linux-kernel - Using an optional regulator in a driver running on a PC

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Date:	Sun, 26 Jan 2014 02:53:07 -0800
From:	Guenter Roeck <groeck7@...il.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>
Subject: Using an optional regulator in a driver running on a PC

Hi all,

I am currently writing a driver which uses an optional regulator
to determine a reference voltage. The regulator is truly optional;
if it is not there, the chip uses an internal reference voltage.

I am testing the driver on a PC which does not really have any
regulators installed or enabled. Kernel version is 3.13.

I noticed that the call to regulator_get() returns -EPROBE_DEFER.
Looking into the regulator core, this appears intentional; there is
obviously no devicetree entry, and there is no regulator device.
The regulator code always returns -EPROBE_DEFER in this case.
I also tried regulator_get_optional(), with the same results.

This leads to an interesting question: How are drivers which require
regulators (optional or not) supposed to run on a system
which does not support devicetree, and does not have any
regulators installed (such as a PC) ? REGULATOR_DUMMY
isn't there anymore, and the dummy code it replaces only
executes on devicetree based systems.

Also, how are non-dt systems supposed to determine if an optional
regulator exists or not ? AFAICS the regulator code always returns
-EPROBE_DEFER, which isn't very helpful. If I just assume that
-EPROBE_DEFER means that the regulator is not there, I end up with
a conflict with a system which _does_ support devicetree, where
-EPROBE_DEFER really means that the probe needs to be deferred.

Thanks,
Guenter

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