Re: Hungry for hardware timers

Clemens Ladisch
2010-09-07T09:04:53Z

Jaswinder Singh Rajput wrote:
> On Tue, Sep 7, 2010 at 1:29 PM, Clemens Ladisch <clemens@ladisch.de> wrote:
>> Jaswinder Singh Rajput wrote:
>>> I am investigating how many hardware timers are available for kernel,
>>> system applications and user applications for x86 platforms.
>>
>> Why would you want to have a separate timer for your application?
>
> I need a programmable periodic interrupt for an embedded project.

And why do you not want to use POSIX timers for this?

>>> HPET have 3 timers :
>>> T0 and T2 is already used by Linux and T1 is used for RTC
>>
>> T2 shouldn't be used.
>
> [ 0.323613] hpet: hpet_msi_capability_lookup(621):
> ..
> [ 0.325323] hpet: T2: CFG_l: 0x0, CFG_h: 0xf00800
> [ 0.325483] hpet: T2: CMP_l: 0xffffffff, CMP_h: 0x0
> [ 0.325639] hpet: T2 ROUTE_l: 0x0, ROUTE_h: 0x0
> [ 0.325958] hpet0: at MMIO 0xfed00000, IRQs 2, 8, 0
> [ 0.326279] hpet0: 3 comparators, 64-bit 14.318180 MHz counter
> [ 0.328003] hpet: hpet_late_init(951):
> ..
> [ 0.331006] hpet: T2: CFG_l: 0x0, CFG_h: 0xf00800
> [ 0.331167] hpet: T2: CMP_l: 0xdfe12a, CMP_h: 0x0
> [ 0.331327] hpet: T2 ROUTE_l: 0x0, ROUTE_h: 0x0
>
> As you can see after hpet_late_init(951) T2 CMP_l is changed.

Strange; nobody should be accessing this.
Do you need to use it before hpet_late_init?

>> What does /proc/interrupts say?
>
> $ cat /proc/interrupts
> CPU0 CPU1
> 0: 192286 0 IO-APIC-edge timer
> 1: 1039 0 IO-APIC-edge i8042
> 8: 50 0 IO-APIC-edge rtc0
> 9: 637 1903 IO-APIC-fasteoi acpi
> 12: 163 515 IO-APIC-edge i8042
> 16: 21285 0 IO-APIC-fasteoi i915, ath9k, ehci_hcd:usb1, uhci_hcd:usb2
> 17: 0 0 IO-APIC-fasteoi uhci_hcd:usb3
> 18: 0 0 IO-APIC-fasteoi uhci_hcd:usb4
> 19: 0 0 IO-APIC-fasteoi uhci_hcd:usb5
> 44: 3511 9569 PCI-MSI-edge ahci
> 45: 515 1569 PCI-MSI-edge hda_intel
> 46: 2 0 PCI-MSI-edge eth0

Nobody is using /dev/hpet, you should be able to grab it for T2.

> By the way why I am getting this :
>
> [ 568.301571] CE: hpet increased min_delta_ns to 7500 nsec
> [ 568.301736] CE: hpet increased min_delta_ns to 11250 nsec
> [ 568.301888] CE: hpet increased min_delta_ns to 16875 nsec

On my AMD machine, this happens only if I have C1E enabled, and only up
to 11250 ns. It seems your BIOS has installed a SMI.

Regards,
Clemens