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

Quentin Schulz
2024-07-01T18:05:46Z

On 7/1/24 7:21 PM, Guenter Roeck wrote:
> On 7/1/24 09:13, Quentin Schulz wrote:
>> Hi Guenter,
>>
>> On 7/1/24 4:37 PM, Guenter Roeck wrote:
>>> 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@roeck-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.
>>
>> Or...... maybe UINT_MAX?
>>
>
> Problem with that is that disconnected fans would report that value as
> fan speed.
> Traditionally drivers report a fan speed of 0 in that case.
>

OK so the issue is that the current fan speed in RPM could be 0 because
it's disconnected, or because it exceeds 6M tach pulses.

> On the other side I agree that reporting "0" as "maximum fan speed" doesn't
> make much sense either because the real limit _is_ unlimited. But reporting
> 4294967295 in that case isn't really any better.
>

Agreed, but I'm also wondering if there really exist fans at 6M+ RPMs?
Maybe we're discussing a scenario that just doesn't exist (yet) and that
we don't need to handle?

[...]
>>> 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.
>>>
>>
>> But didn't you say this means the fan is running at unknown 60 000
>> 000+ RPMs? Do we really want to return 0 even if the fan is actually
>> running? In which case max < current (possibly) but with no event
>> happening (which I would expect, reading the datasheet).
>>
>
> Did I say that ? If so, I must have meant something different. The
> register counts the
> pulse period, so, yes, it would be 0 if rpm is above 6,000,000. But that
> is really not
> realistic. In practice I don't know what the controller reports in the
> register if no
> fan is connected - that would require real hardware which obviously I
> don't have.
>

I'll forage in our shelves tomorrow if I don't forget, trying to find
one... if we have one.

> Overall I think I'll stick with the minimum, at least for now: Permit
> writing 0
> into the high limit register only, and otherwise keep the currently
> permitted ranges.
>

Works for me, we can always revisit later on if needed/desired.

Quentin