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Subject: RE: [PATCH V3] POWER: perf_event: Skip updating kernel counters
	ifregister value shrinks
Date: Fri, 8 Apr 2011 09:38:49 +0100
Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6D8ACE8@saturn3.aculab.com>
From: "David Laight" <David.Laight@ACULAB.COM>
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=20
> +	u64 delta =3D 0;
...
> +	if (((prev & 0x80000000) && !(val & 0x80000000)) || (val >
prev))
> +		delta =3D (val - prev) & 0xfffffffful;
> +
> +	return delta;

The above is incorrect modulo arithmetic.

It is probably intended to do:
    s32 delta =3D val - prev;
    return delta < 0 ? 0 : delta;
which will just ignore the fact that some counts are rolled back.

More accurate would be:
    static u64 val64;
    u32 val =3D read_perf_count();
    s32 delta =3D val - val_64;
    if (delta < 0)
        return;
    val64 +=3D delta;
Which will not double count for rolled back events.
(The low bits of val64 should always match the actual counter.)

	David