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    SubjectRe: sysfs permissions on dynamic attributes (led delay_on and delay_off)
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    On Sat, Jul 21, 2012 at 4:21 AM, Richard Purdie
    <richard.purdie@linuxfoundation.org> wrote:
    > On Sat, 2012-07-21 at 01:26 -0700, Colin Cross wrote:
    >> 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.
    >
    > Having the attributes present all the time leads to some nasty questions
    > like how the on/off delays interact with things like say a network
    > activity trigger. Not all triggers are going to respect these delay
    > values and I can imagine a whole new set of nasty bug reports with no
    > easy solutions if this change is made...

    The delay_on and delay_off files could easily override the values from
    the trigger.

    Sending a KOBJ_CHANGE uevent is not a great solution, it's still
    horribly racy in userspace. This script would never work reliably:
    echo timer > trigger
    echo 1000 > delay_on
    echo 1000 > delay_off
    echo 255 > brightness


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