linux-kernel - Re: [PATCH 02/10] hwmon: (amc6821) Make reading and writing fan speed limits consistent

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Message-ID: <80a7f733-655e-4b00-a802-825d3acaafcb@roeck-us.net>
Date: Mon, 1 Jul 2024 07:37:37 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Quentin Schulz <quentin.schulz@...rry.de>, linux-hwmon@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Farouk Bouabid <farouk.bouabid@...rry.de>
Subject: Re: [PATCH 02/10] hwmon: (amc6821) Make reading and writing fan speed
 limits consistent

On 7/1/24 07:11, Guenter Roeck wrote:
> On 7/1/24 04:05, Quentin Schulz wrote:
>> Hi Guenter,
>>
>> On 6/28/24 5:13 PM, Guenter Roeck wrote:
>>> The default value of the maximum fan speed limit register is 0,
>>> essentially translating to an unlimited fan speed. When reading
>>> the limit, a value of 0 is reported in this case. However, writing
>>> a value of 0 results in writing a value of 0xffff into the register,
>>> which is inconsistent.
>>>  > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>>> ---
>>>   drivers/hwmon/amc6821.c | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/hwmon/amc6821.c b/drivers/hwmon/amc6821.c
>>> index 3c614a0bd192..e37257ae1a6b 100644
>>> --- a/drivers/hwmon/amc6821.c
>>> +++ b/drivers/hwmon/amc6821.c
>>> @@ -601,7 +601,7 @@ static ssize_t fan_show(struct device *dev, struct device_attribute *devattr,
>>>       struct amc6821_data *data = amc6821_update_device(dev);
>>>       int ix = to_sensor_dev_attr(devattr)->index;
>>>       if (0 == data->fan[ix])
>>> -        return sprintf(buf, "0");
>>> +        return sprintf(buf, "6000000");
>>>       return sprintf(buf, "%d\n", (int)(6000000 / data->fan[ix]));
>>>   }
>>> @@ -625,10 +625,10 @@ static ssize_t fan_store(struct device *dev, struct device_attribute *attr,
>>>       int ret = kstrtol(buf, 10, &val);
>>>       if (ret)
>>>           return ret;
>>> -    val = 1 > val ? 0xFFFF : 6000000/val;
>>> +    val = val < 1 ? 0xFFFF : 6000000 / val;
>>>       mutex_lock(&data->update_lock);
>>> -    data->fan[ix] = (u16) clamp_val(val, 1, 0xFFFF);
>>> +    data->fan[ix] = (u16)clamp_val(val, 0, 0xFFFF);
>>
>> This is an unrelated change I believe and I would therefore have this in its own commit with proper documentation in the commit log. Indeed:
>>
>> 1- Change in fan_show handles the default 0x0 register value (which can only currently be achieved via the default value of the registers)
>> 2- Allow (re-)setting unlimited fan speed by allowing the user to pass 6000001+ instead of clamping it to 6000000 RPM.
>>
> 
> Both changes are related.
> 
> The whole point of this commit is to report and permit consistent values when
> the register value is 0. But you do have a point - reading it after my changes
> returns 6000000, but writing the same value sets the register to 1. So I think
> the proper change would be to display 6000001 as speed if the register value is
> 0, and provide a more detailed explanation. Would that address your concerns ?
> 

Ah, never  mind, I'll do it differently:

- If the register value is 0, keep reporting 0.
- If the value written is 0, write 0, otherwise limit the range to 1..6000000
   and write clamp_val(6000000 / val, 1, 0xffff)

This minimizes user visibility of the changes, and also ensures that
the reported fan speed is 0 if the register value is 0 when reading the fan
speed.

Guenter