linux-kernel - Re: [PATCH v2 3/3] Input: pwm-beeper: add optional amplifier regulator

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Date:   Thu, 19 Jan 2017 14:34:31 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     David Lechner <david@...hnology.com>
Cc:     linux-input@...r.kernel.org, devicetree@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] Input: pwm-beeper: add optional amplifier
 regulator

On Sun, Jan 15, 2017 at 07:04:09PM -0600, David Lechner wrote:
> On 01/15/2017 06:34 PM, Dmitry Torokhov wrote:
> >On Sun, Jan 15, 2017 at 06:12:29PM -0600, David Lechner wrote:
> >>On 01/14/2017 01:19 PM, Dmitry Torokhov wrote:
> >>>On Wed, Jan 11, 2017 at 02:02:01PM -0600, David Lechner wrote:
> >>>>This adds an optional regulator to the pwm-beeper device. This regulator
> >>>>acts as an amplifier. The amplifier is only enabled while beeping in order
> >>>>to reduce power consumption.
> >>>>
> >>>>Tested on LEGO MINDSTORMS EV3, which has a speaker connected to PWM through
> >>>>an amplifier.
> >>>>
> >>>>Signed-off-by: David Lechner <david@...hnology.com>
> >>>>---
> >>>>drivers/input/misc/pwm-beeper.c | 29 ++++++++++++++++++++++++++++-
> >>>>1 file changed, 28 insertions(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/drivers/input/misc/pwm-beeper.c b/drivers/input/misc/pwm-beeper.c
> >>>>index 30ac227..708e88e 100644
> >>>>--- a/drivers/input/misc/pwm-beeper.c
> >>>>+++ b/drivers/input/misc/pwm-beeper.c
> >>>>@@ -14,6 +14,7 @@
> >>>> */
> >>>>
> >>>>#include <linux/input.h>
> >>>>+#include <linux/regulator/consumer.h>
> >>>>#include <linux/module.h>
> >>>>#include <linux/kernel.h>
> >>>>#include <linux/of.h>
> >>>>@@ -25,8 +26,10 @@
> >>>>struct pwm_beeper {
> >>>>	struct input_dev *input;
> >>>>	struct pwm_device *pwm;
> >>>>+	struct regulator *reg;
> >>>>	struct work_struct work;
> >>>>	unsigned long period;
> >>>>+	bool reg_enabled;
> >>>>};
> >>>>
> >>>>#define HZ_TO_NANOSECONDS(x) (1000000000UL/(x))
> >>>>@@ -38,8 +41,20 @@ static void __pwm_beeper_set(struct pwm_beeper *beeper)
> >>>>	if (period) {
> >>>>		pwm_config(beeper->pwm, period / 2, period);
> >>>>		pwm_enable(beeper->pwm);
> >>>>-	} else
> >>>>+		if (beeper->reg) {
> >>>>+			int error;
> >>>>+
> >>>>+			error = regulator_enable(beeper->reg);
> >>>>+			if (!error)
> >>>>+				beeper->reg_enabled = true;
> >>>>+		}
> >>>>+	} else {
> >>>>+		if (beeper->reg_enabled) {
> >>>>+			regulator_disable(beeper->reg);
> >>>>+			beeper->reg_enabled = false;
> >>>>+		}
> >>>>		pwm_disable(beeper->pwm);
> >>>>+	}
> >>>>}
> >>>>
> >>>>static void pwm_beeper_work(struct work_struct *work)
> >>>>@@ -82,6 +97,10 @@ static void pwm_beeper_stop(struct pwm_beeper *beeper)
> >>>>{
> >>>>	cancel_work_sync(&beeper->work);
> >>>>
> >>>>+	if (beeper->reg_enabled) {
> >>>>+		regulator_disable(beeper->reg);
> >>>>+		beeper->reg_enabled = false;
> >>>>+	}
> >>>>	if (beeper->period)
> >>>>		pwm_disable(beeper->pwm);
> >>>>}
> >>>>@@ -111,6 +130,14 @@ static int pwm_beeper_probe(struct platform_device *pdev)
> >>>>		return error;
> >>>>	}
> >>>>
> >>>>+	beeper->reg = devm_regulator_get_optional(&pdev->dev, "amp");
> >>>
> >>>If you do not use optional regulator then you will not have to check if
> >>>you have it or not everywhere: regulator core will give you a dummy that
> >>>you can toggle to your heart's content.
> >>
> >>Some months ago, I learned that if you are not using device tree and
> >>you do not call regulator_has_full_constraints(), then you do not
> >>get a dummy regulator. And here, we are only checking if the
> >>regulator exists in one place. We will still need the checks for
> >>beeper->reg_enabled to keep calls to regulator_enable() and
> >>regulator_disable() balanced.
> >
> >Why? You do not have checks for calls to pwm_enable() and pwm_disable(),
> >(or rather beeper->period is used as such flag) why regulator would be
> >any different?
> 
> regulator_enable() has a __must_check attribute on it, so we get
> compiler warnings if we do not check the return value. Also, if
> enabling the regulator fails and returns an error, then calling
> regulator_disable() later would cause an imbalance.
> 
> pwm_enable() and pwm_disable() work differently because they don't
> count how many times they have been called. regulator_enable() and
> regulator_disable(), on the other hand, work like reference
> counting.

Ah, you are right, but it is more than that. It is possible to receive
multiple SND_BELL/SND_TONE events with non-0 value. You need to check if
regulator is already enabled before trying to enable it second time, or
your counting will be off.

Thanks.

-- 
Dmitry