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x86: HPET: read back compare register before reading counter

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After fixing the u32 thinko I sill had occasional hickups on ATI chipsets
with small deltas. There seems to be a delay between writing the compare
register and the transffer to the internal register which triggers the
interrupt. Reading back the value makes sure, that it hit the internal
match register befor we compare against the counter value.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2008-09-06 03:06:08 +02:00
parent f7676254f1
commit 72d43d9bc9
1 changed files with 7 additions and 0 deletions
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7
arch/x86/kernel/hpet.c
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@ -278,6 +278,13 @@ static int hpet_legacy_next_event(unsigned long delta,
cnt += (u32) delta;
hpet_writel(cnt, HPET_T0_CMP);
/*
* We need to read back the CMP register to make sure that
* what we wrote hit the chip before we compare it to the
* counter.
*/
WARN_ON((u32)hpet_readl(HPET_T0_CMP) != cnt);
return (s32)((u32)hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
}