LKML Archive on
help / color / mirror / Atom feed
From: Jacek Anaszewski <>
To: Pavel Machek <>
Cc: Greg KH <>,
	kernel list <>,,,,,,,,
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 09:55:30 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20150129211420.GA21140@amd>

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

  reply	other threads:[~2015-01-30  8:55 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
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 [this message]
2015-01-30 16:40         ` Greg KH
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension)' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).