linux-kernel - Re: [PATCH v1 4/4] hwmon: add driver for the Microchip LAN966x SoC

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Message-ID: <4e5c78ce651c258a4be33c01ec07a0c3@walle.cc>
Date:   Mon, 28 Mar 2022 13:28:12 +0200
From:   Michael Walle <michael@...le.cc>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Jean Delvare <jdelvare@...e.com>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        linux-hwmon@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/4] hwmon: add driver for the Microchip LAN966x SoC

Am 2022-03-27 20:22, schrieb Guenter Roeck:
> On 3/27/22 07:18, Michael Walle wrote:
>> Am 2022-03-27 03:34, schrieb Guenter Roeck:
>> 
>>>> +    /*
>>>> +     * Data is given in pulses per second. According to the hwmon 
>>>> ABI we
>>>> +     * have to assume two pulses per revolution.
>>> 
>>> The hwmon ABI doesn't make any such assumptions. It wants to see RPM,
>>> that is all. Pulses per revolution is a fan property.
>> 
>> There is fanY_pulses according to 
>> Documentation/ABI/testing/sysfs-class-hwmon:
>> 
>>    Should only be created if the chip has a register to configure
>>    the number of pulses. In the absence of such a register (and
>>    thus attribute) the value assumed by all devices is 2 pulses
>>    per fan revolution.
>> 
>> The hardware returns just the pulses per second. Doesn't that
>> mean I have to divide that value by two?
>> 
> 
> The above refers to hardware which reports RPM.
> 
> It is up to the driver to calculate and return RPM. How you do it is 
> your
> decision. Drivers should report the most likely correct RPM value to
> userspace, one that rarely needs manual adjustment. Almost all fans
> report two pulses per revolution, so normally that assumption is used
> to convert PPM to RPM. That isn't mandated (or supposed to be mandated)
> by the ABI. I would call it common sense.
> 
> I'll be happy to accept a patch clarifying this.

Where would that go? into the sysfs abi description of the
fanY_input?

>>>> +     */
>>>> +    *val = FIELD_GET(FAN_CNT_DATA, data) * 60 / 2;
>> 
>> .. otherwise this should then be
>> *val = FIELD_GET(FAN_CNT_DATA, data) * 60;
>> 
> 
> If you really want to do this, make sure it is well documented that 
> users
> will need to adjust the fan speed via sensors3.conf to get the real fan 
> speed.
> 
>> 
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +static int lan966x_hwmon_read_pwm(struct device *dev, long *val)
>>>> +{
>>>> +    struct lan966x_hwmon *hwmon = dev_get_drvdata(dev);
>>>> +    unsigned int data;
>>>> +    int ret;
>>>> +
>>>> +    ret = regmap_read(hwmon->regmap_fan, FAN_CFG, &data);
>>>> +    if (ret < 0)
>>>> +        return ret;
>>>> +
>>>> +    *val = FIELD_GET(FAN_CFG_DUTY_CYCLE, data);
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +static int lan966x_hwmon_read_pwm_freq(struct device *dev, long 
>>>> *val)
>>>> +{
>>>> +    struct lan966x_hwmon *hwmon = dev_get_drvdata(dev);
>>>> +    unsigned long rate = clk_get_rate(hwmon->clk);
>>> 
>>> Is that a dynamic frequency ? If not, it would be better to read it 
>>> once
>>> and store it in struct lan966x_hwmon.
>> 
>> yes it is configurable, actually. See lan966x_hwmon_write_pwm_freq().
>> 
> 
> That is the pwm frequency, not the clock frequency. I don't see any
> code which updates the clock frequency reported by 
> clk_get_rate(hwmon->clk),
> ie I don't see a call to clk_set_rate().

Ahh sorry, missunderstood you. Yeah, in v2 it will be read
during probe.

-michael