linux-kernel - Re: [PATCH v3] dell-rbtn: Ignore ACPI notifications if device is suspended

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Date:	Mon, 28 Mar 2016 19:58:09 +0200
From:	Gabriele Mazzotta <gabriele.mzt@...il.com>
To:	Darren Hart <dvhart@...radead.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Pali Rohár <pali.rohar@...il.com>,
	"D. Jared Dominguez" <Jared_Dominguez@...l.com>,
	"platform-driver-x86@...r.kernel.org" 
	<platform-driver-x86@...r.kernel.org>,
	Alex Hung <alex.hung@...onical.com>,
	Andrei Borzenkov <arvidjaar@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] dell-rbtn: Ignore ACPI notifications if device is suspended

2016-03-28 19:33 GMT+02:00 Darren Hart <dvhart@...radead.org>:
> On Thu, Mar 24, 2016 at 12:24:56PM +0100, Gabriele Mazzotta wrote:
>> 2016-03-24 10:39 GMT+01:00 Pali Rohár <pali.rohar@...il.com>:
>> > On Monday 21 March 2016 16:13:34 Gabriele Mazzotta wrote:
>> >> 2016-03-21 13:17 GMT+01:00 Pali Rohár <pali.rohar@...il.com>:
>> >> > On Friday 18 March 2016 23:44:23 Gabriele Mazzotta wrote:
>> >> >> +#ifdef CONFIG_PM_SLEEP
>> >> >> +static void ACPI_SYSTEM_XFACE rbtn_acpi_clear_flag(void *context)
>> >> >> +{
>> >> >> +     struct rbtn_data *rbtn_data = context;
>> >> >> +
>> >> >> +     rbtn_data->suspended = false;
>> >> >> +}
>> >> >> +
>> >> >> +static int rbtn_suspend(struct device *dev)
>> >> >> +{
>> >> >> +     struct acpi_device *device = to_acpi_device(dev);
>> >> >> +     struct rbtn_data *rbtn_data = acpi_driver_data(device);
>> >> >> +
>> >> >> +     rbtn_data->suspended = true;
>> >> >> +
>> >> >> +     return 0;
>> >> >> +}
>> >> >> +
>> >> >> +static int rbtn_resume(struct device *dev)
>> >> >> +{
>> >> >> +     struct acpi_device *device = to_acpi_device(dev);
>> >> >> +     struct rbtn_data *rbtn_data = acpi_driver_data(device);
>> >> >> +     acpi_status status;
>> >> >> +
>> >> >> +     /*
>> >> >> +      * Clear the flag only after we received the extra
>> >> >> +      * ACPI notification.
>> >> >> +      */
>> >> >> +     status = acpi_os_execute(OSL_NOTIFY_HANDLER,
>> >> >> +                      rbtn_acpi_clear_flag, rbtn_data);
>> >> >> +     if (ACPI_FAILURE(status))
>> >> >> +             rbtn_data->suspended = false;
>> >> >
>> >> > I case when acpi_os_execute success it calls rbtn_acpi_clear_flag,
>> >> > right? And that will set suspended to false. When acpi_os_execute fails,
>> >> > then it set suspended too to false... Then whole acpi_os_execute doing
>> >> > just "barrier" after which suspended flag can be set to false. So I
>> >> > think rbtn_acpi_clear_flag function is not needed here.
>> >> >
>> >> > Cannot you pass NULL or empty function pointer as callback? Or what was
>> >> > reason to do that flag clearing at "two places"?
>> >>
>> >> acpi_os_execute doesn't wait for the callback to be executed, so
>> >> I can't clear the flag from rbtn_resume.
>> >
>> > acpi_os_execute calls callback asynchronously later? Or what exactly do it?
>>
>> In this case, it adds the callback to the kacpi_notify_wq workqueue
>> for deferred execution.
>
> +Rafael for context/advice on the use of acpi_os_execute here.
>
> This is true, but a quick scan through that call path doesn't tell me why we
> would need to call it here instead of just setting rbtn_data->suspended = false.
> The comment suggests waiting for the event, but is that what this is doing? It
> appears to me to be immediately scheduling the function to a work queue, not
> waiting for the event notifier.
>
> Also, since there is no indication to the user that a failure occurs, this
> function is basically equivalent in the success and failure case (the success
> case is just slower).
>
> Am I missing something critical here?

Maybe saying that we are waiting for the extra event is not really
correct. Since the extra ACPI notification is processed by means
of kacpi_notify_wq, or at least that's my understanding, our callback
is likely going to be executed after we received the extra ACPI
notification. This was suggested by Rafael [2].

The problem with setting the flag directly from the resume callback
is that the extra notification might arrive after we cleared the
flag, causing spurious input events [1].

[1] http://article.gmane.org/gmane.linux.drivers.platform.x86.devel/8001
[2] http://article.gmane.org/gmane.linux.drivers.platform.x86.devel/8201

>
>>
>> > --
>> > Pali Rohár
>> > pali.rohar@...il.com
>>
>
> --
> Darren Hart
> Intel Open Source Technology Center