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

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Message-ID: <9fce6789-edc8-4c44-89c0-ae4ca3ec3315@roeck-us.net>
Date: Mon, 1 Jul 2024 07:11:16 -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 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 ?

Thanks,
Guenter