On Fri, 25 Sep 2009, Mikael Pettersson wrote: > Alan Stern writes: > > On Thu, 24 Sep 2009, Mikael Pettersson wrote: > > > > > I'm trying to tune a tickless 2.6.31 kernel for an ARM box > > > (arch/arm/mach-iop32x/n2100.c). Looking at /proc/timer_stats > > > I see that > > > > > > usb_hcd_poll_rh_status (rh_timer_func) > > > > > > is now the #1 cause of timeouts, which prevents the tickless > > > kernel from extending the periods between timer interrupts. > > > > > > The machine has VIA EHCI and UHCI controllers. The polling timer > > > appears if UHCI_HCD is enabled; enabling only EHCI_HCD does not > > > case the timer to appear. > > > > What USB devices are on the UHCI buses? > > There are no devices on the UHCI buses. > > > > Is a polling timer really necessary? If it is, why? UHCI bug or > > > kernel limitation/shortcut? > > > > It is necessary if the the bus is not suspended. This is a UHCI design > > limitation, not a bug and not a kernel limitation. > > > > Even if the bus is suspended, the timer sometimes is necessary. This > > is because of bugs in some UHCI implementations and bugs in some > > motherboard designs. However these bugs are specific to Intel and > > Genesys Logic controllers and Asus motherboards, so they shouldn't > > affect you. > > So an empty bus should be suspended, and thus not need the timer? It should be. Unless you have turned off the "wakeup" attribute under /sys/bus/usb/devices/usbN/power. If you want to get more information about what's happening, build a kernel with CONFIG_USB_DEBUG enabled and see what shows up in the dmesg log. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
