Re: [PATCH v6 1/4] leds: class: Add new optional brightness_hw_changed attribute

[Jacek Anaszewski <jacek.anaszewski@gmail.com>]

On 01/26/2017 09:33 AM, Pali Rohár wrote:
> On Wednesday 25 January 2017 22:35:53 Jacek Anaszewski wrote:
>> On 01/25/2017 05:49 PM, Pali Rohár wrote:
>>> On Wednesday 25 January 2017 17:11:27 Hans de Goede wrote:
>>>> Some LEDs may have their brightness level changed autonomously
>>>> (outside of kernel control) by hardware / firmware. This commit
>>>> adds support for an optional brightness_hw_changed attribute to
>>>> signal such changes to userspace (if a driver can detect them):
>>>>
>>>> What:		/sys/class/leds/<led>/brightness_hw_changed
>>>> Date:		January 2017
>>>> KernelVersion:	4.11
>>>> Description:
>>>> 		Last hardware set brightness level for this LED. Some LEDs
>>>> 		may be changed autonomously by hardware/firmware. Only LEDs
>>>> 		where this happens and the driver can detect this, will
>>>> 		have this file.
>>>>
>>>> 		This file supports poll() to detect when the hardware
>>>> 		changes the brightness.
>>>>
>>>> 		Reading this file will return the last brightness level set
>>>> 		by the hardware, this may be different from the current
>>>> 		brightness.
> 
> In which situation this attribute may be different?
> 
> I think some information should be present in this description...

The first sentence in description says:

"Last hardware set brightness level for this LED" - i.e. it may be
different from the _actual_ brightness, which could have been set by
LED class driver.

>>>> Drivers which want to support this, simply add LED_BRIGHT_HW_CHANGED to
>>>> their flags field and call led_classdev_notify_brightness_hw_changed()
>>>> with the hardware set brightness when they detect a hardware / firmware
>>>> triggered brightness change.
>>>>
>>>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>>>
>>> Just speculation: What about using name 'actual_brightness'? It provides
>>> same output on read as actual_brightness from /sys/class/backlight/.
>>
>> This name is ambiguous. Propagating it to the LED subsystem only because
>> it exists in backlight is not sufficient argument IMHO.
> 
> I thought that having same name could help userspace and also to have
> consistency between similar subsystems. But ok, that was just my
> speculation.

Please also note the "actual" would be ambiguous especially in view
of my above explanation.

> I have just one small suggestion in current name "brightness_hw_changed".
> As it provides regular read() capability I would suggest to not indicate
> "activity" (changed) in name...

Without "changed" one could think that brightness file (the one we have
currently) doesn't reflect hardware state.

-- 
Best regards,
Jacek Anaszewski