Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM based tachometer

[Guenter Roeck <linux@roeck-us.net>]

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.

> >
> >>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");
> >>>
> >>
> >