Re: [PATCH RFC] leds: leds-lp50xx: Handle reg to get correct multi_index

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

On 5/12/25 12:59, Johan Adolfsson wrote:
> 
> From: Jacek Anaszewski <jacek.anaszewski@gmail.com>
> Sent: Saturday, May 10, 2025 17:32
> To: Johan Adolfsson; Lee Jones; Pavel Machek
> Cc: linux-leds@vger.kernel.org; linux-kernel@vger.kernel.org; Kernel
> Subject: Re: [PATCH RFC] leds: leds-lp50xx: Handle reg to get correct multi_index
> 
>> Hi Johan,
>>
>> On 5/6/25 12:39, Johan Adolfsson wrote:
>>> mc_subled used for multi_index needs well defined array indexes,
>>> to guarantee the desired result, optionally use reg for that.
>>>
>>> If devicetree child nodes is processed in random or reverse order
>>> you may end up with multi_index "blue green red" instead of the expected
>>> "red green blue".
>>> If user space apps uses multi_index to deduce how to control the leds
>>> they would most likely be broken without this patch if devicetree
>>> processing is reversed (which it appears to be).
>>
>> Are you trying to solve some real problem that occurred to you?
> 
> Yes! Since the subled is indexed by the processing order, it gets reversed if devicetree processing is reversed
> (which I understand is a "feature"), so instead of "red green blue" I would get "blue green red" in the multi_index file without this patch.
> The mapping to the hardware does not match that, so writing "255 0 0" to multi_intensity will give me red and not blue.

You are expected to write intensities to the multi_intensity file
according to the order of colors listed in multi_index file.

>> The order of DT nodes parsing is not a problem here - we save
>> color index in subled_info to be able to figure out which color
>> is on which position. This information can be retrieved in sysfs
>> by reading multi_index file.
> 
> Maybe the bug is somewhere else in the leds-lp50xx if that is supposed to work,
> but I still think it's a good thing to be able to get the expected order in the multi_index file.

Please make sure you read Documentation/leds/leds-class-multicolor.rst.
There is no such term as "expected order in the multi_index file".
The framework is called multicolor, not rgb. The order of colors does
not need to be RGB.

-- 
Best regards,
Jacek Anaszewski