On Fri, May 11, 2012 at 01:44:26PM -0400, Alan Stern wrote: > On Sat, 12 May 2012, Lan Tianyu wrote: > > > The power saving depends on devices. I test a usb3.0 ssd. The power saving of > > power off is about 2.2w more than just selective suspend. In theory, power > > off can help to save remaining power after selective suspend. > > That's a lot of power! Suspended USB devices aren't supposed to > consume more than 2.5 mA of bus current, which at 5 V amounts to <= > 0.0125 W. Does the port really use that much? Or does the SSD have a > separate power supply that it disables when port power is removed? > > > > The patch did not address the case of powering down ports that have no > > > devices attached. That might be a better place to start, because it's > > > simpler, even though it might not yield as much power savings. > > > > Do you mean internal ports? > > Internal or external. > > > From my opinion, ACPI will tell us whether the port is connectable or not. > > ACPI will tell you about some ports, not others (for example, ACPI > knows nothing about the ports on hubs that the user plugs in). On > other systems, a Device Tree database might provide the same > information. > > > When the internal port is not connectable, this means the port is not used > > and this patch will power down the port. > > ... > > > For external ports, this should be associated with sys file control. The users > > need to determine when they should be power off. > > You don't mean "external", you mean "not described as unconnectable by > ACPI". > > > So I should work on the external ports without devices firstly and > > add the sys file for user to control? > > Yes, I think so. It will be less controversial and probably simpler. > When that mechanism is ready, you should be able to use it > automatically for unconnectable ports. > > One tricky thing: In theory, there should be a separate sysfs file for > each port. That seems like a lot of overhead though; is there any way > to present the information in a single file that won't offend sysfs > purists? Why is that a lot of "overhead"? It's what, 7-9 files max? As Sarah points out, one file for all ports is racy and can get to be a mess. But then again, I'm a "sysfs purist" :) greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
