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    Subject[054/139] x86, hpet: Erratum workaround for read after write of HPET comparator
    2.6.33-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Pallipadi, Venkatesh <venkatesh.pallipadi@intel.com>

    commit 8da854cb02156c90028233ae1e85ce46a1d3f82c upstream.

    On Wed, Feb 24, 2010 at 03:37:04PM -0800, Justin Piszcz wrote:
    > Hello,
    >
    > Again, on the Intel DP55KG board:
    >
    > # uname -a
    > Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
    >
    > [ 1.237600] ------------[ cut here ]------------
    > [ 1.237890] WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80()
    > [ 1.238221] Hardware name:
    > [ 1.238504] hpet: compare register read back failed.
    > [ 1.238793] Modules linked in:
    > [ 1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
    > [ 1.239605] Call Trace:
    > [ 1.239886] <IRQ> [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
    > [ 1.240409] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
    > [ 1.240699] [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
    > [ 1.240992] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
    > [ 1.241281] [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
    > [ 1.241573] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
    > [ 1.241859] [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
    > [ 1.246533] [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
    > [ 1.246826] [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
    > [ 1.247118] [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
    > [ 1.247407] [<ffffffff81029f55>] ? handle_irq+0x15/0x20
    > [ 1.247689] [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
    > [ 1.247976] [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
    > [ 1.248262] <EOI> [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
    > [ 1.248796] [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
    > [ 1.249080] ---[ end trace db7f668fb6fef4e1 ]---
    >
    > Is this something Intel has to fix or is it a bug in the kernel?

    This is a chipset erratum.

    Thomas: You mentioned we can retain this check only for known-buggy and
    hpet debug kind of options. But here is the simple workaround patch for
    this particular erratum.

    Some chipsets have a erratum due to which read immediately following a
    write of HPET comparator returns old comparator value instead of most
    recently written value.

    Erratum 15 in
    "Intel I/O Controller Hub 9 (ICH9) Family Specification Update"
    (http://www.intel.com/assets/pdf/specupdate/316973.pdf)

    Workaround for the errata is to read the comparator twice if the first
    one fails.

    Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    LKML-Reference: <20100225185348.GA9674@linux-os.sc.intel.com>
    Signed-off-by: H. Peter Anvin <hpa@zytor.com>
    Cc: Venkatesh Pallipadi <venkatesh.pallipadi@gmail.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    arch/x86/kernel/hpet.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/arch/x86/kernel/hpet.c
    +++ b/arch/x86/kernel/hpet.c
    @@ -399,9 +399,15 @@ static int hpet_next_event(unsigned long
    * then we might have a real hardware problem. We can not do
    * much about it here, but at least alert the user/admin with
    * a prominent warning.
    + * An erratum on some chipsets (ICH9,..), results in comparator read
    + * immediately following a write returning old value. Workaround
    + * for this is to read this value second time, when first
    + * read returns old value.
    */
    - WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
    + if (unlikely((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt)) {
    + WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
    KERN_WARNING "hpet: compare register read back failed.\n");
    + }

    return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
    }



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