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Date:   Sat, 16 Feb 2019 14:26:11 +0300
From:   Yauhen Kharuzhy <jekhor@gmail.com>
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc:     Pavel Machek <pavel@ucw.cz>, Hans de Goede <hdegoede@redhat.com>,
        linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org
Subject: Re: [PATCH v2 1/2] leds: Add Intel Cherry Trail Whiskey Cove PMIC
 LEDs
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On Fri, Feb 15, 2019 at 10:43:17PM +0100, Jacek Anaszewski wrote:
> 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?

Depends on PMIC and charger configuration. Seems there are no common
way working for all configurations exists.

> 
> > 	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

Thanks, this approach seems easy to implement.

> 
> -- 
> Best regards,
> Jacek Anaszewski

-- 
Yauhen Kharuzhy