linux-kernel - Re: [PATCH 5/6] leds: pca963x: Inform the output that it is inverted

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Date:	Fri, 22 Apr 2016 17:44:09 +0200
From:	Olliver Schinagl <oliver@...inagl.nl>
To:	Rob Herring <robh@...nel.org>
CC:	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Richard Purdie <rpurdie@...ys.net>,
	Jacek Anaszewski <j.anaszewski@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>
Subject: Re: [PATCH 5/6] leds: pca963x: Inform the output that it is inverted



On April 22, 2016 3:09:35 PM CEST, Rob Herring <robh@...nel.org> wrote:
>On Fri, Apr 22, 2016 at 7:38 AM, Olliver Schinagl <oliver@...inagl.nl>
>wrote:
>> Hey Rob,
>>
>> On 21-04-16 17:07, Rob Herring wrote:
>>>
>>> On Tue, Apr 19, 2016 at 09:40:49AM +0200, Olliver Schinagl wrote:
>>>>
>>>> When leds are connected in a totem-pole configuration, they can be
>>>> connected either in a active-high, or active-low manor. The driver
>>>> currently always assumes active-high. This patch adds the
>>>> 'nxp,inverted-out' boolean property to tell the driver that the
>leds
>>>> are driven active-low, or rather, that the behavior is inverted to
>what
>>>> is normally expected.
>>>
>>> How do I know what is normally expected?
>>
>> fair point, and in fact, you don't. The text is bad here.
>
>The problem is not so much the text here, but the property is also
>meaningless without some explanation.
>
Well the datasheet states only that it will invert the output. So would it make sense to just state the same here?
>>>
>>>
>>>> Signed-off-by: Olliver Schinagl <oliver@...inagl.nl>
>>>> ---
>>>>   Documentation/devicetree/bindings/leds/pca963x.txt |  1 +
>>>>   drivers/leds/leds-pca963x.c                        | 20
>>>> +++++++++++++-------
>>>>   include/linux/platform_data/leds-pca963x.h         |  1 +
>>>>   3 files changed, 15 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/leds/pca963x.txt
>>>> b/Documentation/devicetree/bindings/leds/pca963x.txt
>>>> index dafbe99..7b23725 100644
>>>> --- a/Documentation/devicetree/bindings/leds/pca963x.txt
>>>> +++ b/Documentation/devicetree/bindings/leds/pca963x.txt
>>>> @@ -6,6 +6,7 @@ Required properties:
>>>>   Optional properties:
>>>>   - nxp,totem-pole : use totem pole (push-pull) instead of
>open-drain
>>>> (pca9632 defaults
>>>>     to open-drain, newer chips to totem pole)
>>>> +  nxp,inverted-out: the connected leds are active-low, default to
>>>> active-high
>>>
>>> Just state what mode you want: nxp,active-low
>>
>> But that's not what happens, which is why my text is bad :) It
>depends on
>> how the board is wired and if it is push-pull or open-drain. Though
>this
>> goes beyond my electronics knowledge. So I'll reduce the text to say
>exactly
>> what we mean, inverted output (or not).
>>
>> Unless you can explain that it would be unrelated and it is actually
>> active-high/low. I'll be more than happy to oblige.
>
>I'm not familiar with this chip, but googling for "active low LED
>circuit" can give you lots of examples. To put it simply, you are
>defining whether the control/switch is on the cathode or anode side of
>the LED. Open-drain vs. push-pull is also related to how the circuit
>is done, but is probably an independent property. I'd think the only
>reason you would use open-drain here is if you wanted to control the
>LED from 2 different signals. Whether you wanted the controls to
>function as OR or AND logic to turn on would determine what the active
>state needs to be.
I was thinking that the output behaves differently depending whether it is configured as push-pull or open-drain.

So to summarize, does the output always go into active-low mode, or does it depend on other parameters as well?

Olliver
>
>Rob

-- 
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