x86: HPET: read back compare register before reading counter

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>

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 801497a16e0e487e565b29b2c88e3e55bfe6c086..73deaffadd036a578984bb8ad6e0455f1044558a 100644 (file)
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -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;
 }