linux-kernel - Re: [PATCH] video: mxsfb: get supply regulator optionally

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Message-ID: <4b3a7bda2202a536efaf4a693234c273@agner.ch>
Date:   Tue, 06 Sep 2016 11:23:31 -0700
From:   Stefan Agner <stefan@...er.ch>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     plagnioj@...osoft.com, robh+dt@...nel.org, mark.rutland@....com,
        devicetree@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] video: mxsfb: get supply regulator optionally

On 2016-09-06 01:21, Tomi Valkeinen wrote:
> Hi,
> 
> On 04/09/16 07:26, Stefan Agner wrote:
>> The lcd-supply is meant to be optional, there are several device-
>> trees not specifying it and the code handles error values silently.
>> Therefor, avoid creating a dummy regulator (and the associated
>> warning) by using devm_regulator_get_optional.
>>
>> While at it, document that fact also in the device-tree bindings.
> 
> The binding change looks correct, but using
> devm_regulator_get_optional() does not sound correct.
> 
> devm_regulator_get_optional() is to be used when the device in question
> truly can function without the power supply. But if the supply is there,
> it's just not controlled by the SW, devm_regulator_get() is to be used.

The framebuffer device can even function without a display, no problem
there.. Probably not really useful...

devm_regulator_get creates a dummy regulator and a warning. Afaik, the
dummy regulator was meant to be as an aid during development, but not as
a permanent solution. This is what the initial commit of the dummy
regulator says:

> In order to ease transitions with drivers are boards start using regulators
> provide an option to cause all regulator_get() calls to succeed, with a
> dummy always on regulator being supplied where one has not been configured.
> A warning is printed whenever the dummy regulator is used to aid system
> development.

I think we should either make the property mandatory and fix the device
trees or we should fix the driver to support an optional regulator. The
code already supports the reg_lcd being NULL, which is probably mostly
pointless right now as devm_regulator_get always returns a dummy
regulator.

--
Stefan