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SubjectRe: [PATCH] USB: improve ehci_watchdog's side effect in CPU power management
On Thu, 25 Sep 2008 17:25:44 +0800
Yi Yang <yi.y.yang@intel.com> wrote:

> ehci_watchdog will wake up CPU very frequently so that CPU
> stays at C3 very short, average residence time is about 50
> ms on Aspire One, but we expect it should be about 1 second
> or more, so this kind of periodic timer is very bad for power
> saving.
>
> We can't remove this timer because of some bad USB controller
> chipset, but at least we should reduce its side effect to as
> possible as low.
>
> This patch can make CPU stay at C3 longer, average residence time
> is about twice as long as original.
>
> Please consider to apply it, thanks
>
> Signed-off-by: Yi Yang <yi.y.yang@intel.com>
> ---
> ehci.h | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
> index 5799298..9d530d9 100644
> --- a/drivers/usb/host/ehci.h
> +++ b/drivers/usb/host/ehci.h
> @@ -181,14 +181,16 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
> * the async ring; just the I/O watchdog. Note that if a
> * SHRINK were pending, OFF would never be requested.
> */
> - if (timer_pending(&ehci->watchdog)
> - && ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> - & ehci->actions))
> - return;
> + enum ehci_timer_action oldactions = ehci->actions;
>
> if (!test_and_set_bit (action, &ehci->actions)) {
> unsigned long t;
>
> + if (timer_pending(&ehci->watchdog)
> + && ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> + & oldactions))
> + return;
> +
> switch (action) {
> case TIMER_IO_WATCHDOG:
> t = EHCI_IO_JIFFIES;
> @@ -204,7 +206,7 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
> t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;
> break;
> }
> - mod_timer(&ehci->watchdog, t + jiffies);
> + mod_timer(&ehci->watchdog, round_jiffies(t + jiffies));
> }
> }
>

<looks>

<regrets it>


Why does this:

t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;

add "1000" to a jiffies value when it doesn't know what HZ is? It'll
be adding anywhere from one second up to ten seconds to the timeout
interval depending upon compile-time options.

I suspect s/1000/HZ/ would improve things here. Or just delete it -
doesn't the subsequent round_jiffies() do the same thing, only better?
This code needs help, I suspect.


Also, do we really need to inline this large function into at least
five callsites?



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