From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759618AbbA3Izi (ORCPT ); Fri, 30 Jan 2015 03:55:38 -0500 Received: from mailout1.w1.samsung.com ([210.118.77.11]:38140 "EHLO mailout1.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752537AbbA3Izf (ORCPT ); Fri, 30 Jan 2015 03:55:35 -0500 X-AuditID: cbfec7f5-b7fc86d0000066b7-c8-54cb46710c38 Message-id: <54CB4702.1090508@samsung.com> Date: Fri, 30 Jan 2015 09:55:30 +0100 From: Jacek Anaszewski User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130804 Thunderbird/17.0.8 MIME-version: 1.0 To: Pavel Machek Cc: Greg KH , kernel list , 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) References: <1422346028-16739-1-git-send-email-j.anaszewski@samsung.com> <20150127221958.GA18993@amd> <54C8A130.8000807@samsung.com> <20150129211420.GA21140@amd> In-reply-to: <20150129211420.GA21140@amd> Content-type: text/plain; charset=ISO-8859-1; format=flowed Content-transfer-encoding: 7bit X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFrrDLMWRmVeSWpSXmKPExsVy+t/xa7qFbqdDDDasVrfYOGM9q8XRnROZ LOYfOcdqcW7BDEaLs01v2C0u75rDZrH1zTpGi7unjrJZ7N71lNXi8Jt2Vosz+1eyOXB77Jx1 l93j8NeFLB5vHwZ47Jn/g9Wjb8sqRo8Vq7+ze3zeJBfAHsVlk5Kak1mWWqRvl8CV0fj5A3vB XbmKU0/kGhgXS3QxcnJICJhIrL/ylgnCFpO4cG89WxcjF4eQwFJGiYfTGqCcj4wSuxoms4FU 8QpoSRy/s54VxGYRUJW4d+oeWJxNwFDi54vXYJNEBSIk/pzexwpRLyjxY/I9FhBbREBeYmvf CmaQocwC/UwSk3d2soMkhAVaGSXOnuSBW/3tDEQ3p4CmxI7v38C6mQWsJVZO2sYIYctLbF7z lnkCo8AsJEtmISmbhaRsASPzKkbR1NLkguKk9FwjveLE3OLSvHS95PzcTYyQGPm6g3HpMatD jAIcjEo8vDM2nQwRYk0sK67MPcQowcGsJMKran06RIg3JbGyKrUoP76oNCe1+BAjEwenVAPj tjc/rHXXLLh58Ycm53kxcfX7nv6eklWz3nZ91/4p/O3wypjF4rHzCx1f8D9qtdObZpj2dsrj Kd+dDb41S884l3xlJZvfcuZwY78tquvs9E0ML8jHqNx/Vazc9/nM0v6mFzPmqR7xX7Xt407P FzpaOyqZLd4yHTR6OiPqWfOPq23M4pMf2K7VVmIpzkg01GIuKk4EAO/q/yhvAgAA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > I mean... another user can just read the file in loop, and the camera > application will not get any useful information. If the fault is no longer valid at the time of access from camera application, then why it should be reported then? >> 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. And this is OK. If a non-persistent fault was read by grep, then it will not be reported anymore. If someone wanted to maintain the history of flash faults for a device, then they are free to do it on their own by periodically reading the attribute, however I don't think it would be practical during every day use. -- Best Regards, Jacek Anaszewski