linux-kernel - Re: [PATCH v1 11/30] drm/tegra: dc: Support OPP and SoC core voltage scaling

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Message-ID: <20201110203257.GC5957@sirena.org.uk>
Date:   Tue, 10 Nov 2020 20:32:57 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Peter Chen <Peter.Chen@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        linux-samsung-soc@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-usb@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH v1 11/30] drm/tegra: dc: Support OPP and SoC core voltage
 scaling

On Tue, Nov 10, 2020 at 09:29:45PM +0100, Thierry Reding wrote:
> On Thu, Nov 05, 2020 at 02:44:08AM +0300, Dmitry Osipenko wrote:

> > +	/*
> > +	 * Voltage scaling is optional and trying to set voltage for a dummy
> > +	 * regulator will error out.
> > +	 */
> > +	if (!device_property_present(dc->dev, "core-supply"))
> > +		return;

> This is a potentially heavy operation, so I think we should avoid that
> here. How about you use devm_regulator_get_optional() in ->probe()? That
> returns -ENODEV if no regulator was specified, in which case you can set
> dc->core_reg = NULL and use that as the condition here.

Or enumerate the configurable voltages after getting the regulator and
handle that appropriately which would be more robust in case there's
missing or unusual constraints.

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