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Subject: Re: [PATCH -next 2/2] leds: add /sys/class/leds/<led>/current-trigger
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        <linux-leds@vger.kernel.org>, <linux-kernel@vger.kernel.org>
CC:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Pavel Machek <pavel@ucw.cz>
References: <1570029181-11102-1-git-send-email-akinobu.mita@gmail.com>
 <1570029181-11102-3-git-send-email-akinobu.mita@gmail.com>
 <ec4d9d2c-6f8d-97a6-41d7-832a3fc1ca72@ti.com>
 <589ba4d7-31f5-c789-d5ca-c13650cf5b03@gmail.com>
From:   Dan Murphy <dmurphy@ti.com>
Message-ID: <79102915-5510-40e7-b4be-073e1c85bda6@ti.com>
Date:   Wed, 2 Oct 2019 12:57:01 -0500
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Jacek

On 10/2/19 12:46 PM, Jacek Anaszewski wrote:
> Dan,
>
> On 10/2/19 5:47 PM, Dan Murphy wrote:
>> Akinobu
>>
>> On 10/2/19 10:13 AM, Akinobu Mita wrote:
>>> Reading /sys/class/leds/<led>/trigger returns all available LED triggers.
>>> However, this violates the "one value per file" rule of sysfs.
>>>
>>> This provides /sys/class/leds/<led>/current-trigger which is almost
>>> identical to /sys/class/leds/<led>/trigger.  The only difference is that
>>> 'current-trigger' only shows the current trigger name.
>>>
>>> This new file follows the "one value per file" rule of sysfs.
>>> We can find all available LED triggers by listing the
>>> /sys/devices/virtual/led-trigger/ directory.
>>>
>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>>> Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
>>> Cc: Pavel Machek <pavel@ucw.cz>
>>> Cc: Dan Murphy <dmurphy@ti.com>
>>> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
>>> ---
>>>    Documentation/ABI/testing/sysfs-class-led | 13 +++++++++++
>>>    drivers/leds/led-class.c                  | 10 ++++++++
>>>    drivers/leds/led-triggers.c               | 38
>>> +++++++++++++++++++++++++++----
>>>    drivers/leds/leds.h                       |  5 ++++
>>>    4 files changed, 62 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-class-led
>>> b/Documentation/ABI/testing/sysfs-class-led
>>> index 5f67f7a..fdfed3f 100644
>>> --- a/Documentation/ABI/testing/sysfs-class-led
>>> +++ b/Documentation/ABI/testing/sysfs-class-led
>>> @@ -61,3 +61,16 @@ Description:
>>>            gpio and backlight triggers. In case of the backlight trigger,
>>>            it is useful when driving a LED which is intended to indicate
>>>            a device in a standby like state.
>>> +
>>> +What:        /sys/class/leds/<led>/current-trigger
>>> +Date:        September 2019
>>> +KernelVersion:    5.5
>>> +Contact:    linux-leds@vger.kernel.org
>>> +Description:
>>> +        Set the trigger for this LED. A trigger is a kernel based source
>>> +        of LED events.
>>> +        Writing the trigger name to this file will change the current
>>> +        trigger. Trigger specific parameters can appear in
>>> +        /sys/class/leds/<led> once a given trigger is selected. For
>>> +        their documentation see sysfs-class-led-trigger-*.
>>> +        Reading this file will return the current LED trigger name.
>> Why do we need this new file can't we just update the current trigger
>> file implementation?
> We can't change existing ABI. It doesn't matter if it is documented
> or not - it's in place for very long time and you can't guarantee there
> are no users relying on triggers file show format.

So if it has been in place for a very long time why do we need another 
ABI that does sorta the same thing?

This seems to be a bit confusing and extra.

Maybe this ABI should be RO where a user can read the current-trigger as 
a single value per file but writing the trigger still

is done through the old ABI.

Dan


>
>> I don't see any documentation that states that the read of the trigger
>> file will print a list of known triggers.
>>
>> And writing to the trigger file still works so I would think the _show
>> just needs to be fixed.
>>
>> Besides this patch does not fix the issue in the commit message that the
>> trigger file still violates the one value per file rule.
>>
>> Dan
>>
>>