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Date: Tue, 12 Jul 2011 23:34:40 +0300
Message-ID: <CAL409KwV0MYBv-Ju_2p_rx23P7TER6C3D+dVyvbfeg+3bYFQwg@mail.gmail.com>
From: Vitaly Magerya <vmagerya@gmail.com>
To: Andriy Gapon <avg@freebsd.org>
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Cc: freebsd-acpi@freebsd.org
Subject: Re: (Missing) power states of an Atom N455-based netbook
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Andriy Gapon <avg@freebsd.org> wrote:
> on 11/07/2011 19:07 Vitaly Magerya said the following:
>> Can you elaborate? From my reading, the only place cpu_cx_lowest
>> is used is in acpi_cpu_notify, where sc->cpu_cx_lowest is optionally
>> increased to min(cpu_cx_lowest, sc->cpu_cx_count - 1), which should
>> be safe in any situation.
>
> This is exactly the place that I am concerned about.
> Probably my mind is clouded but I can not understand why
> acpi_cpu_set_cx_lowest()
> call is under the condition:
>     if (sc->cpu_cx_lowest < cpu_cx_lowest)
>         acpi_cpu_set_cx_lowest(sc, min(cpu_cx_lowest, sc->cpu_cx_count - 1));
>
> If you or someone else can explain to me why that condition is there...

Now that I think about it, yes it does look like a bug. Here's how to
trigger it:

1. Kill devd.
2. Make sure you've got C3, set hw.acpi.cpu.cx_lowest to C3.
3. Plug the power cord in (only C2 is now reported).
4. Look at dev.cpu.N.cx_lowest: it's still C3, even though
   dev.cpu.N.cx_supported only has C1 and C2.

This happens because normally sc->cpu_cx_lowest == cpu_cx_lowest,
so the update isn't executed, and sc->cpu_cx_lowest remains as
before.

In short, that update should probably be executed unconditionally.