linux-kernel - Re: [PATCH 3/8] mmc: core: Add mmc_regulator_set

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Date:	Wed, 2 Sep 2015 09:20:58 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Heiko Stuebner <heiko@...ech.de>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"tgih.jun@...sung.com" <tgih.jun@...sung.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	linux-mmc <linux-mmc@...r.kernel.org>, linux-clk@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/8] mmc: core: Add mmc_regulator_set_vqmmc()

Ulf,

On Wed, Sep 2, 2015 at 4:38 AM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
>> +/**
>> + * mmc_regulator_set_vqmmc - Set VQMMC as per the ios
>> + *
>> + * For 3.3V signaling, we try to match VQMMC to VMMC as closely as possible.
>
> Looking at the code, I don't think this statement is entirely true.
> Isn't it so that we will be trying with a maximum tolerance of 0.3 V
> towards the VMMC voltage level (then fall-back to the complete range)?
> Perhaps you can find a better way to describe that in the change log.

If regulator_set_voltage_triplet() is ever implemented more correctly
then the description here is correct.  ...the problem is that
regulator_set_voltage_triplet() is still using the same shortcut that
regulator_set_voltage_tol() was using.


>> +int mmc_regulator_set_vqmmc(struct mmc_host *mmc, struct mmc_ios *ios)
>> +{
>> +       int volt, min_uV, max_uV;
>> +
>> +       /* If no vqmmc supply then we can't change the voltage */
>> +       if (IS_ERR(mmc->supply.vqmmc))
>> +               return -EINVAL;
>
> In general vqmmc is considered as an optional regulator and that's
> also how host drivers treat it. So perhaps it would make sense to
> return 0 here instead of an error code or what do you think?

The idea is that since this is intended to be called by
start_signal_voltage_switch() and having no vqmmc should be considered
an error for start_signal_voltage_switch() then it should be an error
here.  What do you think?


>> +
>> +               /* try to stay close to vmmc at first */
>> +               if (!mmc_regulator_set_voltage_if_supported(mmc->supply.vqmmc,
>> +                                               min_uV, volt, max_uV))
>> +                       return 0;
>> +
>> +               return mmc_regulator_set_voltage_if_supported(mmc->supply.vqmmc,
>> +                                               2700000, volt, 3600000);

The whole fact that there are two calls here is really just because of
the limitations of the current implementation of
regulator_set_voltage_triplet().  If that implementation is ever fixed
then we'd just need a single call.  Probably worth a comment saying
that?
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