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

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Message-ID: <72ae6462-13df-9fcb-510e-8e57eee0f035@gmail.com>
Date:   Wed, 11 Nov 2020 00:23:41 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Mark Brown <broonie@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     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

10.11.2020 23:32, Mark Brown пишет:
> 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.
> 

I already changed that code to use regulator_get_optional() for v2.

Regarding the enumerating supported voltage.. I think this should be
done by the OPP core, but regulator core doesn't work well if
regulator_get() is invoked more than one time for the same device, at
least there is a loud debugfs warning about an already existing
directory for a regulator. It's easy to check whether the debug
directory exists before creating it, like thermal framework does it for
example, but then there were some other more difficult issues.. I don't
recall what they were right now. Perhaps will be easier to simply get a
error from regulator_set_voltage() for now because it shouldn't ever
happen in practice, unless device-tree has wrong constraints.