Date:   Fri, 15 Feb 2019 22:43:17 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Yauhen Kharuzhy <jekhor@...il.com>, Pavel Machek <pavel@....cz>
Cc:     Hans de Goede <hdegoede@...hat.com>, linux-kernel@...r.kernel.org,
        linux-leds@...r.kernel.org
Subject: Re: [PATCH v2 1/2] leds: Add Intel Cherry Trail Whiskey Cove PMIC
 LEDs

Hi Yauhen,

On 2/15/19 8:27 AM, Yauhen Kharuzhy wrote:
> On Fri, Feb 15, 2019 at 12:03:07AM +0100, Pavel Machek wrote:
>> Hi!
>>
>>>>>> I suggest that we deal with this special case by adding 3 custom
>>>>>> sysfs attributes:
>>>>>>
>>>>>> 1) "mode" which when read, prints, e.g. :
>>>>>> manual [on-when-charging]
>>>>>
>>>>> While this allows _user on console_ to control everything using echo,
>>>>> it is not suitable for applications trying to control LEDs.
>>>>>
>>>>> As there's nothing special about the case here, I believe we should
>>>>> have generic solution here.
>>>>>
>>>>> My preffered solution would be "hardware" trigger that leaves the LED
>>>>> in hardware control.
>>>>
>>>> As you explained in the parts which I snipped, there are many
>>>> devices which have a similar choice for a LED being under hw or
>>>> user control. I can see how this looks like a trigger and how we
>>>> could use the trigger API for this.
>>>>
>>>> I believe though, that if we implement a "virtual" (for lack of
>>>> a better word) trigger for this, that this should be done in the
>>>> LED core. I can envision this working like this:
>>>>
>>>> 1) Add a:
>>>>
>>>> hw_control_set(struct led_classdev *led_cdev, bool enable_hw_control);
>>>
>>> Please note that we have support for hw patterns in the pattern trigger.
>>> (see how drivers/leds/leds-sc27xx-bltc.c makes use of it for its
>>> breathing pattern).
>>> We have also support for hw blinking in timer trigger via blink_set op.
>>>
>>> In addition to that there is brightness_hw_changed sysfs attribute
>>> with led_classdev_notify_brightness_hw_changed() LED API.
>>>
>>> Couldn't they be used in concert to support the specific features
>>> of the device in question?
>>
>> I believe main issue here is this:
>>
>> Hardware can automatically control the LED according to the charging
>> status, or it can be used as normal software-controlled LED.
>>
>> I believe we should use trigger to select if hardware controls it or
>> not (and then add driver-specific files to describe the
>> details). Other proposal is in the mail thread, too.
> 
> But there are two kinds of 'hardware control' here in discussion:
> 1:
> 	a) PMIC switching LED off/on/breathing/blinking reflected the charging
> 	state (charger is controlled by PMIC entirely without of
> 	software intervention) – 'hw controlled' mode

Can we detect when charging is activated?

> 	b) Software controls when LED is on/off/breathing/blinking but
> 	patterns are generated by PMIC – 'sw controlled' mode.
> 
> 2:
> 	a) parameters of lighting (continuous/blinking/breathing) are
> 	set in the PMIC and the PMIC generates patterns
> 	b) blinking/breathing patterns are generated by the software
> 	entirely.
> 
> It seems that we sometimes confuse this two kinds of hw control definitions
> in the discussion.
> 
> 
> The simple use case question: with this hardware and existing LED class
> API, what user/app should to do to make LED breathing with hw-generated
> pattern?
> 
> As I see, user should activate 'pattern' trigger and write to its hw_pattern attribute...
> hm... big pattern which describes every step of breathing? Some
> simplified pattern which should be interpreted as 'enable the breathing
> an set its frequency' by driver? Which level of simplification will be
> suitable? Which criteria of pattern rejection should be?

Please see my reply to Hans. We need some simplified device specific
syntax of hw_pattern for this device, similarly like in:
Documentation/ABI/testing/sysfs-class-led-driver-sc27xx

-- 
Best regards,
Jacek Anaszewski

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