linux-kernel - Re: [PATCH 1/7] platform/x86: fujitsu-laptop: do not use kfifo for storing hotkey scancodes

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Date:   Sat, 24 Jun 2017 02:25:46 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Darren Hart <dvhart@...radead.org>
Cc:     Michał Kępień <kernel@...pniu.pl>,
        Jonathan Woithe <jwoithe@...t42.net>,
        Andy Shevchenko <andy@...radead.org>,
        platform-driver-x86@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] platform/x86: fujitsu-laptop: do not use kfifo for storing hotkey scancodes

On Wednesday, June 21, 2017 11:15:43 AM Darren Hart wrote:
> On Fri, Jun 16, 2017 at 06:40:52AM +0200, Michał Kępień wrote:
> > All ACPI device notify callbacks are invoked using acpi_os_execute(),
> > which causes the supplied callback to be queued to a static workqueue
> > which always executes on CPU 0.  This means that there is no possibility
> > for any ACPI device notify callback to be concurrently executed on
> > multiple CPUs, which in the case of fujitsu-laptop means that using a
> > locked kfifo for handling hotkeys is redundant: as hotkey scancodes are
> > only pushed and popped from within acpi_fujitsu_laptop_notify(), no risk
> > of concurrent pushing and popping exists.
> 
> Was the kfifo causing a problem currently or for the migration to separate
> modules? Is this purely a simplification?
> 
> Rafael, the above rationale appears sound to me. Do you have any concerns?

I actually do.

While this is the case today, making the driver code depend on it in a hard way
sort of makes it difficult to change in the future if need be.

Thanks,
Rafael