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Subject: Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM
 based tachometer
To:     Rajkumar Rampelli <rrajk@nvidia.com>, robh+dt@kernel.org,
        mark.rutland@arm.com, thierry.reding@gmail.com,
        jonathanh@nvidia.com, jdelvare@suse.com, corbet@lwn.net,
        catalin.marinas@arm.com, will.deacon@arm.com,
        kstewart@linuxfoundation.org, gregkh@linuxfoundation.org,
        pombredanne@nexb.com, mmaddireddy@nvidia.com,
        mperttunen@nvidia.com, arnd@arndb.de,
        gregory.clement@free-electrons.com, timur@codeaurora.org,
        andy.gross@linaro.org, xuwei5@hisilicon.com, elder@linaro.org,
        heiko@sntech.de, krzk@kernel.org, ard.biesheuvel@linaro.org
Cc:     devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pwm@vger.kernel.org, linux-tegra@vger.kernel.org,
        linux-hwmon@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, ldewangan@nvidia.com
References: <1519196339-9377-1-git-send-email-rrajk@nvidia.com>
 <1519196339-9377-6-git-send-email-rrajk@nvidia.com>
From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <5172edff-fe29-7ed1-f0d2-b0359ea2c24d@roeck-us.net>
Date:   Wed, 21 Feb 2018 06:50:47 -0800
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On 02/20/2018 10:58 PM, Rajkumar Rampelli wrote:
> Add generic PWM based tachometer driver via HWMON interface
> to report the RPM of motor. This drivers get the period/duty
> cycle from PWM IP which captures the motor PWM output.
> 
> This driver implements a simple interface for monitoring the speed of
> a fan and exposes it in roatations per minute (RPM) to the user space
> by using the hwmon's sysfs interface
> 
> Signed-off-by: Rajkumar Rampelli <rrajk@nvidia.com>
> ---
>   Documentation/hwmon/generic-pwm-tachometer |  17 +++++
>   drivers/hwmon/Kconfig                      |  10 +++
>   drivers/hwmon/Makefile                     |   1 +
>   drivers/hwmon/generic-pwm-tachometer.c     | 112 +++++++++++++++++++++++++++++
>   4 files changed, 140 insertions(+)
>   create mode 100644 Documentation/hwmon/generic-pwm-tachometer
>   create mode 100644 drivers/hwmon/generic-pwm-tachometer.c
> 
> diff --git a/Documentation/hwmon/generic-pwm-tachometer b/Documentation/hwmon/generic-pwm-tachometer
> new file mode 100644
> index 0000000..e0713ee
> --- /dev/null
> +++ b/Documentation/hwmon/generic-pwm-tachometer
> @@ -0,0 +1,17 @@
> +Kernel driver generic-pwm-tachometer
> +====================================
> +
> +This driver enables the use of a PWM module to monitor a fan. It uses the
> +generic PWM interface and can be used on SoCs as along as the SoC supports
> +Tachometer controller that moniors the Fan speed in periods.
> +
> +Author: Rajkumar Rampelli <rrajk@nvidia.com>
> +
> +Description
> +-----------
> +
> +The driver implements a simple interface for monitoring the Fan speed using
> +PWM module and Tachometer controller. It requests period value through PWM
> +capture interface to Tachometer and measures the Rotations per minute using
> +received period value. It exposes the Fan speed in RPM to the user space by
> +using the hwmon's sysfs interface.
> diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
> index ef23553..8912dcb 100644
> --- a/drivers/hwmon/Kconfig
> +++ b/drivers/hwmon/Kconfig
> @@ -1878,6 +1878,16 @@ config SENSORS_XGENE
>   	  If you say yes here you get support for the temperature
>   	  and power sensors for APM X-Gene SoC.
>   
> +config GENERIC_PWM_TACHOMETER
> +	tristate "Generic PWM based tachometer driver"
> +	depends on PWM
> +	help
> +	  Enables a driver to use PWM signal from motor to use
> +	  for measuring the motor speed. The RPM is captured by
> +	  PWM modules which has PWM capture capability and this
> +	  drivers reads the captured data from PWM IP to convert
> +	  it to speed in RPM.
> +
>   if ACPI
>   
>   comment "ACPI drivers"
> diff --git a/drivers/hwmon/Makefile b/drivers/hwmon/Makefile
> index f814b4a..9dcc374 100644
> --- a/drivers/hwmon/Makefile
> +++ b/drivers/hwmon/Makefile
> @@ -175,6 +175,7 @@ obj-$(CONFIG_SENSORS_WM8350)	+= wm8350-hwmon.o
>   obj-$(CONFIG_SENSORS_XGENE)	+= xgene-hwmon.o
>   
>   obj-$(CONFIG_PMBUS)		+= pmbus/
> +obj-$(CONFIG_GENERIC_PWM_TACHOMETER) += generic-pwm-tachometer.o
>   
>   ccflags-$(CONFIG_HWMON_DEBUG_CHIP) := -DDEBUG
>   
> diff --git a/drivers/hwmon/generic-pwm-tachometer.c b/drivers/hwmon/generic-pwm-tachometer.c
> new file mode 100644
> index 0000000..9354d43
> --- /dev/null
> +++ b/drivers/hwmon/generic-pwm-tachometer.c
> @@ -0,0 +1,112 @@
> +/*
> + * Copyright (c) 2017-2018, NVIDIA CORPORATION.  All rights reserved.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + *
> + */
> +
> +#include <linux/module.h>
> +#include <linux/platform_device.h>
> +#include <linux/err.h>
> +#include <linux/pwm.h>
> +#include <linux/hwmon.h>
> +#include <linux/hwmon-sysfs.h>
> +
> +struct pwm_hwmon_tach {
> +	struct device		*dev;
> +	struct pwm_device	*pwm;
> +	struct device		*hwmon;
> +};
> +
> +static ssize_t show_rpm(struct device *dev, struct device_attribute *attr,
> +			char *buf)
> +{
> +	struct pwm_hwmon_tach *ptt = dev_get_drvdata(dev);
> +	struct pwm_device *pwm = ptt->pwm;
> +	struct pwm_capture result;
> +	int err;
> +	unsigned int rpm = 0;
> +
> +	err = pwm_capture(pwm, &result, 0);
> +	if (err < 0) {
> +		dev_err(ptt->dev, "Failed to capture PWM: %d\n", err);
> +		return err;
> +	}
> +
> +	if (result.period)
> +		rpm = DIV_ROUND_CLOSEST_ULL(60ULL * NSEC_PER_SEC,
> +					    result.period);
> +
> +	return sprintf(buf, "%u\n", rpm);
> +}
> +
> +static SENSOR_DEVICE_ATTR(rpm, 0444, show_rpm, NULL, 0);
> +
> +static struct attribute *pwm_tach_attrs[] = {
> +	&sensor_dev_attr_rpm.dev_attr.attr,
> +	NULL,
> +};

