On Tue, Nov 12, 2013 at 10:19 AM, Kay Sievers [off-list ref] wrote:

On Tue, Nov 12, 2013 at 1:56 AM, Henrique de Moraes Holschuh
[off-list ref] wrote:

quoted

On Tue, 12 Nov 2013, Jingoo Han wrote:

quoted

On Tuesday, November 12, 2013 8:57 AM, Kyungmin Park wrote:

quoted

From: Kyungmin Park <kyungmin.park@samsung.com>

The most mobile phones have Ambient Light Sensors and it changes brightness according lux.
It means it changes backlight brightness frequently by just writing sysfs node, so it generates uevent.

Usually there's no user to use this backlight changes. But it forks udev worker threads and it takes
about 5ms. The main problem is that it hurts other process activities. so remove it.

Kay said
"Uevents are for the major, low-frequent, global device state-changes,
 not for carrying-out any sort of measurement data. Subsystems which
 need that should use other facilities like poll()-able sysfs file or
 any other subscription-based, client-tracking interface which does not
 cause overhead if it isn't used. Uevents are not the right thing to
 use here, and upstream udev should not paper-over broken kernel
 subsystems."

True.

Now, let's take a look at reality: should you poll()/select() on a sysfs
node that doesn't suport it, it will wait until the poll/select timeout
happens (or EINTR happens), and userspace has absolutely NO way to detect
whether a sysfs node has poll/select support.

What happens if the sysfs interface did not provide poll/select support
since day one, but rather added it later?  Nobody will use it for a *long*
time, if ever... unless you actually took pains to version the sysfs
interface, and people actually care.

If that's an issue, we can add a new "event" file, just for that.

quoted

quoted

'thinkpad_acpi.c' uses the 'BACKLIGHT_UPDATE_SYSFS'.
Henrique, can we remove it?

Can't you fix this by rate-limiting, or otherwise adding an attribute that
backlight devices should set when they need to supress change events?

Yeah, great idea, fix a bad hack with another bad one on top. :)
Passing measurement data through uevents is just an utterly broken
idea which cannot be fixed.

quoted

Is there a proper on-screen-display support path for the backlight class
nowadays?  Otherwise, you'd be removing the only way userspace ever had to
do proper OSD of backlight changes...

OSD drawing and event sounds usually happen as a fedback for
keypresses of brightness control, it would be weird to show up when
something else, like a light-sensor, adjusts the brightness in the
background.

Anyway, there might be the need for coordination and a new interface,
but uevents for measurement data need to die entirely; they make no
sense, never made any; and the sooner they are gone the better.

Now power_supply, especially battery uses this scheme. it passes
battery data using uevent.
do you have any idea to kill it?

Thank you,
Kyungmin Park