On Mon, 20 Aug 2007 12:11:34 +0200, Oliver Neukum wrote: > Am Montag 20 August 2007 schrieb Jean Delvare: > > If I rmmod "ehci-hcd" then the power consumption is back to 69 W. This > > confirms that this is really USB-related. I have to admit that I did > > not expect an external drive to eat that much power from the system, > > especially when not used. I am told that VIA chips are notoriously bad > > at this kind of things. I'll try the same external drive on an Intel > > system later today. > > > > The last mystery remaining is how USB "activity" can cause my CPU to > > heat. I would expect the south bridge to heat, not the CPU. > > USB, or strictly speaking EHCI, OHCI and UHCI, use DMA. To allow > that the cache coherency logic has to be active. Therefore your CPU > cannot go to C3. Therefore it draws more power. The problem we are > facing in USB is that to get great savings, our coverage has to be perfect. > One device that cannot be autosuspended and we lose most savings.
Ah, OK, thanks for the clarification, it explains a lot. I've made some more tests on two Intel boards and another VIA board. The bottom line is that both VIA boards see a bump in power consumption when plugging my USB 2.0 hard disk drive (10 W on one board, 4 W on the other) while none of the Intel boards exhibit any change in power consumption. I wonder if I should blame VIA for eating extra power when the disk is plugged, or thank them for saving power when it's not. Or maybe I am looking at things the wrong way, and I should thank AMD for saving more power in C3 than Intel does? -- Jean Delvare - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
