RE: [PATCH V3] POWER: perf_event: Skip updating kernel counters ifregister value shrinks

["David Laight" <David.Laight@ACULAB.COM>]

=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