linux-kernel - Re: [PATCH 0/4] PM: Do not destroy/create devices while suspended (rev. 2)

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Date:	Wed, 02 Jan 2008 09:26:25 -0800
From:	David Brownell <david-b@...bell.net>
To:	rjw@...k.pl, mingo@...e.hu
Cc:	a.zummo@...ertech.it, stern@...land.harvard.edu, pavel@...e.cz,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org, lenb@...nel.org, gregkh@...e.de,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 0/4] PM: Do not destroy/create devices while suspended 
 (rev. 2)

(Alessandro Zummo Cc:-ed too -- RTC subsystem maintainer)

> * Rafael J. Wysocki <rjw@...k.pl> wrote:
>
> > Well, we have the following test script in the userland suspend 
> > package that is supposed to work right now:
> > 
> > #!/bin/bash
> > date
> > cd /sys/class/rtc/rtc0
> > echo $(( $(cat since_epoch) + 20 )) > wakealarm
> > s2ram
> > date
> > 
> > provided that the new rtc driver code is compiled (and the old one is not).

Eventually, swapping driver modules ought to work too.  The
legacy /proc/acpi/wakeup files would ISTR cause problems in
current code.

Of course, one reason to want to use the RTC framework code
is to stop depending on x86-isms like ACPI or "s2ram", and
thus to work on more Linux platforms.  ;)

> ok, will try that. A stupid question. The old RTC driver is in 
> drivers/char/rtc.c, and maps to:
>
>   crw-r--r-- 1 root root  10, 135 Oct 25 18:02 /dev/rtc
>
> the new driver is in drivers/rtc/*, and maps to:
>
>   crw-r--r-- 1 root root 254,   0 Dec 12 02:30 /dev/rtc0
>
> but all the x86 distro boxes i have access to make use of /dev/rtc. 
> There's no symlink set up from /dev/rtc to /dev/rtc0 either.

Current util-linux-ng code uses either RTC device file; and udev
sets up /dev/rtc0 as needed.  (But not /dev/rtc, as I recall...)
Have distros switched away from the old unmaintained util-linux?

>	So it 
> appears to me that the new RTC driver isnt actually utilized on most 
> distributions.

That might be so.  There are some HPET issues, but those show up with
both drivers.

The main other issue I know about which would seem to argue for using
the legacy driver, instead of the RTC framework, is that some BIOSes
don't seem to provide PNPACPI entries for their RTCs.  I got one report
of a newish HP Opteron system that doesn't.  I have no idea how common
that is.  The drivers/acpi/glue.c code could detect that, but maybe a
better place to address that would be in PNP code; in that case, ISTR
that PNP0c01 claimed the RTC ioports, and so would be the natural place
to make provide a real driver model node for that hardware.

> shouldnt we provide a Kconfig way of replacing dev 10:135 with the new 
> driver's 254:0 device? (while keeping all the current modes of operation 
> as well, of course.)

The major number 254 is not statically allocated, ISTR; that should
be managed only by udev.

> 			 It's all supposed to be 100% ioctl ABI compatible 
> with the old driver, right? That way distros could start migrating to it 
> as well, without depending on any udev hackery.

I don't know of any ioctl differences userspace would care about.

- Dave

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