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From: Greg KH <greg@kroah.com>
To: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Pavel Machek <pavel@ucw.cz>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-leds@vger.kernel.org, devicetree@vger.kernel.org,
	kyungmin.park@samsung.com, b.zolnierkie@samsung.com,
	cooloney@gmail.com, rpurdie@rpsys.net, sakari.ailus@iki.fi,
	s.nawrocki@samsung.com
Subject: Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension)
Date: Fri, 30 Jan 2015 08:40:27 -0800	[thread overview]
Message-ID: <20150130164027.GA25830@kroah.com> (raw)
In-Reply-To: <54CB4702.1090508@samsung.com>

On Fri, Jan 30, 2015 at 09:55:30AM +0100, Jacek Anaszewski wrote:
> Hi Pavel,
> 
> On 01/29/2015 10:14 PM, Pavel Machek wrote:
> >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?
> 
> Faults may prevent strobing the flash in case of some devices.
> The example of such a device is ADP1663 (drivers/media/i2c/adp1653.c).
> This driver reads the faults before strobing the flash and if a
> fault preventing strobing has occurred it returns -EBUSY.
> 
> If this driver was made a LED Flash class driver, then it would
> expose flash_faults attribute. The driver would probably need
> redesigning - checking the faults before strobing would have to be
> avoided and it should be left to the userspace.

That's fine, but Pavel's point is that you shouldn't "clear a fault" by
reading a sysfs file as you don't control who reads all sysfs files
(hint, libudev might cache all attributes when they are found / change,
which could prevent anyone else from seeing that fault.)

So please fix this, make a write to clear a fault or some other such
explicit action, not a simple read.  That's not an acceptable api.

thanks,

greg k-h

  reply	other threads:[~2015-01-30 16:40 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1422346028-16739-1-git-send-email-j.anaszewski@samsung.com>
2015-01-27 22:37 ` Pavel Machek
2015-01-28  8:43   ` Jacek Anaszewski
2015-01-29 21:14     ` Pavel Machek
2015-01-30  8:55       ` Jacek Anaszewski
2015-01-30 16:40         ` Greg KH [this message]
2015-02-02  9:07           ` Jacek Anaszewski
2015-02-02  9:44             ` Pavel Machek
2015-02-02 11:55               ` Jacek Anaszewski
2015-02-02 13:51                 ` Pavel Machek
2015-02-02 14:51                   ` Jacek Anaszewski

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