"rpm" is not a standard hwmon sysfs attribute. If you don't provide
a single standard hwmon sysfs attribute, having a hwmon driver is pointless.

> +
> +ATTRIBUTE_GROUPS(pwm_tach);
> +
> +static int pwm_tach_probe(struct platform_device *pdev)
> +{
> +	struct pwm_hwmon_tach *ptt;
> +	int err;
> +
> +	ptt = devm_kzalloc(&pdev->dev, sizeof(*ptt), GFP_KERNEL);
> +	if (!ptt)
> +		return -ENOMEM;
> +
> +	ptt->dev = &pdev->dev;
> +
> +	platform_set_drvdata(pdev, ptt);
> +	dev_set_drvdata(&pdev->dev, ptt);
> +

None of those is used.

> +	ptt->pwm = devm_of_pwm_get(&pdev->dev, pdev->dev.of_node, NULL);
> +	if (IS_ERR(ptt->pwm)) {
> +		err = PTR_ERR(ptt->pwm);
> +		dev_err(&pdev->dev, "Failed to get pwm handle, err: %d\n",
> +			err);
> +		return err;
> +	}
> +
> +	ptt->hwmon = devm_hwmon_device_register_with_groups(&pdev->dev,
> +					 "pwm_tach", ptt, pwm_tach_groups);

Please use the new API - devm_hwmon_device_register_with_info -
for new drivers.

> +	if (IS_ERR(ptt->hwmon)) {
> +		err = PTR_ERR_OR_ZERO(ptt->hwmon);
> +		dev_err(&pdev->dev, "Failed to register hwmon device: %d\n",
> +			err);
> +		return err;
> +	}

Please no noise on failure. The failure is going to be a memory allocation failure,
which is reported already, and user space will be informed with the error code.

> +
> +	return 0;
> +}
> +
> +static const struct of_device_id pwm_tach_of_match[] = {
> +	{ .compatible = "generic-pwm-tachometer" },
> +	{}
> +};
> +MODULE_DEVICE_TABLE(of, pwm_tach_of_match);
> +
> +static struct platform_driver pwm_tach_driver = {
> +	.driver = {
> +		.name = "generic-pwm-tachometer",
> +		.of_match_table = pwm_tach_of_match,
> +	},
> +	.probe = pwm_tach_probe,
> +};
> +
> +module_platform_driver(pwm_tach_driver);
> +
> +MODULE_DESCRIPTION("PWM based Generic Tachometer driver");
> +MODULE_AUTHOR("Laxman Dewangan <ldewangan@nvidia.com>");
> +MODULE_AUTHOR("Rajkumar Rampelli <rrajk@nvidia.com>");
> +MODULE_LICENSE("GPL v2");
>