linux-kernel - Re: RTC_DRV_CMOS can break userspace interface

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Message-Id: <200705271836.55800.david-b@pacbell.net>
Date:	Sun, 27 May 2007 18:36:51 -0700
From:	David Brownell <david-b@...bell.net>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: RTC_DRV_CMOS can break userspace interface

On Sunday 27 May 2007, Matthew Garrett wrote:
> f5f72b46c349fefcfd4421b2213c6ffb324c5e56 appears to break the userspace 
> interface to the CMOS alarm. This could previously be accessed via 
> /proc/acpi/alarm ...

I was a bit surprised the ACPI team didn't have more comments on
that issue, myself.  Thing is, all of /proc/acpi/* is deprecated
(scheduled for removal in barely over one month!) and nobody had
found any actual users of that "alarm" file when they searched for
them a while ago.  I suppose the conclusion then was that there
are no applications using it.

> I'm not actually sure why this is the case. It doesn't look like the two 
> interfaces are fundamentally incompatible.

ISTR the issue is that ACPI only allows one chunk of code to hook
into the relevant notifications.  So:  either /proc/acpi/wakeup;
or /sys/class/rtc/rtc0/wakealarm; but not both.

> I agree that removing the  
> proc code is a good long-term aim, but it'd be nice to be able to test 
> the new RTC code without removing existing functionality.

Coexistence is unfortunately problematic here.  And with "long term"
documented to be a bit over a month ... I guess all I can say is
that if you can come up with a good patch to make both available,
please do so.

- Dave
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