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Date: Sat, 21 Jul 2012 08:07:09 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Colin Cross <ccross@android.com>
Cc: lkml <linux-kernel@vger.kernel.org>, Bryan Wu <bryan.wu@canonical.com>,
        Richard Purdie <rpurdie@rpsys.net>
Subject: Re: sysfs permissions on dynamic attributes (led delay_on and
 delay_off)
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On Fri, Jul 20, 2012 at 10:14:27PM -0700, Colin Cross wrote:
> On Fri, Jul 20, 2012 at 9:08 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Fri, Jul 20, 2012 at 05:46:14PM -0700, Colin Cross wrote:
> >> I'm trying to use the standard ledtrig-timer.c code to handle led
> >> blinking for notifications on an Android device, and I'm hitting some
> >> issues with setting permissions on the dynamically created delay_on
> >> and delay_off attributes.  For most sysfs files, we have userspace
> >> uevent parser that watches for device add notifications and
> >> chowns/chmods attributes.  This doesn't work for delay_on and
> >> delay_off, because they are created later, when "timer" is written to
> >> the trigger attribute.  There is no uevent when the new files are
> >> created, and sysfs doesn't support inotify, so I don't see any way to
> >> receive an event to set the permissions.  This issue exists any time
> >> that device_create_file is called after device_add.
> >>
> >> What is the appropriate way to get an event to set the permissions?
> >> Add inotify support for sysfs file creation?  Send a KOBJ_CHANGE
> >> uevent in device_create_file?
> >
> > No.
> >
> >> Send a KOBJ_CHANGE uevent from the driver after calling
> >> device_create_file?
> >
> > Yes.
> >
> >> Dynamically create a timer device under /sys/class/leds/<led> so a new
> >> add uevent gets sent?
> >
> > Ick.
> >
> >> Promote blinking to be a core led feature instead of a trigger, so the
> >> files are always present?
> >
> > That's the best thing, why not just do that?
> 
> It doesn't solve the general case.  For example, any driver that is
> loaded as a module and then calls device_create_file will suffer the
> same problem.

That's very true, and is why they shouldn't be doing that :)

thanks,

greg k-h