linux-kernel - Re: [PATCH v4 0/6] mfd/regulator/clk: bd71837: ROHM BD71837 PMIC driver

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Message-ID: <20180530110000.GI6920@sirena.org.uk>
Date:   Wed, 30 May 2018 12:00:00 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        mturquette@...libre.com, sboyd@...nel.org, robh+dt@...nel.org,
        mark.rutland@....com, lee.jones@...aro.org, lgirdwood@...il.com,
        linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, mikko.mutanen@...rohmeurope.com,
        heikki.haikola@...rohmeurope.com
Subject: Re: [PATCH v4 0/6] mfd/regulator/clk: bd71837: ROHM BD71837 PMIC
 driver

On Wed, May 30, 2018 at 12:05:12PM +0300, Matti Vaittinen wrote:

> Other 4 can be used on PWM or PFM switching mode. When PWM is used
> voltages can be changed without disabling regulator. On PFM this should
> not be done. These latter 4 regulators can be forced to PWM mode via
> control bit in register. This driver does not support controlling this
> mode though. So this driver version just checks if regulator is enabled
> before changing the voltage and if it is the voltage change fails with
> -EBUSY

It probably should support setting modes (especially if the device isn't
smart enough to automatically shift which sounds like the case) but
that's a separate thing.

> My question is whether it would be good idea to also read the 'force
> PWM' bit when voltage is changed and allow the change if PWM mode is
> forced to be used? Problem is that the check and voltage change can't be
> atomic so there is a chance someone changes the mode (bypassing the
> driver and regulator core) after this check but before we get to modify
> the voltage. Furthermore, I doubt the 'force PWM' is widely used (but I
> can't say for sure as I can't imagine all use cases) as it is not so
> power efficient.

Why would anything else be changing the mode configuration of the
regulator while the system is running?  That sounds like a bad idea.
In any case if the hardware really needs to be manually put into force
PWM to change voltage then the simplest thing would be to just move it
into force PWM mode, do the change, then change back if it wasn't
already in force PWM mode.

The tradeoff with forced PWM mode is that the quality of regulation will
be a lot better, especially if the load changes suddenly (as things like
CPUs often do).  Most hardware that's at all current is able respond to
changes in load and switch modes automatically when it's appropriate,
except possibly in some very low power modes.

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