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Subject: Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM
 based tachometer
To:     Mikko Perttunen <mperttunen@nvidia.com>,
        Rajkumar Rampelli <rrajk@nvidia.com>,
        Mikko Perttunen <cyndis@kapsi.fi>, 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, arnd@arndb.de,
        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
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From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <e85734cd-c91a-a7ed-380e-e2b95fc1632f@roeck-us.net>
Date:   Thu, 8 Mar 2018 06:33:03 -0800
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On 03/07/2018 11:57 PM, Mikko Perttunen wrote:
> 
> 
> On 07.03.2018 16:20, Guenter Roeck wrote:
>> On 03/07/2018 01:47 AM, Rajkumar Rampelli wrote:
>>>
>>>
>>> On Wednesday 28 February 2018 07:59 PM, Guenter Roeck wrote:
>>>> On 02/27/2018 11:03 PM, Mikko Perttunen wrote:
>>>>> On 02/28/2018 08:12 AM, Rajkumar Rampelli wrote:
>>>>>>
>>>>>> On Wednesday 28 February 2018 11:28 AM, Guenter Roeck wrote:
>>>>>>> On 02/27/2018 09:38 PM, Rajkumar Rampelli wrote:
>>>>>>>>
>>>>>>>> On Wednesday 21 February 2018 08:20 PM, Guenter Roeck wrote:
>>>>>>>>> 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.
>>>>>>>> Guenter Roeck,
>>>>>>>> I will define a new hwmon sysfs attribute node called
>>>>>>>> "hwmon_tachometer_attributes" in hwmon.h like below and update
>>>>>>>> the same in tachometer hwmon driver. Is it fine ?
>>>>>>>> enum hwmon_tachometer_attributes {
>>>>>>>
>>>>>>> Are you kidding me ?
>>>>>>>
>>>>>>> Guenter
>>>>>> Sorry, I just wanted to confirm whether my understanding is correct
>>>>>> or not before implementing it actually.
>>>>>> Or, shall I add this attribute as a part of fan attributes with
>>>>>> "hwmon_fan_rpm" ? or any other way to do it ? I need your inputs in
>>>>>> fixing this.
>>>>>
>>>>> I think he wants the attribute to be named according to the
>>>>> properties in this document:
>>>>> https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface. I
>>>>> guess the attribute would then map to fan1_input, though I'm not
>>>>> sure if that's 100% correct either since this could technically be
>>>>> attached to something other than a fan. But I would think in
>>>>> practice that's not a big concern.
>>>>>
>>>>> Guenter,
>>>>> Please correct me as well if I'm wrong.
>>>>>
>>>> You are absolutely correct.
>>>>
>>>> While I am not opposed to ABI changes, the merits of those would need
>>>> to be
>>>> discussed on the mailing list. But replacing "fan1_input" with "rpm" is
>>>> not an acceptable ABI change, even if it may measure something that
>>>> turns
>>>> but isn't a fan.
>>>>
>>>> If this _is_ in fact supposed to be used for something else but fans, we
>>>> would have to discuss what that might be, and if hwmon is the
>>>> appropriate
>>>> subsystem to measure and report it. This does to some degree lead
>>>> back to
>>>> my concern of having the "fan" part of this patch series in the pwm
>>>> core.
>>>> I am still not sure if that makes sense.
>>>>
>>>> Thanks,
>>>> Guenter
>>> I am planning to add tachometer support in pwm-fan.c driver
>>> (drivers/hwmon/) instead of adding new generic-pwm-tachometer.c
>>> driver. Measuring RPM value will be done in pwm-fan driver itself
>>> using pwm capture feature and will add new sysfs attributes under this
>>> driver to report rpm value of fan.
>>
>> There is an existing attribute to report the RPM of fans. It is called
>> fan[1..n]_input.
>>
>> "replacing "fan1_input" with "rpm" is not an acceptable ABI change"
>>
>> Preemptive NACK.
>>
>> Guenter
>>
> 
> I think the idea here (based on personal discussion with Rajkumar) was to add a "new" sysfs attribute the driver exports, but that attribute would be the existing fan[1..n]_input.
> 

Ah, sorry, in this case misunderstanding on my side.

Guenter