Date: Wed, 17 Nov 2004 13:22:33 -0800 From: Nate Lawson <nate@root.org> To: Andrey Smirnov <smir@delit.net> Cc: acpi@FreeBSD.org Subject: Re: Why C3 state isn't entered? Message-ID: <419BC119.4040209@root.org> In-Reply-To: <419BBFAA.3000605@delit.net> References: <419BB14C.1060301@delit.net> <419BB438.3040906@root.org> <419BBFAA.3000605@delit.net>
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Andrey Smirnov wrote: > Nate Lawson wrote: >> This is a FAQ, perhaps we can add it to the handbook entry or man page: >> >> C3 can't be entered while bus masters are active. Typical bus >> mastering devices include USB and sound. Since USB constantly polls >> when loaded, disabling USB support or stopping sound playback should >> allow C3 to be used. A better fix would be to power down the USB host >> controller when idle. >> >> -Nate >> > Thanks for the quick answer, so if I use USB mouse I won't be able to > use C3 state? Is there a way to disable USB polling, if I know I won't > connect new devices, or it is required for normal USB operation? > As I understand it, C3 would give me much more power savings, so it will > extend battery life? (If the effect is less than 10%, it isn't very > important to me). If you have the USB driver (usb.ko) loaded or compiled into the kernel, you can't use C3. The way to disable it is to implement support for idling ports in uhci, ehci, and ohci. C3 doesn't make a huge difference (2-5%?) compared to C2 although it does help. I did some profiling of this a while back and found that the top three power saving features are dimming the display (by far the most), changing CPU frequency (similar but definitely less), and C2/C3 (better than C1 but not nearly as much savings as the first two). -- Nate
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