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

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Message-ID: <332ab946-daee-bb83-24ab-0bda4fd8e1ef@gmail.com>
Date:   Thu, 19 Nov 2020 17:22:43 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Thierry Reding <thierry.reding@...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

16.11.2020 16:33, Mark Brown пишет:
> On Sun, Nov 15, 2020 at 08:42:10PM +0300, Dmitry Osipenko wrote:
>> 13.11.2020 20:28, Mark Brown пишет:
> 
>>>> What should we do?
> 
>>> As I keep saying the consumer driver should be enumerating the voltages
>>> it can set, if it can't find any and wants to continue then it can just
>>> skip setting voltages later on.  If only some are unavailable then it
>>> probably wants to eliminate those specific OPPs instead.
> 
>> I'm seeing a dummy regulator as a helper for consumer drivers which
>> removes burden of handling an absent (optional) regulator. Is this a
>> correct understanding of a dummy regulator?
> 
>> Older DTBs don't have a regulator and we want to keep them working. This
>> is equal to a physically absent regulator and in this case it's an
>> optional regulator, IMO.
> 
> No, you are failing to understand the purpose of this code.  To
> reiterate unless the device supports operating with the supply
> physically absent then the driver should not be attempting to use
> regulator_get_optional().  That exists specifically for the case where
> the supply may be absent, nothing else.  The dummy regulator is there
> precisely for the case where the system does not describe supplies that
> we know are required for the device to function, it fixes up that
> omission so we don't need to open code handling of this in every single
> consumer driver.

The original intention of regulator_get_optional() is clear to me, but
nothing really stops drivers from slightly re-purposing this API, IMO.

Drivers should be free to assume that if regulator isn't defined by
firmware, then it's physically absent if this doesn't break anything. Of
course in some cases it's unsafe to make such assumptions. I think it's
a bit unpractical to artificially limit API usage without a good reason,
i.e. if nothing breaks underneath of a driver.

> Regulators that are present but not described by the firmware are a
> clearly different case to regulators that are not physically there,
> hardware with actually optional regulators will generally require some
> configuration for this case.
> 

I have good news. After spending some more time on trying out different
things, I found that my previous assumption about the fixed-regulator
was wrong, it actually accepts voltage changes, i.e. regulator consumer
doesn't get a error on a voltage-change. This is exactly what is needed
for the OPP core to work properly.

This means that there is no need to add special quirks to work around
absent regulators, we will just add a fixed regulator to the DTs which
don't specify a real regulator. The OPP core will perform voltage
checking and filter out unsupported OPPs. The older DTBs will continue
to work as well.