Re: Hungry for hardware timers

Jaswinder Singh Rajput
2010-09-07T08:23:32Z

Hello,

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.

>> Is there any help available for it so that systems developers can use
>> a hook up an interrupt and use any hardware timer.
>
> The kernel is supposed to abstract away the hardware; just use POSIX
> timers.
>
>> In APIC timer, but each core is having only one timer and it is
>> already utilized by Linux :
>> ... APIC TMICT: 00002078
>> ... APIC TMCCT: 00000b5f
>> ... APIC TDCR: 00000003
>
> Having per-CPU timers allows the kernel to avoid synchronizing between
> CPUs.
>
>> HPET have 3 timers :
>> [    0.328157] hpet: ID: 0x8086a201, PERIOD: 0x429b17f
>> [    0.328315] hpet: CFG: 0x3, STATUS: 0x0
>> [    0.328472] hpet: COUNTER_l: 0x6ff120, COUNTER_h: 0x0
>> [    0.329006] hpet: T0: CFG_l: 0x138, CFG_h: 0xf00000
>> [    0.329165] hpet: T0: CMP_l: 0x701baa, CMP_h: 0x0
>> [    0.329324] hpet: T0 ROUTE_l: 0x0, ROUTE_h: 0x0
>> [    0.329483] hpet: T1: CFG_l: 0x0, CFG_h: 0xf00000
>> [    0.330006] hpet: T1: CMP_l: 0xffffffff, CMP_h: 0x0
>> [    0.330166] hpet: T1 ROUTE_l: 0x0, ROUTE_h: 0x0
>> [    0.331005] hpet: T2: CFG_l: 0x0, CFG_h: 0xf00800
>> [    0.331168] hpet: T2: CMP_l: 0xdf751c, CMP_h: 0x0
>> [    0.331328] hpet: T2 ROUTE_l: 0x0, ROUTE_h: 0x0
>>
>> 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.

> 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
NMI: 0 0 Non-maskable interrupts
LOC: 164087 274764 Local timer interrupts
SPU: 0 0 Spurious interrupts
PMI: 0 0 Performance monitoring interrupts
PND: 0 0 Performance pending work
RES: 56164 64218 Rescheduling interrupts
CAL: 650 484 Function call interrupts
TLB: 1165 625 TLB shootdowns
TRM: 0 0 Thermal event interrupts
THR: 0 0 Threshold APIC interrupts
MCE: 0 0 Machine check exceptions
MCP: 2 2 Machine check polls
ERR: 1
MIS: 0
$

>> System 8254 timer have 3 timers but it seems it is also used by Linux :
>> [ 4923.510233] 0: 83ae 1: 1102 2: 37f2
>> [ 4923.510251] 0: 8382 1: d10 2: 37dc
>
> That thing is horribly slow; nobody would want to use this, if possible.
>

As you seen timers are running may be due to these code :
arch/x86/kernel/apm_32.c: outb_pit(0x34, PIT_MODE);
/* binary, mode 2, LSB/MSB, ch 0 */
arch/x86/kernel/i8253.c: outb_pit(0x34, PIT_MODE);
arch/x86/kernel/i8253.c: outb_pit(0x30, PIT_MODE);
arch/x86/kernel/i8253.c: outb_pit(0x38, PIT_MODE);
arch/x86/kernel/i8253.c: outb_pit(0x00, PIT_MODE); /*
latch the count ASAP */
arch/x86/kernel/i8253.c: outb_pit(0x34, PIT_MODE);
arch/x86/kernel/vmiclock_32.c: outb_pit(0x3a, PIT_MODE); /* binary,
mode 5, LSB/MSB, ch 0 */

It is also used secretly :
arch/x86/include/asm/mach_timer.h: outb(0xb0, 0x43);
/* binary, mode 0, LSB/MSB, Ch 2 */
arch/x86/kernel/acpi/realmode/wakemain.c: outb(0xb6,
0x43); /* Ctr 2, squarewave, load, binary */
arch/x86/kernel/tsc.c: outb(0xb0, 0x43);
arch/x86/kernel/tsc.c: outb(0xb0, 0x43);

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