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

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Message-ID: <1543dfdc-0cde-401c-8ab7-a4f2ee0eb81f@cherry.de>
Date: Mon, 1 Jul 2024 20:05:39 +0200
From: Quentin Schulz <quentin.schulz@...rry.de>
To: Guenter Roeck <linux@...ck-us.net>, 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 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@...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.
>>
>> 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