linux-kernel - Re: [PATCH] gpio: 74x164: add lines-initial-states property

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Message-ID: <CACRpkdYTxvWjLizTr8Qg-FmSPjnnU+kj2+VrUUcz5F0nkhrUrg@mail.gmail.com>
Date:   Thu, 16 Aug 2018 10:11:38 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     mail@...id-bauer.net
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio: 74x164: add lines-initial-states property

Hi David,

On Wed, Aug 15, 2018 at 10:19 PM David Bauer <mail@...id-bauer.net> wrote:

> This adds the ability to define the initial state of each output line on
> device probe.
>
> Signed-off-by: David Bauer <mail@...id-bauer.net>

(...)
>  Optional properties:
>  - enable-gpios: GPIO connected to the OE (Output Enable) pin.
> +- lines-initial-states: Bitmask that specifies the initial state of
> +  each line. When a bit is set to zero, the corresponding output line
> +  is initialized LOW. When a bit is set to one, the corresponding
> +  output line is initialized HIGH. In case this property is not
> +  defined, all lines will be initialized as LOW.

This sounds like something that should be generic, and not use
a bitmask, but offsets. It should work even if the number of
GPIOs from the chip is > 32.

Is the usecase different from hogs?
See Documentation/devicetree/bindings/gpio.txt

There has been extensive discussion about supporting initial values
with something similar to hogs, but I haven't got anything ACKed
by the DT maintainers so it has kind of stalled.

I would make sure to both make it generic, get ACK from the DT
mainatiners, and make sure to implement it in
drivers/gpio/gpiolib-of.c and not locally in drivers.

Yours,
Linus Walleij