linux-kernel - Re: [PATCH 02/24] leds: core: Add support for composing LED class device names

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Date:   Tue, 13 Nov 2018 21:57:38 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, robh@...nel.org,
        Baolin Wang <baolin.wang@...aro.org>,
        Daniel Mack <daniel@...que.org>, Dan Murphy <dmurphy@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Oleh Kravchenko <oleg@....org.ua>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Simon Shields <simon@...eageos.org>
Subject: Re: [PATCH 02/24] leds: core: Add support for composing LED class
 device names

On 11/12/2018 11:06 PM, Pavel Machek wrote:
> On Mon 2018-11-12 21:11:32, Jacek Anaszewski wrote:
>> On 11/12/2018 08:05 PM, Pavel Machek wrote:
>>> Hi!
>>>
>>>>>>> My system looks like this:
>>>>>>>
>>>>>>> input16::capslock    tpacpi::bay_active    tpacpi::standby
>>>>>>> input16::numlock     tpacpi::dock_active   tpacpi::thinklight
>>>>>>> input16::scrolllock  tpacpi::dock_batt	      tpacpi::thinkvantage
>>>>>>> input5::capslock     tpacpi::dock_status1  tpacpi::unknown_led
>>>>>>> input5::numlock      tpacpi::dock_status2  tpacpi::unknown_led2
>>>>>>> input5::scrolllock   tpacpi:green:batt	      tpacpi::unknown_led3
>>>
>>>>> But it is not just for backwards compatibility. See my examples above,
>>>>> it is needed to tell which device the LED is asociated with, and it is
>>>>> absolutely required for USB devices (for example).
>>>>
>>>> For USB devices there is already ledtrig-usbport available, which
>>>> provides sysfs interface for defining and reading the usb ports,
>>>> the status of which the LED indicates. Since the USB devices can be
>>>> attached/removed dynamically, it would be impossible to reflect
>>>> the associations in the LED class device name.
>>>
>>> I'm not talking USB activity. I'm talking USB devices with LEDs on
>>> them, like for example keyboards.
>>>
>>> Please take a look at example above. input16::numlock ;
>>> input5::numlock . You absolutely need device part.
>>
>> It would be redundant since there is "device" symbolic link
>> created in given LED class device sysfs directory, pointing to the
>> corresponding input device directory, like in case of my USB
>> keyboard:
> 
> You are right I forgot about the device symlink, and it partly helps
> with the USB keyboard case... 
> 
> But you still need the device part. Sysfs will not like two
> directories named "::numlock".

LED core has a protection against that. See my reply to Linus.

> 
>> #/sys/class/leds/input5::scrolllock$ ls -l
>> total 0
>> -rw-r--r-- 1 root root 4096 Nov 12 20:22 brightness
>> lrwxrwxrwx 1 root root    0 Nov 12 20:22 device -> ../../input5
>> -r--r--r-- 1 root root 4096 Nov 12 20:22 max_brightness
>> drwxr-xr-x 2 root root    0 Nov 12 20:22 power
>> lrwxrwxrwx 1 root root    0 Nov 12 20:04 subsystem ->
>> ../../../../../../../../../../../class/leds
>> -rw-r--r-- 1 root root 4096 Nov 12 20:22 trigger
> 
>>> Because userspace needs that information?
>>>
>>> Say you have raid array, with "error" leds for each drive (your list
>>> already contains "hdderr"). Now userland detects problem with hdparm
>>> on /dev/sdi. It would like to turn on corresponding LED.
>>>
>>> How do you propose we do that?
>>
>> Similarly as in case of USB keyboard.
> 
> Not really, I'm afraid. Hard drives have no red LEDs on them (and the
> LED would not be visible, anyway) so the "device" symlink in such case
> would point to some kind of i2c LED controller.
> 
> Eventually we'll need to have two devices for each LED. "Controller
> this is on" -- in device symlink and "device this talks about".

After thinking it over I agree - we will still need devicename part
Please refer to my reply to Rob.

-- 
Best regards,
Jacek Anaszewski