linux-kernel - Re: [PATCH] acpi: battery: Always read fresh battery state on update

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Date:   Fri, 5 Jun 2020 12:01:52 -0600
From:   Mat King <mathewk@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Mathew King <mathewk@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH] acpi: battery: Always read fresh battery state on update

On Fri, Jun 5, 2020 at 5:30 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Jun 4, 2020 at 9:57 PM Mathew King <mathewk@...omium.org> wrote:
> >
> > When the ACPI battery receives a notification event it should always
> > read the battery state fresh from the ACPI device and not use the cached
> > state.
>
> Why should it?

According to the ACPI Spec 10.2.1 Battery Events, "When the present
state of the battery has changed or when the trip point set by the
_BTP control method is reached or crossed, the hardware will assert a
general purpose event." So when this event is received we should
assume that the cached state of the battery is no longer valid

>
> > Currently the cached state stays valid and the new state may not
> > be read when a notification occurs. This can lead to a udev event
> > showing that the battery has changed but the sysfs state will still have
> > the cached state values.
>
> Is there a bug entry or similar related to that which can be referred
> to from this patch?

No, I discovered this issue while working on an internal issue where
it was observed that udev events generated when a battery changed did
not accurately reflect the state of the battery. I initially suspected
that the EC may not be updating its state before generating the ACPI
event, however after much debugging I discovered that the battery
driver was caching the state and the state is not always immediately
updated when the event is received. If there is a more formal process
to discuss the issue I will work through that process.

>
> > This change invalidates the update time forcing
> > the state to be updated before notifying the power_supply subsystem of
> > the change.
> >
> > Signed-off-by: Mathew King <mathewk@...omium.org>
> > ---
> >  drivers/acpi/battery.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/acpi/battery.c b/drivers/acpi/battery.c
> > index 366c389175d8..ab7fa4879fbe 100644
> > --- a/drivers/acpi/battery.c
> > +++ b/drivers/acpi/battery.c
> > @@ -981,6 +981,7 @@ static int acpi_battery_update(struct acpi_battery *battery, bool resume)
> >                 acpi_battery_init_alarm(battery);
> >         }
> >
> > +       battery->update_time = 0;
>
> AFAICS this is equivalent to dropping battery->update_time altogether.
> Isn't that a bit too excessive?

It is not the same as dropping the update_time. The cached state is
still used when acpi_battery_get_property() is called which happens
anytime userspace accesses the sysfs properties it is also what is
called by the power_supply subsystem when creating the environment for
the udev events. In those cases the cache still works and makes sense.
The acpi_battery_update() function is only called in a handful of
cases and in all of these cases reading the battery state fresh makes
sense to me. Those cases are:

1. When the battery is added with acpi_battery_add(), this case the
update_time is already cleared
2. On system resume with acpi_battery_resume(), in this case
update_time is cleared before calling acpi_battery_update() so that
static battery info is also updated by calling acpi_battery_get_info()
3. The acpi_battery_update() is called from procfs power functions
which should not be called a frequency where reading fresh battery
state from ACPI will have a performance impact
4. Finally it is called from acpi_battery_notify() when a battery
event is received from firmware that the state has changed

I considered clearing the update_time in acpi_battery_notify() before
acpi_battery_update() is called but if I did that by itself then
acpi_battery_get_info() would also get called and I wasn't sure that
behavior would be wanted. So invalidating the cache where I did seemed
to be the least disruptive way to fix the problem. I can see
opportunities to refactor this driver and I felt that this fix was
acceptable until a refactor could be done.

>
> >         result = acpi_battery_get_state(battery);
> >         if (result)
> >                 return result;
> > --