Message-ID: <546B5EF9.3080102@mentor.com>
Date:	Tue, 18 Nov 2014 17:00:09 +0200
From:	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To:	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>
CC:	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Question about

Hello,

I need to set a GPIO (active high) output high on boot, which enables a
power rail supplying some external devices.

I have a question regarding "regulator-boot-on" and "enable-active-high"
fixed regulator device tree properties (actually AFAIU it applies to
gpio regulator as well, by the way, which one is proper to use in my
situation?)

Here is what we have from the code:

  [...]
  constraints->boot_on = of_property_read_bool(np, "regulator-boot-on");
  [...]
  if (init_data->constraints.boot_on)
          config->enabled_at_boot = true;
  [...]
  config->enable_high = of_property_read_bool(np, "enable-active-high");
  [...]
  cfg.ena_gpio_invert = !config->enable_high;
  if (config->enabled_at_boot) {
          if (config->enable_high)
                  cfg.ena_gpio_flags |= GPIOF_OUT_INIT_HIGH;
          else
                  cfg.ena_gpio_flags |= GPIOF_OUT_INIT_LOW;
  } else {
          if (config->enable_high)
                  cfg.ena_gpio_flags |= GPIOF_OUT_INIT_LOW;
          else
                 cfg.ena_gpio_flags |= GPIOF_OUT_INIT_HIGH;
  }
  [...]
  ret = gpio_request_one(config->ena_gpio,
                         GPIOF_DIR_OUT | config->ena_gpio_flags,
                         rdev_get_name(rdev));
  [...]
  /* Enable GPIO at initial use */
  if (pin->enable_count == 0)
          gpiod_set_value_cansleep(pin->gpiod,
                                  !pin->ena_gpio_invert);
  [...]


If we simplify the matter by assuming GPIOF_OUT_INIT_LOW is inverted
GPIOF_OUT_INIT_HIGH and vice versa, then it is easy to compute by
running over the variants that GPIO output value is set HIGH (regardless
of GPIO active low status) if and only if "regulator-boot-on" is
provided and "enable-active-high" has no effect at all.

This fact confuses me, because from the general regulator and fixed
regulator device tree bindings documentation I get:

  [...]
  - regulator-boot-on: bootloader/firmware enabled regulator
  [...]
  - enable-active-high: Polarity of GPIO is Active high
  If this property is missing, the default assumed is Active low.
  [...]

According to the documentation I'd assume that "regulator-boot-on" does
not touch gpio output value setting (so, if it is controlled by
bootloader or firmware, then it might be out of Linux kernel control).
Also my impression of "enable-active-high" property is that is should
have some effect on the GPIO output value (but it is not, see above),
and actually I don't quite understand why this property exists - there
is a high chance that "enable-active-high" and the real GPIO polarity do
not coincide, it should be more reliable to get GPIO flags of a
particular GPIO right in the regulator driver/framework.

Let's consider two possible configurations:

| regulator-boot-on | enable-active-high | GPIO polarity | GPIO output |
+-------------------+--------------------+---------------+-------------+
|        no         |         yes        |  active high  |    low      |
|        no         |          no        |  active low   |   high      |

I'd rather think that both resulting GPIO outputs are incorrect or
better to say do not correspond to my perception of "regulator-boot-on"
and "enable-active-high" DTS properties described in the documentation,
however above "enable-active-high" and actual GPIO polarity are the same
(when they are not, it is another open topic for discussion).

Do I miss something or have a mistake? Is there a problem in the
implemented logic?

Should documentation be updated to reflect "regulator-boot-on" role that
a regulator is re-enabled by the kernel?

Should "enable-active-high" be replaced by getting GPIO flags directly?

Thank you in advance.

--
With best wishes,
Vladimir
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