linux-kernel - Re: [PATCH v3 1/6] regulator: core: validate selector against linear_min

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Message-ID: <cb096dc5-3757-d72f-41a9-c99007c84b40@nvidia.com>
Date:   Tue, 24 Nov 2020 09:36:52 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Claudiu Beznea <claudiu.beznea@...rochip.com>,
        <lgirdwood@...il.com>, <broonie@...nel.org>
CC:     <s.hauer@...gutronix.de>, <ttynkkynen@...dia.com>,
        <linus.walleij@...aro.org>, <axel.lin@...ics.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        linux-tegra <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v3 1/6] regulator: core: validate selector against
 linear_min_sel

On 13/11/2020 15:21, Claudiu Beznea wrote:
> There are regulators who's min selector is not zero. Selectors loops
> (looping b/w zero and regulator::desc::n_voltages) might throw errors
> because invalid selectors are used (lower than
> regulator::desc::linear_min_sel). For this situations validate selectors
> against regulator::desc::linear_min_sel.

After this commit was merged, I noticed a regression in the DFLL (CPU
clock source) on Tegra124. The DFLL driver
(drivers/clk/tegra/clk-dfll.c) calls regulator_list_voltage() in a loop
to determine the selector for a given voltage (see function
find_vdd_map_entry_exact()).

Currently, the DFLL driver queries the number of voltages provided by
the regulator by calling regulator_count_voltages() and then starting
from 0, iterates through the number of voltages to find the selector
value for the voltage it is looking for by calling
regulator_list_voltage(). It assumes that any negative value returned by
calling regulator_list_voltage() is an error and this will cause the
loop up to terminate.

In this case the regulator in question is the as3722 and the
linear_min_sel for this regulator is 1 and so when the DFLL driver calls
regulator_list_voltage() with a selector value of 0 it now returns a
negative error code, as expected by this change, and this terminates the
loop up in the DFLL driver. So I can clearly see why this is happening
and I could fix up the DFLL driver to avoid this.

Before doing so, I wanted to ask if that is the correct fix here,
because it seems a bit odd that regulator_count_voltages() returns N
voltages, but if the min selector value is greater than 0, then actually
there are less than N. However, changing the number of voltages
supported by the regulator to be N - linear_min_sel does not make sense
either because then we need to know the linear_min_sel in order to
determine the first valid voltage.

In case of the as3722, the value 0 means that the regulator is powered
down. So it is a valid setting and equates to 0 volts at the output AFAICT.

Please let me know your thoughts are the correct way to fix this up.

Thanks
Jon

-- 
nvpublic