[mythtv-users] udev permissions on /dev/rtc

Tom Dexter digitalaudiorock at gmail.com
Thu Mar 27 04:43:44 UTC 2014

On Sat, Apr 7, 2012 at 1:46 PM, Tom Dexter <digitalaudiorock at gmail.com> wrote:
> I figured I'd post a heads up on this, as others may have had this
> happen an aren't aware of it.
> My frontend is running Gentoo (a 3.2.1 kernel and udev sys-fs/udev-171-r5).
> I finally got around to replacing my old nVidia 7100 card with a GT
> 430 and started using VDPAU high quality deinterlacing
> etc...wow...should have done that a long time ago.  The 1080i quality
> is so much better.
> Doing this got me looking at all my playback settings which I hadn't
> touched in just about forever.  In doing so I realized that it was
> using usleep for video sync, and I knew I'd been set up properly for
> RTC a long time ago.
> Apparently, and I'm not sure when, most likely as a result of a udev
> update, my /dev/rtc was getting created readable only to root.  It may
> have been like this for a very long time.  In order to correct it I
> had to add a udev rule like this:
> KERNEL=="rtc|rtc0", MODE="0664"
> In my case there's only /dev/rtc, but apparently in some cases there
> can be rtc0.  For those using RTC, it may be worth checking if you
> still actually are.
> Tom

I'm replying to this old email of mine, mostly as a heads up to anyone
else who might run into this:

Today I discovered *again* that the whole rtc thing had thrown me yet
another curve ball.  I had this in my /etc/sysctl.conf:

dev.rtc.max-user-freq = 1024

...as per many wiki entries etc.  I just discovered today that,
apparently in recent kernels, that no longer works:

sysctl dev.rtc.max-user-freq=1024
sysctl: cannot stat /proc/sys/dev/rtc/max-user-freq: No such file or directory

Apparently since it's no longer under that path, and is now at
/sys/class/rtc/rtc0/max_user_freq, is can no longer be set with sysctl
or via sysctl.conf.  Instead I had to add this:

echo 1024 > /sys/class/rtc/rtc0/max_user_freq

...to the boot process.  On most systems that would be by adding it to
/etc/rc.local or the like.  On Gentoo I added a new script named
rtc.start to the /etc/local.d directory containing that command.

It all makes me wonder what the next thing will be that breaks this stuff...


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