > where there were 3 levels: do it in the CPU, do it in the kernel or do > it in userspace. All three are valid; whichever is best depends on the > exact hardware that you have... > (and you can argue that first everyone started in userspace, then the > hardware improved that made a kernelspace implementation better > (ondemand) and now Turbo Mode is more or less moving this to the > hardware... I wouldn't be surprised if the sata side will show a similar > trend) Currently, ahci is the only one which has controller-side automatic PS but some ATA devices (hdds) implement device initiated PS (DIPS). The sw implementation supports SW HIPS and DIPS. We can add HW HIPS support and hook ALPE/ASP support there but I don't think it would have benefits over SW implementation. I think it's a bit different from cpufreq. ATA is cheaper and more broken and much more diverse. Thanks. -- tejun - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
