Re: uhci vs ehci: 5 wakeups/s saved

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On Wed, Oct 03, 2007 at 02:29:49PM +0100, Alan Jenkins wrote:
> My internet arrives via a USB 2.0 wireless adaptor.
> Using UHCI instead of EHCI saves ~5 wakeup/s:
> With ehci_hcd loaded (and not uhci_hcd):
>   31.1% ( 10.0)     <kernel core> : ehci_work (ehci_watchdog)
>   30.4% (  9.8)       <interrupt> : ehci_hcd:usb5
> With uhci_hcd loaded (and not ehci_hcd):
>   48.0% (  9.8)       <interrupt> : uhci_hcd:usb1
>   19.6% (  4.0)   <kernel module> : usb_hcd_poll_rh_status (rh_timer_func)
> Since ehci_hcd also disagrees with my MP3 player, I'm not sure I want 
> USB 2.0 any more ;-).  I may end up blacklisting the module.

How does it "disagree" with your MP3 player?

> I was suprised, since I've read[1] and seem to remember verifying that 
> UHCI causes a constant 1000 wakeups/s, and that this was unavoidable 
> given the UHCI specification.  I'm running a 2.6.23-rc9-hrt1 kernel 
> (-rc8-hrt1 patch applied cleanly to -rc9).
> Is this expected?
> Could it be USB autosuspend at work?  I've checked and "echo -1 > 
> /sys/module/usbcoreparamters/autosuspend" doesn't change the number of 
> wake-ups, but perhaps I'm missing something.

It's unlikely that your USB wireless adaptor is being autosuspended.  I
don't think there's any kernel support for wireless adaptors.  There is
autosuspend support for USB to ethernet adaptors that run under the
kaweth driver.  Without kernel support, the device won't autosuspend.

> I notice that the number of interrupts is actually the same.   The 
> difference is in the kernel polling routines - ehci_work vs 
> usb_hcd_poll_rh_status.  Does ehci need to poll twice as often, or might 
> it be possible to tune or hack the polling interval?

I haven't read the uhci spec or the code in detail, so I can't give you
an easy answer.  Perhaps someone on the linux-usb-users list can help.

> [1]

I do wonder about their claim that the 1000 wakeups per second is
inherent to the UHCI spec.

Sarah Sharp

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