Re: sysfs permissions on dynamic attributes (led delay_on and delay_off)

From: Colin Cross <ccross@android.com>
To: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Greg KH <gregkh@linuxfoundation.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Bryan Wu <bryan.wu@canonical.com>
Subject: Re: sysfs permissions on dynamic attributes (led delay_on and delay_off)
Date: Sat, 21 Jul 2012 01:26:38 -0700	[thread overview]
Message-ID: <CAMbhsRREVcqf+pnamY62wGcd2mBif8W8_CRXdJQCQvL14Dy3Eg@mail.gmail.com> (raw)
In-Reply-To: <1342856010.21788.47.camel@ted>

On Sat, Jul 21, 2012 at 12:33 AM, Richard Purdie
<richard.purdie@linuxfoundation.org> wrote:
> On Fri, 2012-07-20 at 21:08 -0700, Greg KH 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?
>
> This implies we should make every trigger a core led feature and
> effectively do away with triggers. I'm not sure that makes sense.

Blinking is already effectively a core feature.  It is implemented in
led-core.c so it can be used by other triggers besides timer, it's
state is stored in the led_classdev structure, not in the trigger
data, and the only thing left in ledtrig-timer.c is the sysfs files.