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

From: Olliver Schinagl
Date: Fri Apr 22 2016 - 11:44:31 EST

On April 22, 2016 3:09:35 PM CEST, Rob Herring <robh@xxxxxxxxxx> wrote:
>On Fri, Apr 22, 2016 at 7:38 AM, Olliver Schinagl <oliver@xxxxxxxxxxx>
>> 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
>>>> are driven active-low, or rather, that the behavior is inverted to
>>>> 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@xxxxxxxxxxx>
>>>> ---
>>>> 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
>>>> (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
>> goes beyond my electronics knowledge. So I'll reduce the text to say
>> 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?


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