Messages in this thread | | | Date | Fri, 30 Jan 2015 09:55:30 +0100 | From | Jacek Anaszewski <> | Subject | Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension) |
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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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