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Subject: Re: [PATCH v2 1/2] leds: Add Intel Cherry Trail Whiskey Cove PMIC
 LEDs
To:     Hans de Goede <hdegoede@redhat.com>, Pavel Machek <pavel@ucw.cz>
Cc:     Yauhen Kharuzhy <jekhor@gmail.com>, linux-kernel@vger.kernel.org,
        linux-leds@vger.kernel.org
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From:   Jacek Anaszewski <jacek.anaszewski@gmail.com>
Message-ID: <80971bc3-1193-83ed-913a-12f6217016c8@gmail.com>
Date:   Fri, 15 Feb 2019 23:31:25 +0100
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On 2/15/19 11:26 PM, Hans de Goede wrote:
> Hi,
> 
> On 2/15/19 10:42 PM, Jacek Anaszewski wrote:
>> Hi all,
>>
>> On 2/15/19 12:27 PM, Hans de Goede wrote:
>>> Hi,
>>>
>>> On 15-02-19 00:03, 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.
>>>
>>> Right, so there are really 2 orthogonal issues here:
>>>
>>> 1) With this hardware the LED is either turned on/off automatically
>>> by the PMIC based on charging state; or it is under user control.
>>>
>>> This is different from the led_classdev_notify_brightness_hw_changed
>>> case, where the hardware may update the state underneath the driver,
>>> but the driver can still always update the state itself. In this case
>>> if the LED is in hw-control mode then the driver cannot turn it on/off.
>>>
>>> Pavel suggested modeling this with a new "hardware' trigger, where
>>> setting the trigger to this trigger will enable the hw-controlled
>>> mode and setting any other trigger will switch thing to the 
>>> user-controlled
>>> mode.
>>
>> We already do have hw_pattern file exposed by pattern trigger.
>> It can be used to set hw breathing mode using some device specific
>> syntax semantics, documented in dedicated ABI documentation.
>> It was introduced for similar case, see
>> Documentation/ABI/testing/sysfs-class-led-driver-sc27xx.
> 
> Ah I see, yes that could be used, the pattern would just be a single
> integer specifying the cycle time in milliseconds, as nothing
> else is configurable.

I depends on how the pattern looks like. If it is breathing then
we'd need more than one value.

> I think that should work fine, which means that we can use the timer and
> pattern trigger support for the blinking and breathing modes.
> 
> That still leaves the switching between user and hw-control modes,
> as discussed the hw-controlled mode could be modelled as a new "hardware"
> trigger, but then we cannot choose between on/blink/breathing when
> in hw-controlled mode. As Pavel mentioned, that would require some
> sort of composed trigger, where we have both the hardware and
> timer triggers active for example.
> 
> I think it might be easier to just allow turning on/off the hardware
> control mode through a special "hardware_control" sysfs attribute and
> then use the existing timer and pattern triggers for blinking / breathing.

Pattern trigger exposes pattern file by default and hw_pattern if
pattern_set/get ops are provided. Writing them enables software and
hardware pattern respectively.

-- 
Best regards,
Jacek Anaszewski