Re: [PATCH v3 3/3] leds: tps68470: Add LED control for tps68470

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Hi,

On 3/23/23 12:36, Pavel Machek wrote:
> On Thu 2023-03-23 12:29:29, Hans de Goede wrote:
>> Hi,
>>
>> On 3/23/23 12:26, Pavel Machek wrote:
>>> On Thu 2023-03-23 12:24:05, Hans de Goede wrote:
>>>> Hi Pavel,
>>>>
>>>> On 3/23/23 12:15, Pavel Machek wrote:
>>>>> Hi!
>>>>>
>>>>>> There are two LED controllers, LEDA indicator LED and LEDB flash LED for
>>>>>> tps68470. LEDA can be enabled by setting TPS68470_ILEDCTL_ENA. Moreover,
>>>>>> tps68470 provides four levels of power status for LEDB. If the
>>>>>> properties called "ti,ledb-current" can be found, the current will be
>>>>>> set according to the property values. These two LEDs can be controlled
>>>>>> through the LED class of sysfs (tps68470-leda and tps68470-ledb).
>>>>>
>>>>> If the LED can have four different currents, should it have 4
>>>>> brightness levels?
>>>>
>>>> No this was already discussed with an earlier version. This is in
>>>> indicator LED output. The current setting is a one time boot configure
>>>> thing after which the indicator LED is either on or off.
>>>
>>> Current levels are exponential in that driver. That will result in
>>> rather nice four level. Surely LED does not care if you set it during
>>> boot or later?
>>
>> Well for one there is no guarantee the LED can continuously handle
>> the maximum configurable LED current and as you rightly point out
>> elsewhere in the thread we don't want to be blowing up hw.
> 
> hw can support 16mA -> you expose 0, 2mA, 4mA, 8mA, 16mA levels.
> 
> hw can support 4mA -> you expose 0, 2mA, 4mA.

This is just not how this hw is intended to be used. If you look
at the registers there are on/off bits for LEDA and LEDB in a
single register and then there is a current-limit only for LEDB,
where as LEDA has a fixed current.

So clearly the intention here is on/off use with a fixed current
and dimming something like a privacy-LED on a laptop camera
makes no sense and will never be used since we disable userspace
access of the LED when it is used as a privacy-LED.

Regards,

Hans





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