linux-kernel - sysfs permissions on dynamic attributes (led delay_on and delay

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Message-ID: <CAMbhsRTX=t4UMfmrJRyXidSTWXvaC5L9aFdTFL03wYYKrUE5qw@mail.gmail.com>
Date:	Fri, 20 Jul 2012 17:46:14 -0700
From:	Colin Cross <ccross@...roid.com>
To:	lkml <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Bryan Wu <bryan.wu@...onical.com>,
	Richard Purdie <rpurdie@...ys.net>
Subject: sysfs permissions on dynamic attributes (led delay_on and delay_off)

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? Send a KOBJ_CHANGE uevent from the
driver after calling device_create_file?  Dynamically create a timer
device under /sys/class/leds/<led> so a new add uevent gets sent?
Promote blinking to be a core led feature instead of a trigger, so the
files are always present?
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