RE: Variable ticks

["Brown, Len" <len.brown@intel.com>]

>>>Trouble? Why would USB do DMA unless there was a device activity?
>> 
>> 
>> look here:
>> http://www.google.com/search?hl=en&q=usb+selective+suspend
>> 
>> Linux is working on it too, but it is in development.
>
>Somehow I didn't ask that right... The stuff on selective disable is 
>interesting, but my question is why a USB device, call it a keyboard, 
>would do DMA unless I press a key, at which point response will be 
>better if the CPU wakes up out of C3.
>
>I understand that specialty attachments may send what amounts to keep 
>alives, or gather data (webcam) you don't want, but the typical mouse 
>and KB would seem to be things which generate DMA at user initiation, 
>and would not be blocked for low power, only for suspend.

USB's hardware architecture was optimized for cost and ease of use,
it was not optimized for performance or power savings.

We need software disable because the USB hardware will keep
the memory bus busy constantly even if all the
actual USB devices are idle.  The power cost is that the CPU
is prevented from deep sleep so that it can snoop USB's
(idle) memory activity.

-Len
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