linux-kernel - Re: [PATCH v2 1/2] dt-bindings: leds: gpio: Add optional retain-state-shutdown property

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Message-ID: <20170831212840.ecr2p4nra52xvzrz@rob-hp-laptop>
Date:   Thu, 31 Aug 2017 16:28:40 -0500
From:   Rob Herring <robh@...nel.org>
To:     Andrew Jeffery <andrew@...id.au>
Cc:     linux-leds@...r.kernel.org, rpurdie@...ys.net,
        jacek.anaszewski@...il.com, pavel@....cz, mark.rutland@....com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        joel@....id.au, openbmc@...ts.ozlabs.org
Subject: Re: [PATCH v2 1/2] dt-bindings: leds: gpio: Add optional
 retain-state-shutdown property

On Mon, Aug 28, 2017 at 09:47:10AM +0930, Andrew Jeffery wrote:
> On Baseboard Management Controller (BMC) systems it's sometimes
> necessary for a LED to retain its state across a BMC reset (which is
> independent of the host system state). Add a devicetree property to
> describe this behaviour. The property would typically be used in
> conjunction with 'default-state = "keep"'.

A bit quick on the applying of this...

I don't understand how this works. The BMC usecase is interesting but 
that's fairly irrelevant to the binding. How do you know the GPIO state 
thru a reset? You're doing a core reset, but not reseting the GPIO 
controller?

When you use 'default-state = "keep"' but not this property? Seems like 
the same thing to me. Presumably the bootloader would just distinguish 
between a warm and cold reset to decide whether to re-init the state.

Finally, if we do have this property, why is it GPIO specific. You could 
just as easily have a LED controller IC and want to do the same thing.

Rob