linux-kernel - Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension)

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Date:	Thu, 29 Jan 2015 22:14:20 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	Greg KH <greg@...ah.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
	kyungmin.park@...sung.com, b.zolnierkie@...sung.com,
	cooloney@...il.com, rpurdie@...ys.net, sakari.ailus@....fi,
	s.nawrocki@...sung.com
Subject: Re: Reading /sys with side effects (was Re: [PATCH 1/2]
 Documentation: leds: Add description of LED Flash class extension)

Hi!

> >>+	- flash_fault - list of flash faults that may have occurred:
> >>+		* led-over-voltage - flash controller voltage to the flash LED
> >>+			has exceededthe limit specific to the flash controller
> >>+		* flash-timeout-exceeded - the flash strobe was still on when
> >>+			the timeout set by the user has expired; not all flash
> >>+			controllers may set this in all such conditions
> >>+		* controller-over-temperature - the flash controller has
> >>+			overheated
> >>+		* controller-short-circuit - the short circuit protection
> >>+			of the flash controller has been triggered
> >>+		* led-power-supply-over-current - current in the LED power
> >>+			supply has exceeded the limit specific to the flash
> >>+			controller
> >>+		* indicator-led-fault - the flash controller has detected
> >>+			a short or open circuit condition on the indicator LED
> >>+		* led-under-voltage - flash controller voltage to the flash
> >>+			LED has been below the minimum limit specific to
> >>+			the flash
> >>+		* controller-under-voltage - the input voltage of the flash
> >>+			controller is below the limit under which strobing the
> >>+			flash at full current will not be possible. The condition
> >>+			persists until this flag is no longer set
> >>+		* led-over-temperature - the temperature of the LED has exceeded
> >>+			its allowed upper limit
> >>+
> >>+		Flash faults are cleared, if possible, by reading the attribute.
> >
> >That's bad. Now you can no longer present flash_fault file as readable
> >to non-root users, and grep -ri foo /sys will interfere with your
> >camera application.
> >
> >Bad interface, just fix it.
> 
> In my opinion it isn't crucial for the user to be aware of the
> fact that some non-persistent fault happened right after strobing the
> flash (e.g. over temperature).
> 
> I cannot see anything harmful in the situation when someone does grep
> on /sys and clears non-persistent fault on a flash LED device.

So why export the faults at all?

I mean... another user can just read the file in loop, and the camera
application will not get any useful information.

> Also, not all devices may be able to report the faults that happened
> earlier but are not valid at the time of I2C readout. In that case the
> user will never now that the fault has ever occurred, unless they read
> the flash_fault attribute at the proper moment.
> 
> In this case we cannot enforce consistent policy for all devices.

Too bad. But lets do a good job at least for devices where we can do a
good job, ok?

> Please describe the use case when clearing the fault on read can be
> harmful, if you have any.

while true; grep -ri foo /sys; done

And no, your application trying to read the faults will very probably
read nothing.

									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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