linux-kernel - Re: Where to update regulator register with initial voltage set by HW

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Message-ID: <20170704111935.jwusehzugpi5jl62@sirena.org.uk>
Date:   Tue, 4 Jul 2017 12:19:35 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>
Cc:     "Bartholomae, Thomas" <t.bartholomae@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Liam Girdwood <lgirdwood@...il.com>
Subject: Re: Where to update regulator register with initial voltage set by HW

On Tue, Jul 04, 2017 at 12:58:04PM +0200, Waldemar Rymarkiewicz wrote:
> On 3 July 2017 at 17:36, Mark Brown <broonie@...nel.org> wrote:
> > On Mon, Jul 03, 2017 at 05:33:03PM +0200, Waldemar Rymarkiewicz wrote:

> >> I've asked also on TI forum if this is typical to the regulator not to
> >> determine the startup voltage but still waiting for feedback. Anyway,
> >> if this is the case I guess a driver is a good place to update
> >> register before we register to the regulator framework.

> > It's really unusual to have a device that has the voltage changable by
> > register write at runtime where the current state can't be read back.

> or you did not realise that this is initialised by bootloader for example.

No, it's just incredibly unusual to have hardware that does this.

> After investigating this issue a bit more I've found that this is
> rather typical for power regulators not to update a register with
> startup voltage set by a feedback resistor divider as it would cost
> extra circuit.  So, I assume that most likely a bootloader normally
> initializes power regulator in case it's needed eg. if it's supplying
> CPU which is DVS-enabled.

The majority of things that have configuration based on bias resistors
just wouldn't have any register based control at all, or wouldn't have
any physical readback (for really old devices that were concerned with
the I2C licensing issues).  I'm pretty surprised to see such a
combination in a current system.

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