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

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Date:   Wed, 28 Jun 2017 09:03:38 -0700
From:   Darren Hart <dvhart@...radead.org>
To:     Michał Kępień <kernel@...pniu.pl>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 Wed, Jun 28, 2017 at 06:30:44AM +0200, Michał Kępień wrote:
> > On Sat, Jun 24, 2017 at 02:25:46AM +0200, Rafael Wysocki wrote:
> > > 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.
> > 
> > OK, if we aren't guaranteed for this to run on CPU 0 in the future, and this
> > will be annoying to debug if it does changes, let's skip the kfifo change.
> > 
> > I have removed this patch, and fixed up the merge conflicts of the remaining 6
> > patches here:
> > 
> > http://git.infradead.org/linux-platform-drivers-x86.git/shortlog/refs/heads/fujitsu
> > 
> > Michal / Jonathan, would you please review and let me know if this is what you
> > would have done / approve the rebase?
> 
> The only issue I can see is that the push/pop debug messages in the last
> patch contain the word "buffer" instead of the original "ringbuffer".
> The dropped kfifo patch changed the wording from "ringbuffer" to
> "buffer" as applying it means there is no ringbuffer any more, but since
> it was not applied in the end, I guess the original wording should stay
> in place.
> 
> The rest looks good to me.

Good catch, thank you. The testing branch has been rebased to reflect these
changes.

-- 
Darren Hart
VMware Open Source Technology Center