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    SubjectRe: HPET : Legacy Routing Replacement Enable - 3rd try.
    Patch 03/03 : Documentation

    Signed-Off-by : Om Narasimhan <>

    Documentation/hpet.txt | 56 +++++++++++++++++++++++++++++++++++
    Documentation/kernel-parameters.txt | 9 ++++++
    2 files changed, 65 insertions(+), 0 deletions(-)

    diff --git a/Documentation/hpet.txt b/Documentation/hpet.txt
    index b7a3dc3..0b1b9d9 100644
    --- a/Documentation/hpet.txt
    +++ b/Documentation/hpet.txt
    @@ -298,3 +298,59 @@ members of the hpet_task structure befor
    hpet_control simply vectors to the hpet_ioctl routine and has the same
    commands and respective arguments as the user API. hpet_unregister
    is used to terminate usage of the HPET timer reserved by hpet_register.
    + HPET Legacy Replacement Route option (hpet_lrr)
    +HPET is capable of replacing the IRQ0 (connected INT0 PIN) routing for
    +timer interrupt. The capability register (at offset 0 of HPET
    +base address) has a bit specifying if HPET chip is capbale of doing
    +this. OS can read the bit either from HW or ACPI table. (HPET ACPI
    +description table -> Event Timer block -> bit 15, page 30 of HPET
    +spec). Ideally (I think so!) BIOS should set the ACPI table than letting
    +the OS read H/W, which gives the BIOS a way to configure either legacy
    +or Legacy replacement modes.
    +Typically the motherboard has BIOS configured / hardwired IRQ0 to INT0
    +(pin of APIC) connection. Linux assumes IRQ0 connected to INT0 unless it is
    +supplied using an override parameter in the MPTable. Some NVidia chipsets /
    +BIOS initialization code had configured to override IRQ0 -> INT0 connection
    +and later a parameter was introduce (acpi_skip_timer_override) to get IRQ0 ->
    +INT0 connection right.
    +But a number of bioses (both phoenix and AMI) are not working as
    +expected. (I have an AMI BIOS which sets ACPI table bit 15 to 0 and then
    +connect IRQ0 -> INT2 internally, Another bios I have sets the ACPI table bit
    +15 to 0, but does not connect IRQ0 -> INT2. Both would result in a hang in
    +calibrate_delay() since there would not be any timer interrupts So I have
    +provided a command line parameter which overrides
    +the BIOS ACPI entry. So, irrespctive of the BIOS' HPET ACPI Descriptor
    +table settings, if the parameter hpet_lrr=[0,1] is specified, it takes
    +* When to use this parameter?
    +Some latest versions CK-804 (e.g),(Actually the code initializes the
    +CK804 in the BIOS), would correctly set the HPET such that there would not
    +be any interrupts on INT0. Linux does not handle this situation very well
    +because in linux, if HW is LRR capable, it is enabled from the OS. Still the
    +timer interrupt handler is setup for IRQ0. Under this situation, you can
    +force the parameter hpet_lrr=1, so that IRQ2 is timer interrupts.
    +[root@mophia ~]# cat /proc/interrupts | grep 2:
    + 2: 163 0 0 0 1 7
    +207 955341 IO-APIC-edge timer
    +[root@mophia ~]# uptime
    + 22:52:38 up 15 min, 2 users, load average: 0.00, 0.01, 0.02
    +[root@mophia ~]# dmesg | grep -i MP-BIOS
    +For 15 mts (900 sec), around 95k interrupts on timer looks kinda fine.
    +* Known Bugs:
    +I have tested it only with Nvidia CK-804. There seem to be some kind of timing
    +issue between enabling the HPET with LRR set and start of tinerrupts. As a
    +result of which, calibrate_delay() hangs because there are no interrupts. If
    +you run into such a case, pass lpj=<bogomips * 500> as a work around. i.e, if
    +your bogomips is 5000, pass lpj=2500000
    diff --git a/Documentation/kernel-parameters.txt
    index ff571f9..36e469c 100644
    --- a/Documentation/kernel-parameters.txt
    +++ b/Documentation/kernel-parameters.txt
    @@ -366,6 +366,15 @@ and is between 256 and 4096 characters.
    hpet= [IA-32,HPET] option to disable HPET and use PIT.
    Format: disable

    + hpet_lrr= [IA32,X86_64,HPET] Option to enable/disable the HPET
    + Legacy replacement route. Please read Documentation/
    + hpet.txt for more info.
    + Format : {"0" | "1"}
    + 0 -> Disables Legacy Route Replacement. (Default)
    + 1 -> Enables LRR. Please consult your BIOS
    + documentation before doing this.
    cm206= [HW,CD]
    Format: { auto | [<io>,][<irq>] }
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      Last update: 2006-10-25 22:17    [W:0.023 / U:4.860 seconds]
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