linux-kernel - Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO

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Message-ID: <20180823131816.741c09cc@ipc1.ka-ro>
Date:   Thu, 23 Aug 2018 13:18:16 +0200
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Michal Vokáč <michal.vokac@...ft.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lukasz Majewski <l.majewski@...ess.pl>,
        Fabio Estevam <fabio.estevam@....com>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM
 output between PWM and GPIO

Michal Vokáč <michal.vokac@...ft.com> wrote:
> On 22.8.2018 16:10, Lothar Waßmann wrote:
> > My use case is attaching different displays to the same baseboard,
> > where some displays have the brightness control pin inverted with
> > respect to the others. It's easy to change the compatible string for
> > the simple-panel driver and the PWM polarity setting for the
> > pwm-backlight driver from U-Boot according to the display model, but
> > it's not so easy, to edit the pinctrl settings from pull-up to
> > pull-down or vice versa.  
> 
> OK, I got it. Though that is something different than having two clients,
> right?
>
> You do not actually need to change the pinctrl pull-up/down configuration
> in bootloader. You define the two pinctrl groups as I suggested in the
> example. Or more precisely, you add a new pinctrl group where the PWM
> output pad is configured as a GPIO with pull-up. You add this group to
> all your common device trees. This does no harm as the group is not used
> yet.
> 
> In bootloader you detect the type of the panel. Normal PWM polarity? OK,
> do nothing and boot. Inverted PWM polarity? Set the pinctrl-names property
> to "default", "pwm". Set the pinctrl-0 property to point to the GPIO group
> and set pinctrl-1 property to point to the old PWM group.
> 
> E.g. something like:
> 
> => fdt set /soc/aips-bus@...0000/pwm@...0000 pinctrl-names default pwm
> => fdt get value gpio-phandle /soc/aips-bus@...0000/iomuxc@...0000/pwm1grp-gpio phandle
> => fdt get value pwm-phandle /soc/aips-bus@...0000/iomuxc@...0000/pwm1grp-pwm phandle
> => fdt set /soc/aips-bus@...0000/pwm@...0000 pinctrl-0 ${gpio-phandle}
> => fdt set /soc/aips-bus@...0000/pwm@...0000 pinctrl-1 ${pwm-phandle}  
> 
> Will this work for you?
> 
This would probably work, but it's quite ugly.
I'd still prefer to set the pin output state to the desired level
rather than relying on the chip internal pullup/pulldown facility.


Lothar Waßmann
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