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Subject: Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM
 based tachometer
To:     Guenter Roeck <linux@roeck-us.net>,
        Mikko Perttunen <cyndis@kapsi.fi>
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From:   RAJKUMAR <rrajk@nvidia.com>
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Date:   Fri, 23 Feb 2018 17:15:26 +0530
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On Wednesday 21 February 2018 09:38 PM, Guenter Roeck wrote:
> On Wed, Feb 21, 2018 at 05:20:29PM +0200, Mikko Perttunen wrote:
>> On 21.02.2018 16:46, Guenter Roeck wrote:
>>> On 02/20/2018 11:15 PM, Mikko Perttunen wrote:
>>>> AIUI, the PWM framework already exposes a sysfs node with period
>>>> information. We should just use that instead of adding a new driver
>>>> for this.
>>>>
>>> I am kind of lost. Please explain.
>>>
>>> Are you saying that we should drop the pwm-fan driver as well (which goes
>>> the opposite way), as well as any other drivers doing anything with pwm
>>> signals,
>>> because after all those signals are already exposed to userspace a sysfs
>>> attributes,
>>> and a kernel driver to abstract those values is thus not needed ?
>> The only thing this driver does is do a constant division in kernelspace.
>> I'm not really seeing why that couldn't be done in userspace. But if you
>> think it's appropriate to do the RPM conversion in kernelspace then I'm not
>> greatly opposed to that.
>>
> Isn't that true for any conversion from and to pwm values ? Voltages,
> for example ? Should those be handled in userspace as well ?
>
> Note that I am not entirely convinced that the approach suggested in this
> patch series makes sense. Patch 4 seems to move the notion of a tachometer
> into the pwm subsystem. I am not really convinced that this makes sense
> (maybe all that code should be in the hwmon driver instead, or in a thermal
> driver if the author prefers). But that is a different issue. The question
> you raised is if there should be any abstraction to or from raw pwm values
> in the kernel.
Since tachometer driver implements PWM capture callback feature, I have 
added it under pwm driver as pwm-tegra-tachometer.c driver.
If the best approach is to have this tachometer driver under hwmon 
driver then I will move it to hwmon driver.
>
>>>> In any case, we cannot add something like this to device tree since
>>>> it's not a hardware device.
>>>>
>>> So you are saying there is no means to express in devicetree that
>>> a pwm input is connected to a fan ? How is that not hardware ?
>>>
>>> If so, how do you express in devicetree that a pwm signal is connected
>>> to anything ?
>> If we want to describe that the tachometer is connected to a fan, then we
>> should have a fan node in the board's device tree. We don't have a chip that
>> has a thing called "generic-pwm-tachometer" attached to it. (We have chips
>> that have a "nvidia,tegra186-tachometer", so it's proper to have that.)
>>
> So you are concerned about the property name ? I don't really care how
> it is called.
>
> Guenter
>
>> Thanks,
>> Mikko
>>
>>> Guenter
>>>
>>>> Mikko
>>>>
>>>> On 21.02.2018 08:58, 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,
>>>>> +};
>>>>> +
>>>>> +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);
>>>>> +
>>>>> +    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);
>>>>> +    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;
>>>>> +    }
>>>>> +
>>>>> +    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");
>>>>>