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:	Fri, 30 Jan 2015 09:55:30 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Pavel Machek <pavel@....cz>
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 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
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